Category Archives: Blog

Discovery of Rhenium element


As regards history, rhenium had an undoubted advantage over hafnium: nobody had ever questioned the fact that element No. 75 had to be an analogue of manganese, or tri-manganese in Mendeleev’s terminology. However, in all other respects there was no certainty.

Let us perform an experiment. If we select at random a few monographs and textbooks where rhenium is discussed we shall see that the authors agree on some things while sharply disagreeing on others. They all agree that rhenium was discovered in 1925 but when it comes to the source from which rhenium was extracted, they disagree. Among minerals mentioned as sources of rhenium are columbite and platinum ore, native platinum and tantalite, niobite and wolframite, alvite and gadolinite. Even an experienced geochemist will be at a difficulty finding his way among so varied a group of minerals.

After these introductory remarks, we may name the discoverers of rhenium: V. Noddack, I. Takke (who later married V. Noddack), and the spectroscopist O. Berg. Their authorship was never contested by anybody. This may be the only case when engineers became interested in the yet undiscovered element. They were aware of the uses of the periodic system. Since tungsten was widely used in electrical engineering, there was every reason to believe that element No. 75 would possess properties even more valuable for this industry. It is highly probable that the first attempts of the Noddacks to find this element were prompted by practical needs.

In 1922, after thorough preparations they set to work. First of all, they collected all reports on the discovery of manganese analogues. Since these discoveries remained unconfirmed, it was tempting to check them. The scientists drew up an extensive program of research: they were going to look for two elements at once since unknown manganese analogues included not only element No. 75 but also its lighter predecessor–element No. 43 with an unusual fate (see p. 200). The periodic table made it possible to predict many of their properties. We can now compare the Noddacks’ predictions on rhenium with the actual properties of the element.

            Prediction                                            Modern data

Atomic mass 187-188                                          186.2

Density 21                                                              20.5

Melting point 3300 K                                           3 323 K

The higher oxide formula X27                                  Re2O7

Melting point of the higher

Oxide 400-500oC                                                 220oC

The agreement is, indeed, excellent. Only the melting pint of the oxide proved to be much lower that

the expected one whereas on the whole Mendeleev’s classical method of prediction was fully confirmed. In other words the Noddacks had a perfectly good idea about what element No. 75 (and element No. 43) was going to be. Thus, the history of rhenium was closely related to the history of its light analogue.

But where to search for these element? Predicting the geochemical behavior of rhenium the Noddacks used to the full the capacity of theoretical geochemistry of that time; They even knew that it had to be a very rare element. They could not know, however, that it was a trace element and that, therefore, what seemed unquestionable to them was in effect open to doubt.

The scientists planned to investigate two groups of minerals: platinum ores and so called columbites (tantalites). Four years (from 1921–1925) were spent in searching for the wanted elements but in vain. Then a communication appeared about the discovery of hafnium whose existence in nature was proved by X-ray spectroscopy. Undoubtedly, this event gave the Noddacks the idea to use the same method in order to prove the existence of manganese analogues and they turned for help to O. Berg, a specialist in X-ray spectroscopy.

In June 1925, V. Noddacks, I. Takke, and O. Berg published an article about the discovery of two missing elements, Masurium (No. 43) and rhenium (No. 75). They were found in columbite and in the Uralian platinum and named after two German provinces. The elements X-ray spectra provided the main confirmation of their existence; but there was no question of extracting the elements and the reasoning of the German scientists was, in general, too involved. However, the article attracted attention and other scientists tried to reproduce the results.

However, no such reproduction followed. A year passed and the Soviet scientist O. E. Zvyagintsev and his colleagues proved irrefutably that the Uralian platinum ore contained no new elements. After that the German scientists continued to study columbites which varied considerably in composition but, according to the predictions, had to contain mysterious manganese analogues. They subjective the minerals to complex chemical treatment in order to concentrate the unknown elements and performed X-ray spectral analysis. The data obtained were reassuring but definite conclusions would have been premature: the scientists could not obtain any noticeable amounts of elements No. 43 and No. 75 and experimentally determine their properties.

Nobody could reproduce the results obtained by the Noddacks. Their compatriot W. Prandtl even sent his assistant. A Grimm to the Noddacks’ laboratory to watch them prepare manganese analogues Back home, A. Grimm reproduced the entire procedure, perfected it and…, we do not know the extent of his distress about the wasted time. The English scientists F. Loring and the Czechs Ya. Geirovskii and Y. Druce also doubted the Noddacks’ results. Later, Loring, Geirovskii, and druce claimed the priority of discovering element No. 75 by other methods and from other sources. History has retained their names but not as discoverers of rhenium.

The two German scientists believed to have also isolated element No. 43 (known later as technetium). Now we know that they by no means could detect the presence of technetium at the time but, nevertheless, the Noddacks were more sure of its discovery than of the discovery of rhenium (the fact which is hardly a feather in their cap). As time passed, the Noddacks became convinced that the range of the minerals for analysis had to be considerably enlarged. The previous geochemical prediction did not, apparently, come true. In the summer of 1926 and in 1927 the Noddacks went to Norway to collect minerals among which were: tantalite, gadolinite, alvite, fergusonite, and molybdenite. In the early 1928 the scientists, analysing the minerals, isolated about 120 mg of rhenium mainly from molybdenite (molybdenum sulphide). Earlier it had never been considered as a possible source of manganese analogues.

Thus, rhenium became, at last, a reality. An end was put to doubts and the symbol Re occupied forever box No. 75 in the periodic table; masurium, however, remained an enigma for a long time.

Hence, 1928 is the date of the reliable discovery of rhenium, the final step in the long process of search. As regards the widely accepted date, 1925, it is only a landmark in the prehistory of the element.

Having planned the directions of research, the Noddacks assembled all publications of supposed discoveries of eka-manganese. Their notes were lost during the second World War but, undoubtedly, the name of the Russian Scientists S. F Kern and the name of the element “devium” were mentioned in them. This may be the most reliable discovery of a new element of all unreliable discoveries. And it is equally possible that the history of element No.75 could have begun 50 years earlier.

The events were as follows. In 1877 reports appeared about the discovery of a new metal “devium” named after H. Davy. The reports aroused great interest and Mendeleev suggested inviting S. F.Kern to report to a session of the Russian Chemical Society. The scientists of Bunsen’s laboratory in Heidelberg decided to check Kern’s result carefully. Later his results were confirmed by two or three other scientists the most interesting fact was that some chemical reactions proved to be identical to those found later for rhenium. Does not it point to the identity of devium and rhenium?

For some reason or other S. F Kern lost interest in his discovery and never returned to the problem after 1878. He had extracted the element from platinum ores, which seems impossible from modern point of view (recall Zvyagintsev’s work in 1926). The fact is, however, that platinum ores have a complex and varied composition. The Uralian ore does not contain rhenium but its presence as traces in ores of other deposits has been proven.

  1. F. Kern studied a very rare sample of platinum ore from Borneo where by that time mines had already been abandoned. At the beginning of the 20th century the Russian chemist G. Chernik worked on the island. Analyzing platinum ores he found a constant mass loss in all samples and tried to explain it by the presence of an unknown element. This element- could well be Kern’s “devium”.

In 1950 Y. Druce devoted a large article to devium. He wrote that if rhenium would be discovered in platinum minerals, this would confirm Kern’s discovery. Samples of platinum ores from Borneo can be found now only in a few mineralogical museums of the world. It would be of interest to analyse them thoroughly. This is a case when the history of a chemical element could be partially changed.

Discovery of Hafnium element


The Institute of Theoretical Physics of the Copenhagen University in Denmark was the birthplace of a new element with Z = 72; the date of birth was the end of December, 1992, although the article about the discovery appeared in a scientific journal only in January, 1923. The Dutch spectroscopist D. Coster and the Hungarian radiochemist G. Hevesy named the element after the ancient name of CopenhagenHafnia. N. Bohr, whose role in the discovery of hafnium was decisive, stood at the cradle of the element.

The source of element No. 72 was zircon, a rather common mineral, consisting mainly of zirconium oxide. And it was Bohr who suggested the mineral as a subject of investigation. Why was the Dutch physicist so confident of success? Let us go back to the 1870’s when Mendeleev was drawing up his periodic system. He reserved the box under zirconium for an unknown element with the atomic mass about 180. Using Mendeleev’s terminology, we could name it eka-zirconium. After Mendeleev’s predictions of gallium, scandium, and germanium had come true, the confidence in the existence of eka-zirconium became stronger. The question, however, remained about the properties of this hypothetical element. Mendeleev refrained from definite assessments. Generally speaking, there were two possibilities: either eka-zirconium was part of the IV B-subgroup of the periodic table, i.e. an analogue of zirconium, or it belonged to the rare-earth family as its heaviest element. Now the time has come to recall the name “celtium” (see p. 138).

Having split ytterbium and separated lutetium, the last of the REEs existing in nature, G. Urbain continued the difficult work of separating heavy rare earths. Finally, he succeeded in collecting the fraction whose optical spectrum contained new lines. This event took place in 1911 but at the time did not attract the attention of the scientific community. Perhaps Urbain himself, having suggested the name for it, was not quite sure that he had really discovered a new element. At any rate, he thought it wise to send samples of celtium to Oxford where Moseley worked. Moseley studied the samples by X-ray spectroscopy but the X-ray photographs turned out to be of a poor quality. Nevertheless, in August 1914, Moseley published a communication in which he firmly stated that celium was a mixture of known rare earths. The communication remained practically unnoticed. In a word, the discovery of celtium for a very long time considered to be doubtful, although the symbol Ct sometimes appeared in scientific journals.

Meanwhile N. Bohr was working on the theory of electron shells in atoms which also became the corner-stone of the periodic system theory and, at last, explained the periodic changes in the properties of chemical elements. Bohr also solved the problem which had interested chemists of many years: he found the exact number of rare-earth elements. There had to be fifteen of them from lanthanum to lutetium. Only one REE between neodymium and samarium (later known as promethium, see p.208) remained unknown. Bohr came to this conclusion on the basis of the laws found by him which governed the formation of electron shells of atoms with increasing Z.

