58 THE PROTEINS. [cH. II. 



68. Treat some of the precipitate with o-i per cent. HC1. The 

 mucin dissolves. 



69. Treat some of the precipitate with 2 per cent. Na 2 CO 3 . 

 The mucin dissolves. 



K. The reactions of certain Albuminoids. 



Gelatin is found in the body in the form of its anhy- 

 dride, collagen. This occurs in white fibrous tissue and in 

 the organic substance of bones, and can be converted into 

 gelatin by boiling with a dilute acid. Dried gelatin swells 

 in cold water, but is quite insoluble in it. On warming, a 

 more or less viscid solution is obtained, which solidifies to a 

 jelly on cooling provided the concentration be greater than 

 i per cent. This process is reversible on warming and cool- 

 ing. It is precipitated by half-saturation with ammonium 

 sulphate, by tannic acid, phosphotungstic acid, Esbach's 

 and Briicke's reagents, but not by normal lead acetate. 

 On complete hydrolysis it yields a high percentage of its 

 nitrogen in the form of glycine, but only traces in the form 

 of the aromatic amino-acids, tyrosine, or trytophane, and 

 none as the sulphur-containing compound, cystine. There- 

 fore its solutions fail to give the glyoxylic, Millon's and 

 sulphur colour tests for proteins, and only give a slight 

 xanthoproteic test, which is due, either to an impurity or to 

 a small amount of phenyl-alanine. 



70. Break gelatin up into small pieces and add a small amount 

 of cold water. The gelatin does not dissolve. Immerse the test- 

 tube in a beaker of boiling water and leave it for a short time. The 

 gelatin dissolves. Cool the tube under the tap : the gelatin sets to a 

 jelly. Perform the following tests with an approximately I per 

 cent, solution of gelatin : 



(a) Xanthoproteic reaction : slight. 



(b) Millon's reaction : very slight, showing absence of tyrosine 



from gelatin molecule. (See notes to Ex. 22.) 



(c) Glyoxylic reaction : not obtained, showing absence of 



tryptophane. (Ex. 23.) 





