INTRODUCTION 5 



peratures exceeding 100 C. With some keratins, however, 

 the cystine group is to some extent easily split off by warm 

 water, and on boiling with water hydrogen sulphide is 

 evolved. The sulphur content of keratins is often greater 

 than the average for proteids. All keratins are dissolved 

 with great readiness by solutions containing sulphydrates 

 and hydrates, e.g. a solution of sodium sulphide. In solu- 

 tions of the hydrates of the alkali and alkaline earth metals, 

 keratins behave differently. Some dissolve with great ease, 

 some with difficulty, some only on heating and some not 

 even if digested with hot caustic soda. They are dissolved 

 (with hydrolysis) by heating with mineral acids, yielding 

 peptones and eventually amido acids, acid amides, etc. 

 Many keratins have a comparatively low content of nitrogen. 

 Gelatins are very difficult to distinguish from one 

 another, their behaviour being closely similar to reagents. 

 They are also very readily hydrolyzed even with water, and 

 the products of hydrolysis are even more similar. The 

 gelatins are known together, commercially, under the 

 general name of gelatine. Gelatins of different origin, how- 

 ever, have undoubtedly a different composition, the nitrogen 

 content being variable. If the gelatins are not bleached 

 whilst they are being manufactured into commercial gelatine, 

 they are called "glue." Gelatine is colourless, transparent, 

 devoid of taste and smell. It is usually brittle. Its S.G. is 

 about 1-42, and it melts at 140 C. and decomposes. It is 

 insoluble in organic solvents. When swelling in cold water 

 it may absorb up to 12 times its own weight of water. The 

 swollen product is called a "jelly." Jellies easily melt on 

 heating and a colloidal solution of gelatine is obtained. 

 This " sets " again to a jelly on cooling, even if only i per 

 cent, gelatin (or less) be present. The solution is optically 

 active and Isevorotatory, but with very variable specific 

 rotation. Some observers have thought that the different 

 gelatins have different specific rotations and may so be 

 distinguished. .Gelatins are precipitated from solutions by 

 many reagents, such as alcohol, formalin, quinone, meta- 

 phosphoric acid, tannins, and many salt solutions, e.g. those 



