From the above facts it is obvious that gallic acid does not exist ready formed in gall nuts, but that it is produced by the reaction of atmospheric oxygen upon the tannin of these concretions.
Gallic acid is a solid, feebly acidulous and styptic to the taste, inodorous, crystallizing in silky needles of the greatest whiteness; soluble in about 100 times its weight of cold, and in a much smaller quantity of boiling water; more soluble in alcohol than in water, but little so in sulphuric ether.
Gallic acid does not decompose the salts of protoxide of iron, but it forms, with the sulphate of the peroxide, a dark blue precipitate, much less insoluble than the tannate of iron. Gallic acid takes the oxide from the acetate and nitrate of lead, and throws down a white gallate unchangeable in the air, when it is mixed with that acetate and nitrate. It occasions no precipitate in solutions of gelatine (isinglass or glue), by which criterion its freedom from tannin is verified.
Gallic acid occurs but seldom in nature; and always united to brucine, veratrine, or lime. Its constituents are, carbon 49·89; hydrogen 3·49; oxygen 46·62. In the crystalline state it contains one atom of water, which it loses by drying.
Scheele obtained gallic acid by infusing pounded galls for 3 or 4 days in 8 times their weight of water, and exposing the infusion to the air, in a vessel covered loosely with paper. At the end of two months, the liquor had almost all evaporated, leaving some mouldiness mixed with a crystalline precipitate. The former being removed, the deposit was squeezed in a linen cloth, and then treated with boiling water. The solution being gradually evaporated, yielded crystals of gallic acid, granular or star-like, of a grayish colour. These crystals might be whitened by boiling their solution along with a little animal charcoal. About one fifth of gallic acid may be obtained by Scheele’s process from good gall-nuts.
From a decoction of 500 parts of galls, Sir H. Davy obtained 185 parts of solid extract; which consisted of 130 parts of tannin; 31 parts of gallic acid with extractive; 13 parts of mucilage; 12 parts of lime and salts. Hence gall-nuts would seem to contain, by this statement, more than two-thirds of their weight of tannin. This result is now seen, from the above experiments of Pelouze, to have been incorrect, in consequence of the admixture of yellow extractive in Davy’s tannin.
The uses of galls in many processes of dyeing, and in making [black ink], are detailed under their respective heads.
GALL OF ANIMALS, or OX-GALL, purification of. Painters in water colours, scourers of clothes, and many others employ ox-gall or bile, but when it is not purified, it is apt to do harm from the greenness of its own tint. It becomes therefore an important object to clarify it, and to make it limpid and transparent like water. The following process has been given for that purpose. Take the gall of newly killed oxen, and after having allowed it to settle for 12 or 15 hours in a basin, pour the supernatant liquor off the sediment into an evaporating dish of stone ware, and expose it to a boiling heat in a water bath, till it is somewhat thick. Then spread it upon a dish, and place it before a fire till it becomes nearly dry. In this state it may be kept for years in jelly pots covered with paper, without undergoing any alteration. When it is to be used, a piece of it of the size of a pea is to be dissolved in a table spoonful of water.
Another and probably a better mode of purifying ox-gall is the following. To a pint of the gall boiled and skimmed, add one ounce of fine alum in powder, and leave the mixture on the fire till the alum be dissolved. When cooled, pour into a bottle, which is to be loosely corked. Now take a like quantity of gall also boiled and skimmed, add an ounce of common salt to it, and dissolve with heat; put it when cold into a bottle, which is likewise to be loosely corked. Either of these preparations may be kept for several years without their emitting a bad smell. After remaining three months, at a moderate temperature, they deposit a thick sediment, and become clearer, and fit for ordinary uses, but not for artists in water colours and miniatures, on account of their yellowish-green colour. To obviate this inconvenience, each of the above liquors is to be decanted apart, after they have become perfectly settled, and the clear portion of both mixed together in equal parts. The yellow colouring matter still retained by the mixture coagulates immediately and precipitates, leaving the ox-gall perfectly purified and colourless. If wished to be still finer, it may be passed through filtering paper; but it becomes clearer with age, and never acquires a disagreeable smell, nor loses any of its good qualities.
Clarified ox-gall combines readily with colouring matters or pigments, and gives them solidity either by being mixed with or passed over them upon paper. It increases the brilliancy and the durability of ultramarine, carmine, green, and in general of all delicate colours, whilst it contributes to make them spread more evenly upon the paper, ivory, &c. When mixed with gum Arabic, it thickens the colours without communicating to them a disagreeable glistering appearance; it prevents the gum from cracking, and fixes the colours so well that others may be applied over them without degradation. Along with lamp black and gum, it forms a good imitation of China ink. When a coat of ox-gall is put upon drawings made with black lead or crayons, the lines can no longer be effaced, but may be painted over safely with a variety of colours previously mixed up with the same ox-gall.