Thus, if celtium were indeed a rare-earth element, Bohr’s theory would eliminate it completely. Why couldn’t it be eka-zirconium? Having proved that lutetium completed the REE series, Bohr firmly established that element No. 72 had to be a zirconium analogue and could be nothing else. Bohr advised D. Coster and G. Hevesy to look for the missing element in zirconium minerals. Now all this seems to us quite logical and clear but at that time many things were at stake: if element No. 72 could not be proved to be a complete analogue of zirconium, the whole of Bohr’s periodic system theory would have been questioned. Having separated hafnium from zirconium Coster and Hevesy confirmed this theory experimentally just as the discovery of gallium had been a confirmation of Mendeleev’s periodic system than half a century before.

When Urbain read the communication about the discovery of hafnium, he understood that this was the end of celtium. Not everybody can take the bitterness of defect with dignity. Urbain was reluctant to part with celtium and continued his attempts to identify it with element No.72. The French spectroscopist A. Dauvillier came to help; he tried to prove the originality of celtium spectra thus making the “element” one of the rare earths.

Moreover, Urbain and Dauvillier declared that Coster and Hevesy had only rediscovered celtium but nothing much came of it, since hafnium soon came into its own. It was prepared in pure form and new spectral investigations showed that there was nothing in common between hafnium and celtium. What an irony of history! Urbain had everything to be the first to discover hafnium. At the beginning of 1992 he and his colleague C. Boulange analysed thortveitite, a very rare mineral from Madagascar. The mineral contained 8 per cent of zirconium oxide and the content of hafnium oxide was even higher. It is the only case when hafnium is contained in the mineral in amounts greater than those of zirconium and, nevertheless, Urbain and Boulange failed to uncover element No. 72. The reason for this lies in the great chemical similarity between zirconium and hafnium.

Prediction of Unknown Chemical Elements

Prediction of Unknown Chemical Elements

The history of gallium, scandium and germanium shows that their discoveries were practically unaffected by the periodic law and periodic system. However, the properties predicted by D.I Mendeleev for eka-aluminium, eka-boron and eka-silicon coincided with those of gallium, scandium, and germanium. Mendeleev had determined the main features of these elements long before they were discovered in nature. Is not this fact a striking evidence of the periodic system’s power of prediction?

The discovery of gallium and its identity with eka-aluminium became milestones in the history of the periodic law and in the history of discovery of elements. After 1875 even those scientists who had disregarded the periodic system had to recognize its value. And among them there were top researchers, such as R. Bunsen, the creator of spectral analysis (he once said that to classify elements is the same thing as to search for regularities in the stock-exchange quotations) or P. Cleve who had never mentioned the periodic system in his lectures. The discovery of scandium and germanium meant further triumph of Mendeleev’s theory of periodicity.

In addition to the classic triad Mendeleev predicted the existence other unknown elements. On the whole, as early as 1870 Mendeleev saw about ten vacant places in his table. He saw them, for instance, in the seventh group where there were neither manganese analogues nor a heavy iodine’s analogue (the heaviest halogen which had to possess metallic properties).

In Mendeleev’s papers we find mention of eka-, dvu-, and tri-manganese and eka-iodine. The scientist firmly believed in their existence. And here we encounter a very interesting fact in the predictions. Eka-manganese (known subsequently as technetium) and eka–iodine (astatine) were synthesized later. Mendeleev, naturally, could not know that they did not exist in nature and firmly believed in their existence since these elements filled in the gaps in the periodic system and made it more logical.

The prediction consists of two stages: prediction of the existence of an element and prediction of its main properties. The first stage was in many respects guess-work for Mendeleev. As yet unknown was the phenomenon of radioactivity making some elements so short-lived that their earthly existence is impossible at all or they exist only because they are products of radioactive transformations of long-lived elements (thorium and uranium).

The second stage was completely within Mendeleev’s power and depended on his confidence. Sometimes Mendeleev predicted boldly and resolutely. This was the case with eka-aluminum, eka-boron, and eka-silicon: these elements had to be placed in that part of the periodic table where many well-known and well-studied elements had already been located–the region of reliable prediction. Sometimes Mendeleev predicted the properties of unknown elements with the extreme caution. Among them were analogues of manganese, iodine, and tellurium as well as the missing elements of the beginning of the seventh period: eka-cesium, eka-barium, eka-lanthanum, and eka-tantalum. Here Mendeleev was groping in the dark, darling only to estimate atomic masses and suggest formulas of oxides. Mendeleev thought that it was difficult to predict the properties of the unknown elements (including those of REEs) whose places were at the boundaries of the system because there were few known elements around them. This was the “grey” area of uncertain prediction. Of course, they included the rare–earth elements. Finally, in some parts of the periodic table prediction was completely unreliable. They included those mysterious stretches extending in the directions of hypothetical elements lighter than hydrogen and heavier than uranium. Mendeleev never thought that the periodic system had to begin with hydrogen. He even wrote a paper in which he described two elements preceding hydrogen. Only when physicists explained the meaning of the periodic law, his mistake became clear: the nucleus of the hydrogen atom had the smallest charge equal to 1. As regards elements which are heavier than uranium, Mendeleev conceded the existence of a very restricted number of them and never took the liberty of predicting, even approximately, their possible properties. Predictions of this kind did not come until much later when they signaled important events in the history of science.


Discovery of Elements : Germanium


Among the three elements predicted by Mendeleev eks-silicon was the last to be discovered and its discovery was to a greater extent than in the case of the two others, due to a chance. Indeed, the discovery of gallium by P Lecoq de Boisbaudran was directly related to his spectroscopic investigations, and the separation of scandium by L. Nilson and P. Cleve was associated with thorough investigation of REEs, which was going on at the time.

Predicting the existence of eka-silicon, Mendeleev assumed that it would be found in minerals containing Ti, Zr, Nb, and Ta; he himself was going to analyse some rare minerals in search for the predicted element. Mendeleev, however, was not fated to do it and 15 years had to pass before eka-silicon was discovered.

In summer 1885, a new mineral was found in the Himmels–furst mine near Freiberg. It was named “argyrodite” since chemical analysis showed the presence of silver the Latin for which is argentum. The Freiberg Academy of Mining asked the chemist C. Winkler to determine the exact composition of the mineral. Analysis was comparatively easy and soon Winkler found the mineral to contain 74.72% silver, 17.43% Sulphur, 0.66% iron (II) oxide, 0.22% zinc oxide, and 0.31% mercury. But what surprised him was that the percentage of all the elements found in argyrodite added up to only 93.04 per cent instead of 100 per cent. No matter how many times Winkler repeated the analysis 6.96 per cent was missing.

Then Winkler made an assumption that the elusive amount had to be an unknown element. Inspired by the idea he began to study the mineral carefully and in February 1886 the principal events in the discovery of eka-silicon took place.

On February 6, Winkler reported to the German Chemical Society that he had succeeded in preparing some compounds of the new element and isolating it in a free state. The scientist’s report was published and sent to many scientific institutions all over the world. Here is the text received by the Russian Physico-Chemical Society: “The signatory has the honour to inform the Russian Physico-Chemical Society that he found in argyrodite a new non-metal element close in its properties to arsenic and antimony which he named “germanium”. Argyrodite is a new mineral found by Weisbach in Freiberg and consisting of silver, Sulphur, and germanium.”

Three points in this letter deserve attention: firstly, Winkler considered the new element to be a non-metal; secondly, he assumed its analogy with arsenic and antimony, and, thirdly, the element had already been named. Originally, Winkler wanted to name it “neptunium” but the name had already been given to another element–a false discovery–and the scientist proposed the name “germanium” after “Germany”. The name became widely accepted although not immediately.

Later it become clear that germanium is to a great extent amphoteric in nature and, hence, Winkler’s description of germanium as a non-metal cannot be considered completely erroneous. Much sharper debates revolved around the question the analogue of which element in the system germanium was. In his first report Winkler suggested arsenic and antimony but the German chemist Richter disagreed with Winkler saying that germanium, most likely, was identical to eka-silicon. Richter’s opinion seemed to affect the opinion of the discoverer of germanium and in his letter of February 26 to Mendeleev Winkler wrote: “At first I thought this element would fill the gap between antimony and bismuth in your remarkable and thoughtfully composed periodic system and that the element would coincide with your eka-antimony, but the facts indicate that here we are dealing with eka-silicon.”

Such as Winkler’s reply to Mendeleev’s letter of congratulation. It is interesting that the antimony-germanium analogy was considered erroneous by Mendeleev but he did not think of germanium as eka-silicon either. Probably, Mendeleev was surprised that the natural source of the new element proved to have nothing in common with that predicted by him earlier (titanium and zirconium ores). The discoverer of the periodic law proposed another hypothesis: germanium is an analogue of cadmium, namely eka–cadmium. It the nature of gallium and scandium was established beyond any doubt, as regads germanium, Mendeleev was less certain. This uncertainty, however, soon gave way to certainty and already on March 2 Mendeleev wired to Winkler conceding the identity of germanium and eka-silicon.

Soon an exhaustive article by Winkler entitled “Germanium–a new element” was published in the “Journal of Russian Physico-Chemical Society”. It was a new illustration of the brilliant similarity between the predicted properties of eka-silicon and real properties of germanium.



We have already briefly mentioned the discovery of scandium in the chapter devoted to REEs (see p. 130). Although many of scandium’s properties are similar to those of rare earths, D. I. Mendeleev predicted that the element would be a boron analogue in the third group of the periodic system. His prediction proved to be accurate enough. Scandium was discovered by the Swedish chemist L. Nilson; on March 12, 1879, his article “On Scandium, a New Rare Metal” was published and on March 24 it was discussed at a session of the Paris Academy of Sciences.

Nilson’s results, however, were in many respects erroneous. He considered scandium to be tetravalent and gave, therefore, the formula of its oxide as ScO2. He did not measure the atomic mass and gave only its probable range (160-180). And, finally, Nilson suggested that scandium should be placed in the periodic table between tin and thorium, which ran counter to Mendeleev’s prediction.

