90 



CHEMISTRY. 



takes up the gallic acid and other matters ; hence the liquid which 

 has passed through will consist of two distinct strata, the lower one, 

 which is a concentrated aqueous solution of tannic acid, and the 

 upper etherial solution. The latter having been carefully removed, 

 the tannic acid may be obtained by evaporation. It has a light yel- 

 lowish colour, of a porous, feathery texture ; has a very astringent 

 but not bitter taste ; very soluble in water, less so in alcohol ; insolu- 

 ble in pure ether ; has an acid reaction. 



Tannic acid yields with the sesqui-salts of iron a deep bluish-black 

 precipitate ; it also precipitates the solution of tartar emetic, nitrate 

 of silver, sulphate of copper, and acetate of lead ; with gelatin it 

 forms a dense whitish compound — tannate of gelatin, the basis of 

 leatheri 



The variety of tannin yielded by kino, catechu, and krameria, 

 gives a greenish-hlack precipitate with the salts of iron. 



Leather is made by soaking hides, which have been deprived of 

 their hair, for a considerable time in an infusion of oak-bark : the 

 tannin which it contains combines with the gelatin of the skins. 



Gallic acid, CyH03 + 2HO, is usually found combined with tannic 

 acid. It is believed to result from the action of the oxygen of the 

 air upon tannic acid. It is not so soluble in water as tannic acid, 

 nor does it precipitate gelatin, but it yields with sesqui-salts of iron a 

 bluish-black compound. — Writing ink is a tanno-gallate of iron. 



The remaining vegetable acids are of less importance; the most 

 interesting of them are : 



Formic acid — the acid of ants ; — prepared from sugar or starch by 

 the action of sulphuric acid and peroxide of manganese. It some- 

 what resembles acetic acid. 



Benzoic acid, — oxide of benzule, — exists in the various balsams. 

 It is best procured from gum benzoin by sublimation. It is in the 

 form of white crystals, very light and feathery ; exhales a fragrant 

 odour, not due to the acid itself, but to some of the volatile oil ; it 

 forms benzoates. 



Meconic acid, — exists in opium combined with morphia and codeia. 

 It is characterized by forming a blood-red coloured compound with 

 a sesqui-salt of iron. 



SECTION VII. 



THE VEGETABLE ALKALIES VBGETO-ALKALIES. 



The vegeto-alkalies, or alkaloids, constitute a peculiar group of 

 compounds. They are met with in various plants, always in combi- 

 nation with an acid, which, in many cases, is itself peculiar in its 

 nature, not occurring elsewhere in the vegetable kingdom. They 



