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”.