The discovery of scandium excited the scientific community and Nilson’s compatriot P. Cleve set out to study the newly discovered element. He studied it thoroughly for almost five months and came to the conclusion that many results obtained by Nilson were erroneous. Cleve reported to the Paris Academy of Science on August 18, and the academicians learnt much new about scandium. It turned out to be trivalent; its oxide’s formula was Sc2O; its properties differed somewhat from those determined by Nilson. According to Cleve (and this was especially important) scandium was the eka-boron predicted by Mendeleev; Cleve showed a table in the left-hand column of which eka-boron properties were given and in the right-hand one those of scandium the following day Cleve sent a letter to Mendeleev in which he wrote: “I have the honour to inform you that your element eka–boron, has been obtained”. It is scandium discovered by L. Nilson this spring.

And, finally, on September 10 Cleve published a long article about scandium from which it is clear that he had a much better understanding of the new element than Nilson. Therefore, those historians are who consider Cleve and Nilson as co–discoverer of scandium right.

For a long time Nilson was working under an illusion about some of scandium’s properties and refused to recognize its identity with eka-boron. Cleve’s investigations, however, impressed Nilson very much; in the long run he was forced to admit that he was wrong, thus doing justice to the prediction power of the periodic system.

All of Mendeleev’s predictions were confirmed in the long run. The last to be confirmed was the prediction of the density of metallic scandium; only in 1937 did the German chemistry W. Fischer succeed in preparing 98 per cent scandium. Its density was 3.0 g/cm3, that is exactly the figure predicted by Mendeleev.

Discovery of Elements : Gallium


The time of discovery of gallium is known to an hour. “One Friday of August 27, 1875, between 3 p.m. and 4 p.m. I discovered some signs that there can be a new simple body in the by-product of chemical analysis of zinc blende from the Pierfitt mine in the Argele valley (Pyrenees).” With these words P. E. Lecoq de Boisbaudran began his report to the Paris Academy of Sciences. He described some of the new element’s properties and noted that its presence in the ores was ascertained by spectra; analysis just as predicted by Mendeleev five years before. Boisbaudran extracted an extremely small amount of the substance and, therefore, could not study its properties properly.

On August 29, Boisbaudran suggested to name the element “gallium” after Gaul, the ancient name of France. The scientist continued the investigation of the new element and obtained additional information which he included into his report to the Paris Academy and then sent it to be academic journal. In the middle of November the journal with the article reached Petersburg where Mendeleev was impatiently waiting for it. There is every reason to believe that Mendeleev had already learnt about gallium though at second hand. Two weeks earlier the Russian Chemical Society had received a report from Paris signed by P. de Clermont. It recounted the discovery of gallium and contained a brief description of its properties. However, it was much more important for Mendeleev to read what the discoverer himself had written. Mendeleev’s reaction was prompt; on November 16, he delivered a report to the Russian Physical Society. According to the minutes of the session, Mendeleev declared that the discovered metal was, most probably, eka-aluminium. Next day he wrote an article in French entitled “Note on the discovery of Gallium”. And finally, on November 18, Mendeleev spoke about gallium at a session of the Russian Chemical Society. Such a spurt of activity is understandable: the great chemist saw an element predicted by him becoming a reality. Mendeleev believed that if further investigation confirmed the similarity of eka-aluminium properties of those of gallium, this would be an instructive demonstration of the periodic law’s usefulness.

Six days later (a surprisingly short time!) the “Note on the Discovery of Gallium” appeared in the journal of the Paris Academy of Sciences. Boisbaudran’s reaction to it is of particular interest. He continued his experiments and prepared the new results for publication. The next article by the French scientist was published on December 6. As before, he complained of the difficulties caused by the extreme scarcity of gallium, described the preparation of the metal by the electrochemical method and discussed some of its properties, and suggested that the formula of gallium oxide had to be Ga2O3.

Only at the end of the article were there a few words about Mendeleev’s note. Boisbaudran admitted that he had read it with great interest since classification of simple substances interested him for a long time. He had never known about Mendeleev’s prediction of eka-aluminium properties but it did not matter; Boisbaudran believed that his discovery of gallium was facilitated by his own laws of spectral lines of elements with similar chemical properties. In his opinion, spectral analysis played a decisive role. And not a word that Mendeleev in his prediction of eka-aluminium also underlined the prominent role of spectral analysis in the discovery of the new element. According to Boisbaudran, Mendeleev’s predictions had nothing to do with the discovery of gallium.

However, as Boisbaudran went on studying the properties of metallic gallium and its compounds, his results continued to coincide with Mendeleev’s predictions. For instance, in May 1876, the Franch scientist established that gallium was readily fusible (its melting point is 29.5oC), its appearance remained the same after storage in air, and it was slightly oxidized when heated to redness. The same properties of eka-aluminium were predicted by Mendeleev in 1870, who calculated the density of eka-aluminium to be 5.9-6.0 on the basis of the periodic system and the densities of eka-aluminium’s neighbours. Lecoq de Boisbaudranm, however, making use of his spectral laws, found that the density of eka-aluminium was 4.7 and confirmed the value experimentally. Such a difference (less than two units) might seem small to a layman but it was essential for the future of the periodic law. Up to that time only qualitative characteristics of the predicted properties had been confirmed and density was the first quantitative parameter. And it turned out to be erroneous.

There is a widely known story that Mendeleev, having received Boisbaudran’s article citing a low (4.7) density of gallium, wrote him that the gallium obtained by the French chemist was contaminated most likely by sodium used in the process of gallium preparation. Sodium has a very low density (0.98), which could substantially decrease the density of gallium. Hence, it was required to purify gallium thoroughly.

This letter has not been found either in French or in the Mendeleev’s archives. There is only indirect evidence from Mendeleev’s daughter and the eminent historian of chemistry B. Menshutkin that the letter did exist. However, that may be Mendeleev’s views became known to Boisbaudran who decided to repeat the measurements of gallium’s density. This time he took into account that Mendeleev’s calculations for the hypothetical element’s density this time he took into account that Mendeleev’s calculations for the hypothetical element’s density gave 5.9. And be obtained this value at the beginning of September, 1876. His report about this fact needs no comments. The French scientist became firmly convinced of the extreme importance of the confirmation of Mendeleev’s predictions about the density of the new element. Sometime later Lecoq de Boisbaudran send his photo to the great Russian chemist with the inscription: “With profound respect and an ardent wish to count Mendeleev among my friends. L. de B.” Mendeleev wrote under it: “Lecoq de Boisbaudran. Paris. Discovered eka-aluminium in 1875 and named it “gallium”, Ga=69.7.”

In autumn 1879, F. Engels became acquainted with a new detailed chemistry textbook by H. Roscoe and C. Shorlemmer. For the first time it contained the story about the prediction of eka-aluminium by Mendeleev and its discovery as gallium. In an article to be later included in his Dialectics of Nature Engels quoted the corresponding text from the book and concluded: “by means of the unconscious application of Hegel’s law of the transformation of quantity into quality, Mendeleev achieved a scientific feat which is not too bold to put on a par with that of Leverrier in calculating the orbit of the still unknown planet Neptune”.

Discovery of Inert Gasses

Inert Gases 

The discovery of inert gases ranks among the four great scientific events of the end of the 19th century that led to revolutionary changes in natural sciences, the other three being the discovery of X-rays by Roentgen, discovery of radioactivity, and the discovery of electron.

This prominence given by scientists to inert gases has many reasons.The history of their discovery is colourful and exciting. Helium, the mysterious solar element, was discovered on the earth and this fact alone illustrates how inventive and penetrating man’s mind became in his striving for deeper and better understanding of nature.

No less mysterious argon sowed confusion among scientists. Its chemical inertness made it impossible to be classified as a chemical element in the ordinary sense of the term since it revealed no chemical properties. There was nothing left for the researchers but to grow accustomed to the idea that there can be elements unable to enter into chemical reactions. The idea proved extremely fruitful. The discovery of inert gases contributed to development of the zero valence concept. Moreover, forming an independent zero group they added harmony to the periodic system. Almost twenty five years after their discovery the inert gases helped N. Bohr to develop his theory of the electron shells of atoms. This theory, in its turn, explained the chemical inactivity of the inert gases and their atomic structure became the basis of the concepts of ionic and covalent bonds. Thus, the discovery of inert gases contributed greatly to the development of theoretical chemistry.

In the early 60’s they surprised the scientific community once more. Scientists showed that Xenon (mainly) and krypton can form chemical compounds. Now more than 150 such compounds are known. Such late “debunking” of the myth about the complete chemical inactivity of inert gases is a paradoxical and interesting feature in their history.

Inert gases are among the rarest stable elements on the earth. Here are the data given by Ramsay: there is one part by volume of helium per 245 000 parts of atmospheric air, one of neon per 81 000 000, and one of argon per 106, one of krypton per 20 000 000 and one of xenon per 170 000 000. Since then these figures have remained almost unchanged. Ramsay said that xenon content in air is less than that of gold is sea water. This alone shows how excruciatingly difficult was the discovery of inert gases.

Discovery of Elements: Krypton, Neon, and Xenon

Krypton, Neon, and Xenon

In the history of inert gases, many problems stuck in starting. Out of many problems and their several reasons ; one of them was that scientists were dealing with very small amounts of Argon and Helium. To isolate them from air, one had to chemically remove oxygen, nitrogen, hydrogen, and carbon dioxide. All inert gases constitute a negligible part of the earth’s atmosphere.

Detection and isolation of so minor traces was difficult specially when Helium and Argon were known.  

Another reason was chemical inactivity of Argon and Helium. Even the most active reagents (for instance, fluorine) were powerless to combine with these gases and isolate them . Chemists had no way of studying inert gases and only physical methods could bring some results. Therefore better physical methods were required and they were being developed during this discovery period.

Scientists developed experimental techniques for analysing small amounts of gases, perfected spectroscopes and devices for determining gas densities.

Finally, an event took place that was of extreme importance for the history of inert gases. Two engineers, U. Hampson from England and G. Linde from Germany, invented and effective process for liquefaction of gases. Hampson built an apparatus that produced on liter of liquid air per hour. The success gave an impetus to the creative though of scientists.

