Purification. Gallic acid, as obtained by either of the above forms, is never quite pure; but it may be rendered absolutely pure by combining it with oxide of lead, and decomposing the compound (gallate of lead) by sulphuretted hydrogen. The sulphuret of lead acts like animal charcoal in removing the colour. (Liebig.) Commercial gallic acid “may be rendered nearly white by dissolving it in 20 times its weight of boiling distilled water, and causing the solution to traverse a stratum of prepared animal charcoal, spread upon a calico filter. When the liquid passes through colourless, it should be evaporated to 1-6th its volume, and then suffered to cool, in order to the separation of the crystallised acid.” (Ph. D.)
GALLIC FERMENTATION. This name has been given to the peculiar process by which tannic acid is converted into gallic acid, under the joint influence of moisture and atmospheric oxygen. According to the researches of M. Antoine Larocque, the peculiar ferment of nut-galls which operates this change also converts sugar into alcohol and carbonic acid, in the same way as yeast does; whilst beer yeast, muscular flesh, and caseous matter change tannin into gallic acid. The similarity of the gallic and vinous fermentation may hence be reasonably inferred.
GALLIUM. A new metal discovered in August, 1875, by means of the spectroscope, by M. Lecoq Boisbaudran, in a specimen of blende from the mines of Pierrefitte, in the Pyrenees. The new element was named gallium in honour of France, the discoverer’s native country.
Gallium gives a spectrum composed of two bands in the violet, one of the bands being brilliant, and of wave length 417, and the other, a feeble one of wave length, 403·3.
The Pierrefitte blende contains one part of gallium in four hundred thousand. It is, however, found much more abundantly in a black blende from Bensberg, on the Rhine, one hundred thousand parts of this latter yielding one part of gallium.
Gallium resembles lead in appearance, but is less blue in colour. Exposed to moist air it tarnishes slightly. It is a little harder than lead, is flexible, malleable, and may be easily cut with a knife. If melted and poured upon glass, it adheres to it, and forms a mirror which is whiter than that caused by mercury. A red heat fails to volatilise it to any appreciable extent, and it is only slightly oxidised at that temperature; therefore it is not tarnished when exposed to the air. Hot nitric acid dissolves it, but the cold acid has scarcely any action on it. It melts at 30·15 C. When once fused, it preserves the liquid condition even for several months at 0° C.,[324] until it is touched by some solid body, or by a piece of solid gallium, when it congeals to a crystalline solid, having a specific gravity of 5·93; when fused it has a specific gravity of 6·08. It crystallises in square octohedra. In properties gallium is more or less intermediate between the metals aluminium and indium.
[324] In consequence of this curious property gallium was first described as a liquid metal.
Chemical reactions of gallium:—The following are the chief reactions of the salts of gallium when in solution. With ammonia they give a white gelatinous precipitate, soluble, but not readily in excess of the precipitant; potash gives a similar precipitate, soluble in excess; acetate of ammonia, on boiling in a solution free from excess of acid, precipitates a basic compound; barium carbonate readily precipitates gallium salts in the cold. A sulphate and a chloride of gallium have already been obtained. These salts are both very soluble; the sulphate is a non-deliquescent substance, the chloride, on the contrary, is excessively so, and decomposed by a large excess of water. Gallium also forms an alum consisting of the double sulphate with ammonium. Gallium alum is a beautifully crystalline body, more soluble in cold than in hot water.
At a meeting of the ‘Academie des Sciences’ in March, 1878, M. de Baubradon stated that he had determined the atomic weight of gallium. The mean of two experiments showed it to be 69·9.
The ‘Comptes Rendus’ for February, 1878 (No. 7), contains a communication from MM. Lecoq de Boisbaudran and E. Jungfleisch, on the extraction of gallium from the ores in which it is found associated with indium.