

CHAP. 



I.] GELATINE. 41 



is vegetable albumen. And in peas, beans, lentils, and similar legu- 

 minous seeds, we find a substance similar to caseine. It is vegetable 

 caseine, which, like the animal principle of the same name, does not 

 coagulate by heat alone, but yields a coagulum on the addition of 

 an acid, as in milk. " The chemical analysis of these three sub- 

 stances," says Liebig, " has led to the very interesting result, that 

 they contain the same organic elements, united in the same pro- 

 portions by weight ; and, what is still more remarkable, that they 

 are identical in composition with the chief constituents of blood, 

 animal fibrine, and albumen. They all three dissolve in concen- 

 trated muriatic acid with the same deep purple colour ; and, even in 

 their physical characters, animal fibrine and albumen are in no 

 respect different from vegetable fibrine and albumen. It is especi- 

 ally to be noticed, that by the phrase, identity of composition, we 

 do not here imply mere similarity ; but that, even in regard to the 

 presence and relative amount of sulphur, phosphorus, and phosphate 

 of lime, no difference can be observed." 



4. Gelatine. This substance exists in a peculiar combination 

 with the tissues of which it forms a constituent, and can only be 

 obtained by artificial means. If the cutis or true skin, tendon, or 

 bone, be subjected to continued boiling, this substance is obtained 

 in solution in the hot water, and upon cooling assumes the form of a 

 solid jelly, which is the more solid as the quantity of water con- 

 tained in it is less. The textures which yield gelatine are, the 

 white fibrous tissue, areolar tissue, skin, serous membranes, bone. 

 Glue prepared from hides, &c. ; size, from parchment, skin, &c. ; 

 and isinglass from the swimming-bladder of the sturgeon, are various 

 forms of gelatine used in commerce. 



Gelatine, obtained by boiling, is in combination with a consider- 

 able quantity of water : by a slow and gentle heat this may be 

 driven off, and the gelatine obtained in a dry state. Dry gela- 

 tine is hard, transparent, colourless, without smell or taste, of 

 neutral reaction ; in cold water it softens and swells up, and dis- 

 solves in warm water. It is insoluble in alcohol and ether, but 

 very soluble in the dilute acids and alkalies. When tannin, or the 

 tincture or infusion of galls, is added to its solution in water, a 

 brownish precipitate is thrown down the tanno-gelatine, which 

 may be precipitated from a solution of gelatine in 5000 times its 

 weight of water. 



The process of tanning leather results from the affinity of gelatine 

 for tannin. The skins of the animals having been first freed from 