In early 1898 M. Travers, Ramsay’s assistant, began to design a refrigerating apparatus for preparing large amounts of liquid argon. Since atmospheric gases liquefy at different temperatures, they can easily be separated from one another. The discoveries of argon and helium are remarkable also in that they set the chemists thinking not only about the nature of chemical inertness ( the phenomenon was understood only about a quarter of a century later) but about the periodic law and periodic system which were under a serious threat. Three most important characteristics of argon and helium (atomic masses, zero valence, monatomic molecule) put both gases outside the system. That is why Mendeleev was so readily attracted by the convenient thought about N3.

History has a striking power of prediction. Argon had not been properly discovered yet, when on May 24, 1894, Ramsay wrote a letter to Rayleigh in which he asked whether it had ever occurred to him that there was indeed a place in the periodic table for gaseous elements. For instance:

Li         Be        B         C          N         O          F          X          X          X


                                                                                    Mn       Fe        Co        Ni


                                                                                    ?           Rd        Ru        Pd…

Ramsay assumed that the system’s small period could contain a triad to elements similar to those of iron and platinum metals in the great periods. The discoveries of argon and helium gave rise to an idea that these gases could occupy the places of two Xs in Ramsay’s graph. The atomic masses of these elements, however (4 and 40, respectively), proved to be too different for He and Ar to be placed in the same period. Gradually, the idea about new triads was relegated to the background and Ramsay proposed to place inert gases at the end of each period. In this case one could even expect the discovery of an element with the atomic mass 20, an intermediate between helium and argon. Ramsay’s report at the session of British Association in Toronto in August 1897, was devoted just to this element. The report was entitled “Undiscovered Gas”. Ramsay wanted to describe interesting properties of the gas but though it unwise not to mention its most remarkable property: the gas had not been discovered yet.

And here again we see the same certainty which permeated Ramsay’s letter to his wife on the eve of argon’s discovery. But not it was not audacity of a romantic but conviction multiplied by experience. The undiscovered gas turned out to be neon. Owing to a whim of fate (a frequent thing in science) the discovery was preceded by another event. The new gas could, obviously, be discovered by gradual evaporation of liquid air and by analysis of the resulting fractions, the ones lighter than argon being especially interesting. On May 24, 1898, Ramsay and Travers received a Dewar flask with liquid air. Unfortunately (or, rather, fortunately) the amount of air was too small to search for argon’s predecessor and the scientists decided to use the material for perfecting the procedure of liquid air fractionation. Having done so, Ramsay and Travers discovered by the end of the day that the fraction remained was the heaviest one. For a week the fraction remained neglected until on May 31 Ramsay decided to investigate it. The gas was scrubbed from possible impurities of nitrogen and oxygen and subjected to spectral analysis. Ramsay and Travers were dumbfounded when they saw a bright yellow line which could belong neither to helium nor sodium. Ramsay wrote down in this diary: “May 31. A new gas. Krypton.” Recall that this name was previously given to undiscovered helium. Now the name found its place in the history of inert gases. Krypton, however, was not the gas about which Ramsay made a report. Its density and atomic mass were higher than the predicted ones.

The discovery of neon promptly followed. Ramsay and Travers selected light fractions formed on the distillation of air and discovered a new inert gas in one of them. Ramsay later recollected that the name “neon” (from the Greek neos for “new”) had been proposed by Ramsay’s twelve-year-old son. In this case the experiment was performed by Travers alone since Ramsay was away. It was on the 7th of June. Then a whole week was required to confirm the result, obtain greater amounts of neon, and determine its density. Neon, as had been expected, turned out to be an intermediate between helium and argon although t had not yet been isolated as a pure gas. The problem of complete separation of neon and argon was solved later.

Still another inert gas was to be discovered by Ramsay and Travers. The scientists, however, did not feel as certain as in the case of neon. One day in July, 1898, the colleagues were busy with distilling liquid air and separating it into fractions. By midnight they collected more than 50 fractions discovering krypton in the last of them (No. 56). After that upon heating the apparatus one more fraction was collected (No. 57) consisting, mainly, of carbon dioxide traces. Ramsay and Travers argued about the expediency of studying it and at last decided to proceed with the of experiment. Next morning the scientists observed the spectrum of fraction No. 57, which turned out to be highly unusual. Ramsay and Travers concluded that it could be attributed to a new gas. pure xenon, however was prepared only in the middle of 1900. The name “xenon” originates from the Greek xenos, which means “stranger”

Discovery of Element : Argon


If you saw the statement “Inert gases were discovered by H. Cavendish in 1785” you would treat it as a joke. But no matter how paradoxical it seems, it is essentially true. Only the word “discovered” is misused here. One would be equally justified in declaring that hydrogen was discovered by R. Boyle in 1660 or by M.V Lomonosov in 1745. In his experiments Cavendish only observed “something” whose nature became clear on hundred years later. In one of his laboratory records Cavendish wrote that, passing an electric spark through a mixture of nitrogen with an excess of oxygen, he obtained a small amount of residue, no more than 1/125 the initial volume of the mixture. This mysterious gas bubble remained unchanged under the subsequent action of the electric discharge. It is clear now that it contained a mixture of inert gases, the fact which Cavendish could neither understand nor explain.

The famous English physicist’s experiment was described in 1849 by his biographer H. Wilson in the book Life of Herny Cavendish. In the early 80’s of the 19th century Ramsay studied the reaction of gaseous nitrogen with hydrogen and oxygen in the presence of a platinum catalyst. Nothing came out of these experiments and Ramsay did not even publish his results. As he recalled later, he had just read the book by Wilson and wrote “Pay attention” against the description of Cavendish’ experiment. He even asked his assistant C. Williams to repeat the experiment but we do not know the result of the attempt. Most likely, nothing came out of it. The episode, however, turned out to be unforgettable for Ramsay (his “hidden memory”, as he called it) and played a certain role in the prehistory of argon’s discovery. At first, the English physicist J. Rayleigh was the main character in it and the need for a further development of the atomic and molecular theory was its historic background. It was essential to specify the atomic masses of the elements for the development of the theory. Numerous experiments showed that in the majority of cases the atomic masses were not integers. Meanwhile, as early as 1815–1816 the English physician W. Prout advanced a hypothesis, a landmark in the history of natural sciences, that atoms of all chemical elements consist of hydrogen atoms; thus, atomic masses had to be integers. Therefore, either Prout was wrong, or the atomic masses were determined incorrectly.

To remove the discrepancy, new studies of the composition and nature of the gases were required. Rayleigh thought it necessary to determine, first of all, the densities of the main atmospheric gases, nitrogen and oxygen, since their atomic masses could then be calculated on the basis of the density values.

Rayleigh published a short article in the influential English journal Nature on September 29, 1892. It might seem that the article was about a mere trifle; the density of nitrogen separated from atmospheric air differed from that of nitrogen obtained by passing a mixture of air and ammonia over a red-hot copper wire. The difference was very small, only 0.001, but it could not be explained by an experimental error. Atmospheric nitrogen was heavier. Thus, a mystery appeared which was described as “an anomalously high density of atmospheric nitrogen”. Nitrogen obtained by any other chemical techniques was always lighter by the same value.

What was the cause of the discrepancy? Ramsay became interested in the problem. On April 19, 1894, he met with Rayleigh and discussed the situation. Each of them, however, remained firm in his previous conviction. Ramsay believed that atmospheric nitrogen contained an admixture of a heavier gas and Rayleigh, on the contrary, felt that an admixture of a lighter gas in “chemical” nitrogen was responsible for the discrepancy.

Rayleigh’s view seemed more attractive. The composition of atmosphere had been thoroughly studied for more than a hundred years and it was hardly possible that some components of the air could have remained undetected. It is just the time to remember Cavendish’s experiment and for Ramsay’s “hidden memory” to work. On April 29, Ramsay sent a letter to his wife in which he wrote that nitrogen, probably, contained some inert gas which had escaped their attention; Williams is combining nitrogen with magnesium and is trying to establish what remains after the reaction. “We can discover a new element.”

The latter breathes confidence: an unknown gas is a new element which, like nitrogen, is inactive, i.e., it hardly enters into chemical reactions. To separate the “stranger” from nitrogen, Ramsay tried to bond nitrogen chemically and used the reaction of nitrogen with red-hot magnesium shaving (3Mg+N2 = Mg3N2); this is the only example when chemistry played a role in the discovery of inert gases. Entering into polemics with himself Ramsay, however, assumed another possibility: the unknown gas is not a new element but an allotropic variety of nitrogen whose molecular consists of three atoms (N3) like oxygen (O2–molecular oxygen and O3–ozone). The absorption of nitrogen with magnesium must be accompanied with the decomposition of the N2 molecule into atoms; the single N atom could then be added to N2 forming N3. Such was Ramsay’s thinking and later the assumption about the existence of N3 became a trump card in the hands of argon’s opponents. Fruitless attempts to separate an ozone-like nitrogen continued for more than two months but by the 3rd of August Ramsay had 100 cm3 of a gas which was nitrogen with a density of 19.086.

The scientist wrote about his success to Crookes and Rayleigh. He send an ampoule with the gas Crookes for spectroscopic investigations; Rayleigh himself collected a small amount of the new gas. In the middle of August Ramsay and Rayleigh met at a scientific session and made a joint report. They described the spectrum of the gas and underlined its chemical inactivity. Many scientists listened to the report with interest but were surprised: how could it be that air contained a new component? The eminent physisist O. Lodge even asked: “Didn’t you, gentlemen, discover the name of the new gas as well?

The difficulty about the name was settled in early November when Ramsay suggested to Rayleigh to name it argon (from the Greek for “inactive”) taking into account its exceptional chemical inactivity and to assign the symbol A to it (which later became Ar.) On November 30, the president of the Royal Society Lord Kelvin (W. Thomson who in 1871 was the first to use the name “helium”) Publicly described the discovery of a new constituent of the atmosphere as the outstanding scientific event of the year. The nature of the constituent, however, was unclear. Was it a chemical element? Such authorities as D. I. Mendeleev and J. Dewar, the inventor of the flask for storage of liquid air, believed that argon was N3. The absolute chemical inactivity of argon was a new property previously unknown to chemists and, therefore, it was difficult to study the gas (in particular, to determine its atomic mass). In addition, it became clear that argon, unlike all known elemental gases, is monatomic, i.e. its molecule consists of one atom. At a session of the Russian Chemical Society on March 14, 1895, Mendeleev declared: argon’s atomic mass of 40 does not fit the periodic system, hence, argon is condensed nitrogen N3.

