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.