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.