TISSUES YIELDING GELATIN. 509 



from these substances by boiling them with water, 

 coagulates on cooling, forming a thick jelly, which, 

 when dried, constitutes ordinary carpenter's glue. 

 Pure gelatin is obtained most readily by boiling rasped 

 deer-horns, isinglass, or pure bone-cartilage freed of 

 earthy matter by means of hydrochloric acid, with 

 water, and filtering the solution at about 50. Glutin 

 is colorless, transparent, hard, tasteless, and inodorous ; 

 softens when heated, and is then destroyed. In cold 

 water it swells up, and when heated dissolves. The 

 solution forms, on cooling, a clear jelly, even when it 

 contains but one per cent, of gelatin ; this however 

 varies in the gelatin from different tissues. It is inso- 

 luble in alcohol and ether, and is precipitated by alco- 

 hol from its aqueous solution as a flocculeiit mass. 

 When subjected to combustion it always leaves behind 

 some earthy matter. 



A solution of this gelatin is not precipitated by alum, 

 neutral iron sulphate, neutral and basic lead acetate. 



Tannic acid precipitates it completely from its solu- 

 tion. The precipitate, which is at first white and 

 flocculent, generally contracts, forming a thick, tough, 

 sticky mass. Tissues, which have the power to yield 

 gelatin, and are not yet converted into it, take up tannic 

 acid completely from its aqueous solution ; upon this 

 property is founded the process of tanning (converting 

 hides into leather). Acetic acid readily dissolves gela- 

 tin ; the solution possesses the properties of glue but 

 does not gelatinize. 



Glutin contains about 18 per cent, of nitrogen and 

 a very small quantity (J per cent.) of sulphur. Its 

 composition cannot be expressed by a probable formula. 



When boiled for a long time and particularly at a 

 temperature above 100, its solution loses the property 

 of gelatinizing. On evaporation it then dries up, 

 forming a yellowish, gummy mass, which is easily 

 soluble in cold water. The change that thus takes 

 place is not understood. Subjected to dry distillation, 

 it yields a large number of products, among which the 

 most remarkable are ammonium carbonate and the 



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