60 GELATIN. 



picric acid, by mercuric chlorid in the presence of sodium 

 chlorid and hydrochloric acid, by tannic acid in the pres- 

 ence of sodium chlorid, and by saturation with ammonium 

 sulphate. On the other hand, it is not coagulated by boil- 

 ing; it is not precipitated by mineral acids; and it does not 

 give a brown color when warmed with an alkaline solution 

 of lead, as albumin does, the sulphur being apparently too 

 firmly united to be split off and form lead sulphid. It 

 does not give the xanthoproteic reaction when pure. 



138. Prepare collagen from bone by dissolving out 

 the mineral constituents with dilute hydrochloric acid 

 (HC1, 1 part; water, 8 parts) until they are flexible, then 

 wash out the acid. Notice that the collagen is not soluble 

 in dilute acid nor cold water. To remove all the albumins 

 it may be necessary to soak awhile in 5-per-cent. NaOH 

 solution, then wash again. 



139. Convert the collagen into gelatin by boiling it 

 a few minutes with water. Notice that it gelatinizes upon 

 cooling the solution, especially after standing. 



140. Boil a portion of the solution for some time, 

 and notice that it is thus decomposed, so that it will not 

 form a jelly upon cooling. 



141. Test a portion of the gelatin solution with the 

 biuret test. It gives a purple color like albumin. 



142. Show that it is precipitated by tannic acid in 

 the presence of NaCl. 



143. Show that gelatin is not precipitated by nitric or other 

 mineral acids, but is by saturation with ammonium sulphate and 

 also by mercuric chlorid in the presence of HC1 and NaCl. 



144. Show that gelatin contains sulphur by heating with 

 dry sodium carbonate in the reducing flame, then testing with 

 sodium nitroprussid as in the case of albumin (Experiment 74), 

 but that it gives no black sulphid of lead when heated in a solution 

 of lead acetate in an excess of sodium hydrate (Experiment 73). 



