A significant achievement of Lecoq de Boisbaudran was his discovery of the element gallium in 1875. Beginning in 1874, Lecoq de Boisbaudran investigated a sample of 52 kg of the mineral ore sphalerite obtained from the Pierrefitte mine in the Pyrenees. From it, he extracted several milligrams of gallium chloride. Using spectroscopic methods, he observed what appeared to be two previously unreported lines in the sample's spectrum, at wavelengths of 4170 and 4031 angstroms.
He continued his experiments using several hundred kilograms of crude zinc ore and in the same year isolated more than one gram of a near-pure metal by electrolysis of a solution of the metal in its form as a hydroxide compound, dissolved in potassium hydroxide solution. Later he prepared 75 grams of gallium using more than 4 tonnes of crude ore. He confirmed its spectral characteristics, consisting of two spectral lines in the violet portion of the spectrum of the mineral sphalerite. In this way, he ruled out the possibility that the spectral characteristics were an accident of the extraction process, rather than being an indication of a new element.
He named his discovery "gallium", from the Latin Gallia meaning Gaul, in honor of his native land of France. It was later suggested that Lecoq de Boisbaudran had named the element after himself, since gallus is the Latin translation of the French le coq. Lecoq de Boisbaudran denied this suggestion in an article in 1877. He published an account of his investigations on the new element in Sur un nouveau metal, le gallium (1877).
De Boisbaudran calculated the atomic mass of gallium as 69.86, close to the currently accepted value of 69.723. Unknown to Lecoq de Boisbaudran, the existence of gallium had been predicted during 1871 by Dmitri Mendeleev, who gave it the name eka-aluminium. De Boisbaudrans discovery of gallium was significant support for Mendeleevs theory of the periodicity of the elements.