TISSUES YIELDING GELATIN. 509 



from these subsfances 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 floeculent 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. 



Grlutin 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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