Gallium is one of the three great vindications of the periodic table. In 1871 Dmitri Mendeleev had predicted an undiscovered element below aluminium, which he called eka-aluminium, and specified its density, melting point and chemistry. When Lecoq de Boisbaudran characterised gallium, Mendeleev wrote to point out that the density the Frenchman first reported was wrong and that his own predicted value should be trusted; on repurifying his sample, Lecoq de Boisbaudran found Mendeleev right. The episode, in which a theorist corrected a discoverer about a substance he had never seen, became one of the most celebrated demonstrations that the periodic system could genuinely predict nature. Some historians doubt the coq pun was intended at all, since Lecoq de Boisbaudran himself denied choosing the name as a play on his own; whatever the truth, the coincidence has been retold ever since.
The three confirmations of Mendeleev's predictions, scandium in 1879, gallium in 1875 and germanium in 1886, all arrived within seventeen years of his original 1869 table, a remarkably fast vindication for a scheme that many senior chemists had initially dismissed as speculative or premature. Mendeleev's own priority over the German chemist Lothar Meyer, who had devised a similar periodic arrangement at almost the same time but published his predictions less boldly, is itself one of the enduring footnotes of the table's history.