Much time had passed before the many problems presented by the discovery of argon were solved. A certain role was played here by the discovery of helium, which also turned out to be an inert and monatomic gas. The argon-helium pair allowed an assumption to be made that the existence of such gases is a regularity rather than a mere chance and one could expect the discovery of new representatives of this family. However, they were not discovered until three years passed. In the meantime scientists thoroughly studied the properties of helium and argon, made precise determination of their atomic masses, and put forward ideas about the location of both elements in the periodic table.

Discovery of Helium element


Helium’s unusual story attracted attention of many scientists and science historians, but the real sequence of events was distorted in numerous descriptions which overgrew with a lot of fictional details. Even a legend was invented beautiful and impressive–about the discovery of the sun element but it was far from the truth.

The French astronomer J. Janssen and the English astronomer N. Lockyer are considered to be the discoverers of helium. They studied the total solar eclipse of 1868 which was especially convenient to observe on the Indian ocean shores. In letters sent to the Paris Academy of Sciences and read out at one of its sessions they wrote that the spectra of the sun photographed during the eclipse contained a new yellow line D3 corresponding to an unknown element. To commemorate this remarkable event (the discovery of a new element existing on the sun but not on the earth) a special medal was minted.

Everything is wrong in this fascinating story except two dates. First of all, in August 1868, Lockyer was not on the Indian Ocean coast and did not observe the total solar eclipse. Janssen made his observations after the eclipse. They were of great importance for astronomy but not for the history of helium. The French astronomer was the first to observe solar prominences (gigantic ejections of solar matter) not during an eclipse and to describe their nature. Here is the text of the telegram sent by him to the Paris Academy of Science: “the eclipse and prominences were observed, the spectrum is remarkable and unexpected; Prominences are of a gaseous nature.”

Up to that time scientists had known nothing about the nature of prominences. Now it became clear they were clouds of gaseous matter and had a complex chemical composition. A detailed description of his observation was given by Janssen in a letter which reached Paris only 40 days later and was two weeks behind the letter of another French astronomer S. Raye. The latter also observed the prominences and made certain conclusions about them. And what was Lockyer doing at the time? Without leaving England, he observed the prominences with the help of a specially designed spectroscope and determined the positions of lines in their spectra. On October 23 he sent a letter to the Paris Academy of Sciences; by a surprising coincidence it was received on the same day as J. Janssen’s letter.

On October 26 the letters of Janssen and Lockyer were read to the session of the Academy but they did not contain a word about either the hypothetical sun element or the line which was later identified as the characteristic line of the helium spectrum. It was only pointed out in the letters that prominences had been observed when the sun was not eclipsed. And the medal was minted precisely to mark this event. Thus, no helium was discovered on August 18, 1868, either by Janssen or by Lockyer. Their observations provided an impetus for an intensive study of prominences by many astronomers. And only then was it noticed that the spectra of prominences contained a line which could be assigned to none of the elements known on the earth. Most clearly the line was observed by the Italian astronomer A. Secci who later designated it as D3. Secci’s name ought to be placed side by side with those of Janssen and Lockyer. His role in discovering helium was no less than that of his predecessors. Secci, however, assumed that the D3 line could belong to some known element, for instance, hydrogen, under high pressures and temperatures. If this assumption had not been confirmed, Secci would have agreed to consider D3 line as corresponding to some element unknown on Earth. N. Lockyer and E. Frankland tried to solve the problem posed by Secci but they did not notice any changes in the hydrogen spectrum. Therefore, in his article of April 3, 1871, Lockyer already used the expression “a new element X”. There are indications that the name “helium” (from the Greek helios for “solar”) was proposed by Frankland. The word “helium” was first uttered at a British Association Session by its president V. Thomsov (Lord Kelvin) on August 3, of the same year. Even if we regard the discovery of helium as “fait accompli”, then, it still remained unusual. It was the only element which could not be isolated in a material form. What is helium under ordinary conditions–gas, liquid, or solid? What are its properties? What is its atomic mass and where is its place in the natural series of elements?

None of these questions could be answered even approximately. Besides, Secci’s doubt was still not cleared. Thus, a period began in the history of helium when it was only a hypothetical element. There was no consensus on helium. Mendeleev firmly supported Secci’s point of view, feeling that the bright yellow line could belong to some other known element at high temperatures and pressures. W. Crookes, however, completely recognized helium’s independence and considered it to be a primary matter which gave rise to all other elements via successive transformations.

Sometimes it seemed that helium was not unique in its mysteriousness. Astronomers discovered new lines in the spectra of various cosmic objects: the sun, the stars, and nebulae. A number of hypothetical elements appeared, namely coronium, arconium, nebulium, protofluorine. Several years later they were all recognized to be nonexistent and only helium survived.

To receive recognition, helium had to show its “earth face” and its “earth” history began with a chance event.

On February 1, 1895, W. Ramsay received a short letter from K. Miers, a British museum employee. By that time Ramsay had already been acclaimed as the discoverer of argon and we may think Miers did not choose him by chance. Miers wrote about the experiments of the American researcher. W. Hildebrand, performed at the US Geological Institute as early as 1890. Upon heating of some thorium and uranium minerals (for instance, cleveite) a chemically inactive gas was liberated; its spectrum was similar to that of nitrogen and contained new lines.

Later Hilderbrand himself confessed to Ramsay that he had a temptation to attribute these lines to a new element. However, his colleagues were sceptical about the results and Hildebrand stopped his experiments. Miers, however, believed that in the light of numerous cases of nitrogen presence in natural uranates it was reasonable to stage another experiment.

Evidently, Ramsay believed that Hildebrand’s inactive gas could be argon; therefore, he agreed with Miers and on February 5, he acquired a small amount of cleveite. Ramsay himself, however, was busy with studying argon and attempting to prepare its compounds and, therefore, asked his pupil D. Matthews to carry out the experiment. Matthews treated the mineral with hot sulphuric acid and, like Hildebrand, observed the formation of bubbles of a gas resembling nitrogen.

When a sufficient amount of the gas was collected, Ramsay performed its spectral analysis (March 14). The picture was unexpected: the spectrum had a bright band whose lines were not found in the spectra of nitrogen and argon.

Although Ramsay had no sufficient facts to make definitive conclusions he assumed that cleveite contained, in addition to argon, another unknown gas. Ramsay spent a whole week to obtain this gas in as pure a form as possible. On March 22, he compared the spectra of argon and the unknown gas in the presence of B. Brauner. Ramsay provisionally named this gas “krypton” from the Greek for “secret”, “covered”. The name later passed to another inert gas. On March 23. The scientist wrote down in his diary that the bright yellow line of “krypton” did not belong to sodium and was not observed in the argon spectrum. (In the late sixties it was necessary to prove that the D3 line of solar helium was not the bright yellow line of sodium; history, as we see, repeated itself.)

Not quite sure of his result, Ramsay sent an ampoule with the gas to W. Crookes. A day later a telegram was received from Crookes which read: “krypton is helium, 587.49; come and see.” The figure 587.49 corresponded to the wavelength of the solar helium on a specially calibrated scale. Although these data facilitated the identification of helium on the earth, otherwise this discovery was independent. It became possible for the scientists to comprehensively study helium–a new chemical element which was no longer hypothetical. Helium’s complete chemical inactivity was not suspicious: similar inactivity of argon had already been known by that time (1894).

A brief communication about the discovery of helium on the earth was first published by Ramsay on March 29, 1895, in the “Chemical News” edited by Crookes. It is interesting that almost simultaneously terrestrial helium was discovered in cleveite by the Swedish scientist P. Cleve (in whose honour the mineral had been named) and by his assistant. A Lunglet. They, however, were a little too late with their experiments and could only express their disappointment, by no means claiming their priority. Terrestrial helium received full recognition and no attempts were made to refute Ramsay’s results. A little time passed and helium was discovered in other minerals and mineral spring waters. In 1898 helium was found in the earth atmosphere.

Discovery of Elements: Indium


In the history of chemical elements the discovery of a new element often directly affected the discovery of another one. Thus, the discovery of thallium was a catalyst for the discovery of indium–the last of the classic group of four elements identified by spectral analysis.

The stage was set in the German town of Freiberg; and the main characters were F. Reich, professor of physics in the Mining Academy and his assistant Th. Richter. The time was the year of 1863. Interested in some properties if thallium, discovered two year earlier, F Reich decided to obtain a sufficient amount of the metal for his experiments. Searching for natural sources of thallium, he analysed samples of zinc ores mined at Himmelsfürst. In addition to zinc the ores were known to contain Sulphur, arsenic, lead, silicon, manganese, tin, and cadmium, in a word, quite a number of chemical elements. Reich believed that thallium could be added to the list. Although time-consuming chemical experiments did not produce the desired element, he obtained a straw-yellow precipitate of an unknown composition. It was told that when C. Winkler (subsequently the discoverer of germanium) entered Reich’s laboratory the latter showed him a test-tube with the precipitate and said that it contained sulphide of a new element.

It would have been surprising if F. Reich had not used spectroscopy to prove his assumption. Of course, Reich did use it but, unfortunately, he was colour-blind and, therefore, asked his assistant Richter to perform spectral analysis. Th. Richter succeeded in the very first attempt: in the spectrum of the sample he saw an extremely bright blue line which could not be confused with either cesium blue line or any other line. In a word, the observation was quite definite. Reich and Richter came to the conclusion that the ores of Himmelsfurst contained a new chemical element. They named it “indium” after “indigo”, a bright blue dye. There is an interesting fact that does credit to F. Reich. The first reports about the discovery of indium were signed by the two scientists. Reich, however, believed that this was unjust and that the honour of the discovery belonged solely to Richter.

