GELATIN. 1037 



acetic acid is very gluey, but does not gelatinize. Gelatin is not 

 precipitated by corrosive sublimate, which throws down al- 

 bumen. 



Gelatin forms a white compound with tannic acid, tanno- 

 gelatin, which is precipitated by a strong infusion of gall-nuts, from 

 a solution of gelatin in 5000 times its weight of water. 

 The white flocks adhere to each on stirring, aud form a soft, 

 tenacious, and elastic mass, which is of the same composition 

 with, and has considerable resemblance to leather. The skins 

 of animals are tanned, after being cleaned and deprived of the 

 cuticle and hair by lime-water, and allowed to enter into a 

 degree of putrefaction to soften them, by submitting them to 

 the action of infusion of oak bark, or other astringent vegetable 

 matter, the strength of which is gradually increased until a com- 

 plete combination takes place. The tannin is taken up by the 

 skin, of which the weight is considerably increased by this treat- 

 ment. In the tanning of thick sole-leather many months 3 

 digestion in the tan-pit is required, but the process has of late 

 been considerably shortened by slightly heating the infusion of 

 oak-bark by means of steam. The skin is greatly altered by its 

 combination with tannic acid, losing its solubility in boiling 

 water, and becoming nearly indestructible by atmospheric agen- 

 cies ; the animal matter it contains is no longer suitable for the 

 preparation of prussiate of potash, by fusion with an alkali. 



" Tawed leather is made by impregnating the skin duly prepared 

 by washing in potash liquor, with a solution of alum and com- 

 mon salt ; it is afterwards trodden in a mixture of yolk of eggs 

 and water. The alum and salt re-act on each other so as to 

 produce sulphate of soda and chloride of aluminum ; the latter 

 salt combines with the skin. White glove leather is thus pre- 

 pared. 



" Wash leather is another important manufacture; in this, the 

 skin, after being prepared and softened, is imbued with oil, and 

 afterwards subjected to a weak alkaline solution. 



" Curried leather, is made by besmearing the skin, or leather, 

 while yet moist, with common oil, which, as the humidity 

 evaporates, penetrates into the pores of the skin, giving it a 

 peculiar suppleness, and making it to a considerable extent 

 waterproof. As familiar examples of these processes, the thick 

 sole leather for shoes and boots is tanned ; the upper leather is 

 tanned and curried ; the white leather for gloves is tawed ; and 



