CH. II.] GELATIN AND KERATIN. 59 



(d) Biuret reaction : violet colour. 



(e) Sulphur reaction : not obtained, showing absence of cystine. 



(Ex. 25.) 



(/) Add acetic acid : no precipitate. 

 (g) Add acetic acid and potassium ferrocyanide : very slight or 



no precipitate. 



(h) Add tannic acid: white precipitate. 



(i) Add lead acetate : very slight or no precipitate. 



(j) Half saturate with ammonium sulphate. The whole of the 

 gelatin is precipitated, as shown by a negative biuret test 

 in the nitrate (distinction from peptones). 



(k) Add Esbach's or Briicke's solution : yellow or white precipi- 

 tate (distinction from peptones). 



Keratin. An insoluble body found in the hair, skin, 

 nails, and horns. Remarkable for the high percentage of 

 cystine it yields on acid hydrolysis. 



71. Perform the following tests by using horn shavings, or hair. 

 Note insolubility in hot or cold water, dilute acids, and dilute 

 alkalies. 



(a) Xanthoproteic reaction : well marked. 



(b) Millon's reaction : well marked. 



(c) Glyoxylic reaction: well marked. 



(d) Biuret reaction : not obtained, owing to insolubility. 



(e) Sulphur reaction : well marked. 