Soon after the two scientists had proved the existence of natural indium with the help of spectroscopy, they obtained a small amount of it. Indium compounds turn the flame of a Bunsen burner blue-violet and so bright that presence of the new element could be established without a spectroscope. Subsequently Reich and Richter studied some properties of indium, with Winkler giving them considerable help.

When metallic indium, although contaminated, was prepared, Richter submitted the samples to the Paris Academy of Sciences in 1867 and estimated their value at 800 pounds sterling which was quite a lot of money at the time.

Chemical properties of indium were described soon after its discovery but its atomic mass was at first determined incorrectly (75.6). Mendeleev saw that this atomic mass would not correctly place indium in the periodic table and suggested to increase it by about 50 per cent. Mendeleev proved to be right and indium occupied its place in the third group of the periodic table.

Discovery of Thallium Element


Thallium became the third element whose presence in the earth minerals was established by spectroscopy. Some of its properties proved to be similar to those of alkali metals and, therefore, there were scientists who believed that thallium was not an independent chemical element but a mixture of alkali metals, namely unknown heavy analogues of rubidium and cesium. Time was required to dispel the doubts, while Bunsen and Kirchhoff continued to investigate the newly discovered elements their method of spectral analysis attracted attention of the English chemist and physicist W. Crookes. By that time, he had been known to the scientific community mainly as the editor and publisher of the Chemical News journal. There was nothing glamorous in the way Crookes started on his way to the discovery. Back in 1850 he received ten pounds of sludge remaining in lead chambers after production of sulphuric acid in Tilkerod plant (Germany). The scientist separated selenium from the sludge for the study of compounds called selenocyanides to which his first published paper was devoted. After the extraction of selenium and its purification a certain amount of the material remained ad there was every reason to suspect the presence of tellurium, a direct analogue of selenium in terms of chemical properties. However, with the methods he used to could not extract tellurium. The investigation was stopped and it was just a lucky chance that the scientist kept the residue after the processing of the sludge (and, perhaps, the belief that the residue contained tellurium).

The discovery of cesium and rubidium impressed W. Crookes very much. Being not only impressionable but practical as well, the scientist understood at once how very promising the spectral method was for analytical purposes. Having obtained a spectroscope, Crookes decided to test it immediately. The time came for the samples of the sulphuric acid sludge (or, to be more exact, its residue after removal of selenium) which had been kept for more than ten years. Crooks introduced the sample into the flame of a burner and was instantly disappointed: no hint of tellurium lines in the spectrum. The selenium lines appeared and the gradually faded. However instead of them a magnificent green line appeared which Crookes had never observed before. Of course, there was a temptation to assign the line to a new chemical element and the scientist did so naming it “thallium” from the Greek thallos, which means “a new green branch”.

The first publication about Crookes’ discovery appeared in Chemical News on March 30, 1861, under the title “On the Existence of a new Element Probably from the Sulphur Group”. Here the author was wrong since, as we know, thallium has nothing in common either with Sulphur or with its analogues. A year later Crookes recognized his mistake and published another paper titled “Thallium, a New Chemical Element” where no analogy with Sulphur was drawn. In this way was thallium discovered. The word “discovered” means here the establishing of the existence of thallium by the new method. After having observed the element’s spectrum Crookes neither separated the pure element nor prepared its compounds. This was done by the French chemist C. Lamy who is often credited with being an independent discoverer of thallium.

For the first time C. Lamy observed the green thallium line in a sample of selenium extracted from the sludge of sulphuric acid production (the raw material used by Crookes). This took place in March 1862, a year after Crookes’ observations, and already on June 23 Lamy submitted a sample of metallic thallium with a mass of about 14g to the Paris Academy of Science. Crookes also succeeded in preparing metallic thallium but in the form of powder. C. Lamy, however, declared that the thallium of Crookes was nothing other than the metal sulphide. Controversy went on. Crookes said that he had obtained the metal powder before May 1, 1862, but did not dare to fuse the powder into an ingot because of the product’s volatility. A special committee organized by the Paris Academy of Sciences, including such prominent scientists as A. Saint Claire Deville, T Pelouze, and J. Dumas, recognized the priority of G. Lamy.

The French chemist undoubtedly studied thallium in much greater detail than W. Crookes. He showed that the metal formed trivalent and monovalent compounds. Monovalent thallium has much in common with alkali metals; trivalent thallium resembles aluminium. J. Dumas named it “the paradoxical metal”. It was the similarity of thallium with sodium and potassium that gave rise to the idea that thallium was a mixture of unknown alkali metals with large atomic masses. It is regrettable that all the credit for the discovery of thallium is given to W. Crookes, while the French chemist’s significant achievements are often ignored.

In 1866 E. Nordenshöld, a well-known traveller, mineralogist and one of the explorers of Greenland, found a new mineral containing silver, copper, selenium, and thallium. He proposed to name it crookesite (in honour of W. Crookes). For a long time this mineral was believed to be the only one containing noticeable amounts of thallium.

Discovery of Rubidium element


The discovery of the second “spectral element” occurred in the studies of a rare mineral, lepidolite (called also lilalite because of its lilac colour). For the first time a detailed chemical analysis of lepidolite was performed by M. Klaproth at the end of the 18th century. But the experienced analyst did not discover alkalis in the mineral. Doubting his own results, Klaproth decided to repeat the analysis and this time (1797) he found the following components: 54.5% silicon dioxide, 38.25% aluminium oxide, 4% potassium oxide, and 0.75% manganese oxide. The missing 2.5 per cent Klaproth ascribed to the loss of water contained in the mineral. However, no matter what ingenious techniques the chemist tried, he could not determine the content of the two most important components: lithium (it had not been discovered yet by that time) and fluorine; thus, the nature of lepidolite remained obscure.

At the beginning of 1861 a sample of this mineral from Saxony fell into the hands of R. Bunsen and G. Kirchhoff, who separated alkaline components form it and precipitated potassium in the form of chloroplatinate. After a thorough washing the precipitate was subjected to spectral analysis. On February 23, 1861, the chemists reported the existence of a new alkali metal in lepidolite to the Berlin Academy of Sciences. The Scientists asserted that magnificent dark red colour of the line of the new metal gave them every reason to name the element “rubidium” and assign to it the symbol Rb from the Latin word rubidus, which meant a deep red colour. Then Bunsen and Kirchhoff discovered rubidium in the same mineral spring water in which cesium was found a year before. The rubidium content turned out to be only slightly higher than that of cesium. Metallic rubidium was prepared by R. Bunsen in 1863.

Quantum Mechanics Series – 6 Planck’s Theory

Max Planck Quantum Theory
Plank’s story begins in the physics department of the Kaiser Wilhelm Institute in Berlin, just before the turn of the century.

Plank was repeatedly being confronted with reliable experimental data on Black-Body Radiation. He was trying to explain black body radiation, but could not explain with available theoretical tools at that time.

Planck was a very conservative member of the Prussian Academy, steeped in traditional methods of classical physics and a passionate advocate of thermodynamics. In fact, from his PhD thesis days in 1879 (the year Einstein was born) to his professorship at Berlin twenty years later, he had worked almost exclusively on problems related to the laws of thermodynamics. He believed that the Second Law, concerning entropy, went deeper and said more than was generally accepted.

Planck was attracted by the absolute and universal aspects of the black-body problem. Plausible arguments showed that at equilibrium, the curve of radiation intensity versus frequency should not depend on the size or shape of the cavity or on the materials of its walls. The formula should contain only the temperature, the radiation frequency and one or more universal constants which would be the same for all cavities and colours.
Finding this formula would mean discovering a relationship of quite fundamental theoretical interest.
1. This radiation law, whenever it is found, will be independent of special bodies and substances and will retain its importance for all times and cultures… even for non-terrestrial and non-human ones.

History has proved Planck’s insight to be more profound

than even he thought. In 1990, scientists using the COBE satellite measured the background radiation at the edge of the universe (i.e left over from the Big Bang), and found a perfect fit to his Black-body radiation Law.

Pre-Atomic Model of Matter
Planck knew the measurements by his friends Heinrich Rubens and Ferdinand Kurlbaum were extremely reliable.
Planck’s oscillators in the walls of the cavity

Planck started by introduced the idea of a collection of electric oscillators in the walls of the cavity, vibrating back and forth under thermal agitation.
(*Note! Nothing was known about atoms.)
Planck assumed that all possible frequencies would be

present. He also expected the average frequency to increase at higher temperatures as heating the walls caused the oscillators to vibrate faster and faster until thermal equilibrium was reached.

The electromagnetic theory could tell everything about the emission, absorption and propagation of the radiation, but nothing about the energy distribution at equilibrium. This was a thermodynamics problem.
Planck made certain assumptions, relating the average energy of the oscillators to their entropy, thereby obtaining a formula for the intensity of the radiation which he hoped would agree with the experimental results.

Planck tried to alter his expression for the entropy of the radiation by generalizing it, and eventually arrived at a new formula for the radiation intensity over the entire frequency range.

The constants C1 and C2 are numbers chosen by Planck to make the equation fit the experiments.
Among those present at the historic seminar was

Heinrich Rubens. He went home immediately to compare his measurements with Planck’s formula. Working through the night, he found perfect agreement and told Plank early next morning.

Planck had found correct formula for the radiation law. Fine. But could he now use the formula to discover the underlying physics ?

Planck’s Predicament
1. …..From the very day I formulated the radiation law, I began to devote myself to the task of investing it with true physical meaning.
2. After trying every possible approach using traditional classical applications of the laws of thermodynamics, I was desperate.
3. I was forced to consider the relation between entropy and probability according to Boltzmann’s ideas. After some of the most intense weeks of my life, the light began to appear to be.

Boltzmann’s statistical version of the Second law based on probabilities seemed Planck’s only alternative. But he rejected the underlying assumption of Boltzmann’s approach which allows the second law to be violated momentarily during fluctuations.

S = k log W

(Boltzmann’s version of the second law of thermodynamics.)
Not once in any of the forty or so papers that Planck wrote prior to 1900 did he use, or even refer to, Boltzmann’s statistical formulation of the second Law!

