78 THE CHEMISTRY OF CATTLE FEEDING 



substance set as a jelly on cooling to ordinary temperatures. 

 The commercial product is usually obtained in the form of thin, 

 yellowish, transparent plates. These contain from 10 to 15 per 

 cent, of water. The pure, anhydrous substance is a colourless, 

 amorphous powder. It varies slightly in composition, but the 

 average of numerous analyses is as follows : carbon, 50 ; 

 hydrogen, 7; nitrogen, 18; oxygen, 25 per cent. Sulphur is 

 absent or present only in traces, probably as impurity. 



Under hydrolytic treatment with acids, or by the action of 

 ferments, gelatine is converted first into a proteose, and subse- 

 quently into peptone. Tyrosine and tryptophane are not 

 present, and the xanthoproteic and Millon's reaction conse- 

 quently give negative results. Gelatine gives a well-marked 

 biuret colour, but, owing to the absence of sulphur, it does not 

 give Vogel's reaction. It is not precipitated by acids or by 

 normal salts of lead or copper, but mercuric chloride, basic 

 acetate of lead, and most of the alkaloid reagents produce pre- 

 cipitates. It is insoluble in alcohol, and is thrown down by 

 that reagent and by tannic acid from salt-free solutions. 



Keratin is the chief constituent of horn, hoofs, hair, wool, 

 feathers, etc. It is quite insoluble in water, and is strongly 

 resistant to the action of acids, alkalis, and digestive ferments. 

 Dilute (10 per cent.) acids and alkalis have no effect upon it 

 in the cold. Stronger solutions decompose it. Keratin is 

 therefore easily prepared from the substances mentioned. Fat 

 ami other adventitious matters are removed by extraction with 

 alcohol and ether, and the residue is then boiled successively 

 with dilute acids and alkalis, which hydrolyse all other 

 proteins. 



Analyses of keratin from various sources differ consider- 

 ably, but they all show a large amount of sulphur usually 

 about 5 per cent. The dissociation products include excep- 

 tionally large amounts of cystin and tyrosine, and the sub- 

 stance gives very strong colorations with the xanthoproteic, 

 Millon's and Vogel's reactions. Proteoses and peptones have 

 been formed by the action of superheated steam on keratin. 



Conjugated Proteins. The conjugated proteins may be 

 regarded as analogous to the compound celluloses (p. 29). 



