BIOLOGICAL METHODS 329 



hoi soluble proteins, form a distinct class ; they are found only 

 in the vegetable kingdom, and have no analogues amongst 

 proteins from animal sources. 



Of twenty-three different seed-proteins which have so far 

 been systematically hydrolysed, all were found to contain 

 leucine, proline, phenylalanine, asparagine, glutamic acid, 

 tyrosine, histidine, arginine and ammonia ; two gave no 

 glycine ; two gave no alanine ; four gave no lysine ; and 

 one gave no tryptophane. One, namely zaein, gave neither 

 glycine, lysine nor tryptophane. Three gave no cystine, and 

 two others only traces. 



It is, on the whole, unlikely that there is any protein 

 entirely free from sulphur, although in the case of vicilin the 

 amount is actually as low as O'l per cent. If it is assumed 

 that the sulphur is contained in the molecule in the form of 

 cystine, it follows that there must be at least two atoms of 

 sulphur present. Calculations based on this assumption give 

 a value for the molecular weight of at least 15,000, but 

 although from other considerations the molecular weight of 

 proteins is known to be high, it is unlikely that the value is 

 as high as this. 



While it is possible by means of general reactions to place 

 a given protein in the class of albumins or globulins, there are 

 no distinctive chemical or physical methods by which the iden- 

 tity of any particular albumin or globulin may be established ; 

 thus it not infrequently occurs that two substances which have 

 been obtained from different sources, and are described under 

 different names, are eventually found to give the same figures 

 on analysis, and are therefore regarded as identical. This is 

 notably the case with albumins obtained from different plant 

 seeds, and the serum albumin derived from different animals. 

 Within the last few years, however, a biological method has 

 been discovered which promises to become of the very greatest 

 value in distinguishing the various compounds from each other. 

 Following upon the researches of Wassermann and Uhlenhuth, 

 Tstistowitch found that serum drawn from a rabbit, which had 

 been inoculated for some time with the serum of a horse, had 

 acquired the property of producing a precipitate when added 

 to normal horse serum ; this is due to the formation in the 

 rabbit's blood of a substance known as a precipitin, which 