Chopping Up the Energy
So, Planck applied three to Boltzmann’s ideas about entropy.
1. His statistical equation to calculate the entropy.
2. His condition that the entropy must be a maximum (i.e. totally disordered) at equilibrium.
3. His counting technique to determine the probability W in the entropy equation.
To calculate the probability of the various possible arrangements, Planck followed Boltzmann’s method of dividing the energy of the oscillators into arbitrarily small but finite chunks. So the total energy was written as E = N e where N is an integer and e an arbitrarily small amount of energy. e would eventually become infinitesimally small as the chunks became infinite in number, consistent with the mathematical procedure.

A Quantum of Energy
1. I found that I Had To Choose Energy units proportional to the oscillator frequency, namely e = h f, In Order To obtain the Correct form for the total energy. F is the Frequency and h is a constant which would eventually decrease to zero.

Eureka! Planck had stumbled across a mathematical method which at last gave some theoretical basis for this experiment radiation law – but only if the energy is discontinuous.
Even though he had no reason whatsoever to propose such a notion, he accepted it provisionally, for had nothing better. He was thus forced to postulate that the quantity e = h f must be a finite amount and h is not zero.
Thus, it this is correct, it must be concluded that it is not possible for an oscillator to absorb and emit energy in a continuous range. It must gain and lose energy discontinuously, in small indivisible units of e = h f, which Planck called “energy quanta”.

Now you can see why the classical theory failed in the high frequency region of the Black-Body Curve. In this region the quanta are so large (e = h f) that only a Few vibration modes are excited.
With a decreasing number of modes to excite, the oscillators are suppresses and the radiation frequency end. The ultraviolet catastrophe does not occur.

Planck’s quantum relation thus inhibits the equipartition of energy and not all modes have the same total energy. This is why we don’t get sunburn from a cup of coffee. (Think about it!)

The classical approach of Rayleigh-Jeans works fine at low frequencies, where all the available vibrational modes can be excited. At high frequencies, even though plenty of modes of vibration are possible (recall it’s easier of stuff short waves into a box). Not many are excited because it costs too much energy to make a quantum at a high frequency since e = h f.

During his early morning walk on 14 December 1900. Planck told his son that he may have produced a work as important as that of Newton. Later that same day. He presented his result to the Berlin Physical Society signaling the birth of quantum physics.

It had taken him less than two months to find an explanation for his own black-body radiation formula. Ironically, the discovery was accidental, caused by an incomplete mathematical procedure. An ignominious start to one of the greatest revolutions in the history of physics !
From this start would come an understanding of why statistical rules must be used for atoms, why atoms don’t glow all the time and why atomic electrons don’t spiral into the nucleus.

In early 1901, the constant h – today called Planck’s constant – appeared in print for the first time. The number is small –
h = 0.000 000 000 000 000 000 000 000 006 626
-but it is not zero! If it were, we would never be able to sit in front of a fire. In fact, the whole universe would be different. Be thankful for the things in life.
Surprisingly, in spite of the important and revolutionary aspects of the black-body formula, it did not draw much attention in the early years of the 20th century. Even more surprisingly, Planck himself was not convinced of its validity.

I was so sceptical of the universality of boltzmann’s entropy law that I spent years trying to explain my results in a less revolutionary way.
Now of the second experiment which could not be explained by classical physics. It is more simple, yet inspired a more profound explanation.

Discovery of Cesium Element

Cesium, a rare alkaline-earth metal, was fated to become the first chemical element whose presence on Earth was established by spectroscopy, although its fate could have been different.

Back in 1846 the mineralogist A. Breithaupt, studying minerals and ores from the island of Elba, noted a coloured variety of quartzite, which he named pollux (or pollycite). The sample of pollux then fell into the hands of the German chemist K. Plattner from Freiberg, a professor of metallurgy in the Mining Academy. Plattner had a minute amount of pollux sufficient only for one analytical experiment. Having separated the constituents of the mineral and finding nothing new, Plattner, to his surprise, noted that the sum total of the constituents was only 92.75 per cent.
The reason for this remained unclear since Plattner had no pollux left. The scientist, however, established the following: pollux had the highest alkali content among all known silicates. It is now clear that cesium was safely masked by the much large amounts of sodium and potassium and Plattner was not able to extract it.
In 1860 R. Bunsen and G. Kirchhoff studied the chemical composition of various mineral spring waters by spectroscopy. After the separation of calcium, strontium, magnesium, and lithium from a sample of Dürkheim mineral water, a drop of the evaporated solution was studied spectroscopically. The scientists observed two pronounced blue lines close to each other. One of them almost coincided with the strontium line. Bunsen and Kirchhoff asserted that since no substance was known to have such spectral lines it had to be an unknown substance, belonging to the group of alkali metals. They proposed to name it “cesium” (symbol Cs) from the Latin caesius: in ancient times this word was used to describe the blueness of the upper part of the firmament. The beautiful blue vapour of cesium helped to prove the presence of a few millionths of a milligram of this substance in a mixture with sodium, lithium, and strontium.
On April 11, 1860, R. Bunsen wrote to G. Roskoe (his collaborator in a study in photochemistry) about his investigation of the new alkali metal. On May 10 he reported the discovery of cesium to the Berlin Academy of Sciences. Six months later Bunsen already had 50 g of almost pure cesium chloroplatinate. To obtain such an amount of the product, it was required to process nearly 300 tons of mineral water; about one kilogram of lithium chloride was isolated as a side product. These figures show how small was the cesium content in mineral spring waters.
Four years later the Italian Analyst F. Pizani set to study pollux, earlier investigated by Plattner. Pizani had a stroke of luck; he discovered cesium in the mineral and demonstrated that the German scientist had erroneously taken cesium sulphate for a mixture of sodium and potassium sulphates. Pure cesium, however, was separated only in 1882 by the German chemist K. Satterberg via electrolysis of a mixture of cyanides CsCN and Ba(CN)2.

In Russia Beketov prepared cesium almost at the same time and independently of Satterberg by reducing cesium aluminate (CsAlO2) with magnesium in a hydrogen flow.

Quantum Mechanics Series -5 : Thermal Equilibrium and Fluctuations

The Thirty Year War (1900−30) – Quantum Physics Versus Classical Physics

There were three critical experiments in the pre-quantum era which could not be explained by a straightforward application of classical physics.
Each involved the interaction of radiation and matter as reported by reliable, experimental scientists.

The measurements were accurate and reproducible, yet paradoxical… the kind of situation a good theoretical physicist would die for.
We will describe each experiment step-by-step, pointing out the crisis engendered and the solution advanced by Max Planck, Albert Einstein and Niels Bohr respectively.

In putting forward their solution, these scientists made the first fundamental contributions to a new understanding of nature. Today the combined work of these three men, culminating in the Bohr model of the atom in 1913, is known as the Old Quantum Theory.

Black-Body Radiation
When an object is heated, it emits radiation consisting of electromagnetic waves, i.e. light with a broad range of frequencies.

1. Measurements made on the radiation escaping from a small hole in a closed heated oven – which in Germany we call a cavity – shows that the intensity of the radiation varies very stronger with the frequency of the radiation.

The dominant frequency shifts to a higher value as the temperature is increased, as shown in the graph drawn from measurements made in the late 19th century.

A black-body is a body that completely absorbs all the electromagnetic radiation failing on it.

Inside a cavity the radiation has nowhere to go and is continuously being absorbed and re-emitted by the walls. Thus, a small opening will give off radiation emitted by the walls, not reflected, and thus is characteristic of the black body.
When the oven is only just warm, radiation is present but we can’t see it because it does not stimulate the eye. As it gets hotter and hotter, the frequencies reach the visible range and the cavity glows red like a heating ring on an electric cooker.

This is how early potters determined the temperature inside their kilns. They would notice the color of fire where pots are heated and thr color gave them idea of temperature. Already in 1792, the famous porcelain maker Josiah Wedgwood had noted that all bodies become red at the same temperature.

In 1896, a friend of Planck’s Wilhelm Wien, and others in the Berlin Reichsanstalt (Bureau of Standards) physics department put together an expensive empty cylinder of porcelain and platinum.
At Berlin’s Technische Hochschule, another of Planck’s close associates, Heinrich Rubens, operated a different oven.
These radiation curves – one of the central problem of theoretical physics in the late 1890s – were shown to be very similar to those calculated by Maxwell for the velocity (i.e. energy) distribution of heated gas molecules in a closed container.

Paradoxical Results
Could this black-body radiation problem be studied in the same way as Maxwell’s ideal gas… electromagnetic waves (instead of gas molecules) bouncing around in equilibrium with the walls of a closed container?
Wien derived a formula, based on some dubious theoretical arguments which agreed well with published experiments, but only at the high frequency part of the spectrum.
The English classical physicist Lord Rayleigh (1842−1919) and Sir James Jeans (1877−1946) used the same theoretical assumptions as Maxwell had done with his kinetics theory of gases.
The equation of Rayleigh and Jeans agreed well at low frequencies but they got a real shock at the high frequency region. The classical theory predicted an infinite intensity for the ultraviolet region and beyond, as shown in the graph. This was dubbed the ultraviolet catastrophe.
What does this experimental result actually mean and What Went Wrong ?
The Rayleigh-Jeans result is clearly wrong, otherwise anyone who looked into the cavity would have eyeballs burned out.

This ultraviolet Catastrophe became a serious Paradox for classical physics.
If Rayleingh and Jeans were right, it would be dangerous for us even to sit in front of a fireplace.

If classical physicists had their way, the romantic glow of the embers would soon turn into life-threatening radiation. Something had to be done!

The Ultraviolet Catastrophe
Everyone agreed that Rayleigh and Jeans’ method was sound, so it is instructive to examine what they actually did and why it didn’t work.
1. We applied the statistical physics method to the waves by Analogy with Maxwell’s gas particles using the equipartition of energy, i.e. we assumed that the total energy of radiation is distributed equally among all possible vibration frequencies.
2. But there is one big difference in the case of waves. There is no limit on the number of modes of vibration that can be excited…
3. …Because It’s easy to fit more and more waves into the container at higher and higher frequencies (i.e. the wavelengths get smaller and smaller).
4. Consequently, the amount of radiation predicated by the theory is unlimited and should keep getting stronger and stronger as the temperature is raised and the frequency increases.
5. No wonder it was known as the ultraviolet catastrophe.

