Eka-aluminium

On 27th August 1875, Lecoq de Boisbaudran observed a new violet line, λ4170, in the spark spectrum of some material he had separated from the zinc blende of Pierrefitte; further work led to the discovery of another new line, λ4031, and established the fact that both lines belonged to the spectrum of a new metallic element, to which the discoverer patriotically gave the name of gallium in honour of his native country

The first researches on gallium were conducted with only a few milligrams of material; but in 1878 Lecoq de Boisbaudran and Jungfleisch, starting with 2400 kilograms of Bensberg blende, obtained 62 grams of nearly pure gallium.

The discovery of gallium marked the inauguration of the periodic classification. In putting forward that scheme for the classification of the elements, Mendeleeff was obliged to assume the existence of several elements not known at that period (1871). One of these hypothetical elements, of atomic weight about 68, he called eka-aluminium, and described the properties it should possess; and it was noticed very shortly after the discovery of gallium that gallium closely resembled the hypothetical eka-aluminium in its behaviour. The following tabular statement will serve to illustrate how closely Mendeleeff succeeded in foretelling the properties of gallium: -

Eka-aluminium	Gallium
Atomic weight, c. 68.	Atomic weight, 69.9.
Metal of density 5.9 and low melting- point; not volatile; unaffected by air; should decompose steam at a red heat and dissolve slowly in acids and alkalies.	Metal of density 5.94; melting at 30.15°; not volatile; unchanged in air; action on steam not known; dissolves slowly in acids and alkalies.
Oxide should have formula El2O3, density 5.5, and dissolve in acids to form salts of the type ElX3. The hydroxide should dissolve in acids and alkalies.	Oxide, Ga2O3; density not known; dissolves in acids, forming salts GaX3. The hydroxide dissolves in acids and alkalies.
There should be a tendency towards the formation of basic salts. The sulphate should form alums. The sulphide should be precipitated by H2S or (NH4)2S. The anhydrous chloride should be more volatile than zinc chloride.	Salts readily hydrolyse and form basic salts. Alums are known. The sulphide can be precipitated by H2S or (NH4)2S, but only under special circumstances. The anhydrous chloride is more volatile than zinc chloride.
The element will probably be discovered by spectrum analysis.	Was discovered spectroscopically.