12 CHEMICAL COMPOSITION OF HUMAN. BODY. 



and in its purest state, from isinglass, which is the swim- 

 ming bladder of the sturgeon, and which, with the excep- 

 tion of about 7 per cent, of its weight, is wholly reducible 

 into gelatin. The most characteristic property of gelatin 

 is that already mentioned, of its solution being liquid when 

 warm, and solidifying or setting when it cools. The tem- 

 perature at which it becomes solid, the proportion of gela- 

 tin which must be in solution, and the firmness of the 

 jelly when formed, are various, according to the source, 

 the quantity, and the quality of the gelatin ; but, as a 

 general rule, one part of dry gelatin dissolved in 100 of 

 water, will become solid when cooled to 60. The solidi- 

 fied jelly may be again made liquid by heating it, and the 

 transitions from the solid to the liquid state by the alter- 

 nate abstraction and addition of heat, may be repeated 

 several times ; .but at length the gelatin is so far altered, 

 and, apparently, oxydized by the process, that it no longer 

 becomes solid on cooling. Gelatin in solutions too weak 

 to solidify when cold, is distinguished by being precipitable 

 with alcohol, ether, tannic acid, and bichloride of mercury, 

 and not precipitable with the ferrocyanide of potassium. 

 The most delicate and striking of these tests is the tannic 

 acid, which is conveniently supplied in an infusion of oak- 

 bark or gall-nuts ; it will detect one part of gelatin in 

 5,OOO of water; and if the solution of gelatin be strong 

 it forms a singularly dense and heavy precipitate, which 

 has been named tanno -gelatin, and is completely insoluble 

 in water. 



Chondrin, the kind of gelatin obtained from cartilages, 

 agrees with gelatin in most of its characters, but its 

 solution solidifies on cooling much less firmly, and, unlike 

 gelatin, it is precipitable with acetic and the mineral and 

 other acids, and with alum, persulphate of iron and acetate 

 of lead. 



Albuminous substances, or proteids, as they are sometimes 

 called, exist abundantly in the human body. The chief 