Discovery of Calcium Element

Many calcium minerals, for instance, limestone, gypsum, alabaster, that is, mainly carbonate and sulphate minerals, have been known for a very long time. In the old days people already knew how to transform limestone into lime by calcination, as was reported by Pliny the Elder. However, it was only in 1755 that J. black showed that the weight (mass) losses during calcination were completely caused only by the removal of fixed air, i.e. carbon dioxide.
The name “alabaster” served in antiquity to denote two minerals. For one of them (a variety of calcium sulphate) the name survived up to our days, but in Egypt, for example, “alabaster” meant a variety of calcite (calcium carbonate). Gypsum has also been used from times immemorial; as a construction material. Gypsum-based solutions found application in building pyramids, temples, and other edifices. Theophrastos applied the name “gypsum” to two minerals gypsum itself and the product of its partial dehydration. Pure calcium oxide was described by the German chemists I. Pott back in 1746; however, attempts to obtain metal from it with the acid of various reducing agents failed. The right approach was suggested by H. Davy. First, he attempted to obtain calcium by passing electric current through humid earth insulated from the air by a kerosene layer. (In a similar way he had tried to prepare barium and strontium.) As a result of his experiments, Davy developed the following method of preparing pure alkaline-earth metals. He mixed humid earth with 1/3 (by mass) of mercury oxide and placed the mixture into a platinum vessel connected to the positive pole of a high-voltage battery. Then he introduced a drop of mercury at the centre of the mixture. The platinum electrode placed in the drop was connected with the negative pole of the battery. Amalgam obtained in this way was then separated into mercury and silvery-white metal, calcium. Davy prepared pure calcium in 1808. In the same year J. Berzelius and M. Pontin obtained calcium independently of Davy using a similar method. The name of the element originates from the Latin word calx, which means “lime”. ..

Quantum Mechanics Series – 4 : The Existence of Atoms

The Existence of Atoms

India Philosopher “Kanad” before 600 B.C. said that each matter consists of small particles which are called “Kan”. He gave those small particle his own name.

A Greek philosopher named Democritus (c. 460−370 B.C.) also first proposed the concept of atoms (means “indivisible” in Greek).

The idea was questioned by Aristotle and debated for hundreds of years before the English chemist John Dalton (1766 – 1844) used the atomic concept to predict the chemical properties of elements and compounds in 1806.

But it was not until a century later that a theoretical calculation by Einstein and experiments by the Frenchman Jean Perrin (1870−1942) persuaded the sceptics to accept the existence of atoms as a fact.

However, during the 19th century, even without physical proof of atoms, many theorists used the concept.


Averaging Diatomic Molecules

The Scottish physicist J.C. Maxwell, a confirmed atomist, developed his kinetic theory of gases in 1859.


This was qualitatively consistent with physical properties of gases, if we accept the notion that heating causes the molecules to move faster and bang into the container walls more frequently.


Maxwell’s theory was based on statistical averages to see if the macroscopic properties (that is, those properties that can be measured in a laboratory) could be predicted from a microscopic model for a collection model for a collection of gas molecules.


Maxwell made for assumptions :

Maxwell : gave distribution of velocity for gas particles








This last assumption was the most unusual and revolutionary showing a great deal of physical insight by Maxwell.

It would be impossible by try to compute the individual motions of many particles. But Maxwell’s analysis, based on Newton’s mechanics, showed that temperature is a measure of the microscopic mean squared velocity of the molecules. 

The real importance of Maxwell’s theory is the prediction of the probable velocity distribution of the molecules, based on his model. In other words, this gives the range of velocities…how the whole collection deviates from the average.

Postulates of Maxwell Theory helps to calculate probability that a molecule chosen at random would have a particular velocity.

Maxwell velocity distribution curve:

This is the well known curve which physicists today call the Maxwell Distribution. It gives useful information about the billions and billions of molecules, even though the motion of an individual molecule can never be calculated. This is the use of probabilities when an exact calculation is impossible in practices.





Ludwig Boltzmann and Statistical Mechanics

In the 1870s, Ludwig Boltzmann (1844−1906) – inspired by Maxwell’s kinetic theory – made a theoretical pronouncement.

  • He presented a general probability distribution law called the canonical or orthodox distribution which could be applied to any collection of entities which have freedom of movement, are independent of each other and interact randomly.
  • He formalized the theorem of the equipartition of energy.

This means that the energy will be shared equally among all degree of freedom if the system reaches thermal equilibrium.

  • He gave a new interpretation of the Second Law.


When energy in a system is degraded (as Clausius said in 1850), the atoms in the system become more disordered and the entropy increases. But a measure of the disorder can be made. It is the probability of the particular system – defined as the number if ways it can be assembled from its collection of atoms.

More precisely, the entropy is given by :

          S = k Log W −−−−

Where k is a constant (now called Boltzamann’s constant) and W is the probability that a particular arrangement of atoms will occur. This work made Boltzmann the creator of statistical mechanics, a method in behavior of their constituent microscopic parts.

Quantum Mechanics Series- 3 : What is Thermodynamics

Quantum Mechanics 3

What is Thermodynamics ?

The word means the movement of heat, which always flows from a body of higher temperature to a body of lower temperature, until the temperatures of the two bodies are the same. This is called thermal equilibrium.

Heat is correctly described as a form vibration…


The First Law of Thermodynamics


Steam Engines

James Watt (1736 – 1819), A Scot , who had built a working steam engine in 19th century.  





Soon after, the son of a Manchester brewer, James Prescott Joule (1818−19), showed that a quantity of heat can be equated to a certain amount of mechanical work.

Then somebody said…. “since heat can be converted into work, it must be a form of energy” (the Greek word energy means “containing work”) But it wasn’t until 1847 that a respectable academic scientist, Hermann von Helmholtz (1821-94), stated…..









This is called the law of the conservation of energy.  It remains a foundation of modern physics, unaffected by modern theories.


Rudolf Clausius: Two Laws

In 1850, the German physicist Rudolf Clausius (1822-88) published a paper in which he called the energy conservation law The First Law of Thermodynamics. At the same time, he argued that there was a second principle of thermodynamics in which there is always some degradation of the total energy in the system, some non-useful heat in a thermodynamic process.

Clausius introduced a new concept called entropy – defined in terms of the heat transferred from one body to another.

Entropy is measurement of disorderness of any system. The entropy of an isolated system always increases, reaching a maximum at thermal equilibrium, i.e. when all bodies in the system are at the same temperature.

Quantum Mechanics Series – 2: Solvay Conference 1927 – Formulation of Quantum Theory

The Solvay Conference 1927 – Formulation of Quantum Theory

A few years before the outbreak of World War I, the Belgain industrialist Ernest Solvay (1838-1922) sponsored the first of a series of international physics meetings in Brussels. Attendance at these meeting was by special invitation, and participants – usually limited to about 30 – were asked to concentrate on a pre-arranged topic.

The first five meeting held between 1911 and 1927 chronicled in a most remarkable way the development of 20th century physics. The 1927 gathering was devoted to quantum theory and attended by no less than nine theoretical physicists who had made fundamental contributions to the theory. Each of the nine would eventually be awarded a Nobel Prize for this contribution.

This photograph of the 1927 Solvay Conference is a good starting point for introducing the principal players in the development of the most modern of all physical theories. Future generations will marvel at the compressed time scale and geographical proximity which brought these giants of quantum physics together in 1927.


There is hardly and period in the history of science in which so much has been clarified by so few in so short a time.

Look at the sad-eyed Max Planck (1858−1947) in the front row next to Marie Curie (1867−1934). With his hat and cigar, Planck appears drained of vitality, exhausted after years of trying to refute his own revolutionary ideas about matter and radiation.


A few year later in 1905, a young patent clerk in Switzerland named Albert Einstein (1879−1955) generalized Planck’s notion.


That’s Einstein in the front row centre, sitting stiffly in his formal attire. He had been brooding for over twenty years about the quantum problem without any real insight since his early 1905 paper. All the while, he continued to contribute to the theory’s development and endorsed original ideas of others with uncanny confidence. His greatest work – the General Theory of Relativity – which had made him an international celebrity, was already a decade behind him.


In Brussels, Einstein had debated the bizarre conclusions of the quantum theory with its most respected and determined proponent, the “great Dane” Niels Bohr (1885-1962). Bohr – more than anyone else – would become associated with the struggle to interpret and understand the theory. At the far right of the photo, in the middle row, he is relaxed and confident – the 42 year old professor at the peak of this powers.

In the back row behind Einstein, Erwin Schrodinger (1887−1961) looks conspicuously casual in his sports jacket and how tie. To his left but one are the “young Turks”, Wolfgang Pauli (1900−58) and Werner Heisenberg (1901−76) – still in their twenties – and in front of them, Paul Dirac (1902−84), Louis de Broglie (1892−1987), Max Born (1882−1970) and Bohr. These men are today immortalized by their associate with the fundamental properties of the microscopic world: the Schrodinger wave equation; the Pauli exclusion principle ; the Heisenberg uncertainty relation, the Bohr Atom…. and so forth.

They were all there – from Planck, the oldest at 69 years, who started it all in 1900 – to Dirac, the youngest at 25 years, who completed the theory in 1928.

The day after this photograph was taken – 30 October 1927 – with the historic exchanges between Bohr and Einstein still buzzing in their minds, the conferees boarded trains at the Brussels Central Station to return to Berlin, Paris, Cambridge, Gottingen, Copenhagen, Vienna and Zurich.

They were taking with them the most bizarre set of ideas ever concocted by scientists. Secretly, most of them probably agreed with Einstein that this madness called the quantum theory was just a step along the way to a more complete theory and would be overthrown for something better, something more consistent with common sense.


But how did the quantum theory come about? What experiments compelled these most careful of men to ignore the tenets of classical physics and propose ideas about nature that violated common sense ?

Before we study these experimental paradoxes, we need some background in thermodynamics and statistics which are fundamental to the development of quantum theory.