GELATIN. 59 



That from the bones was formerly called ossein. It is com- 

 posed of the same chemical elements as the albumins, but 

 contains a little more oxygen. It is probably an oxidation- 

 product of some of the albumins. It is insoluble in water, 

 but by boiling with water it is converted into gelatin or 

 glue. By the action of tannic acid collagen is changed to 

 a form which does not putrefy. This is the action which 

 takes place when leather is tanned. 



GELATIN. 



Gelatin may be considered as the hydrate of collagen, 

 as it is formed by the union of collagen with water. It 

 can be changed back into collagen by heating for some 

 time at 130. It swells up in cold water and dissolves 

 when the water is warmed. On allowing the solution to 

 cool it gelatinizes, or becomes a semi-solid. It acts in this 

 respect oppositely to albumin, which is soluble in the cold, 

 but becomes a solid by the action of heat. 



After it has been boiled a long time with water it is 

 decomposed, and does not gelatinize on cooling, peptone 

 being formed. The sulphur of gelatin is united in the 

 molecule in a different manner from that of the albumin 

 molecule, as is proved by its decomposition-products. 



Gelatin is decomposed by the gastric juice, giving 

 products similar to those from albumin. It has been 

 found, however, that it cannot take the place of the al- 

 buminous materials of food, though it is of value when 

 used with them. 



Gelatin does not give all the reactions of the albumi- 

 nous substances, although it does give the same results with 

 some of them. Like the albuminous compounds, it gives 

 a purple color with the biuret test; it is precipitated by 



