ACCESSORY FOOD-FACTORS 41 



as far as can be ascertained, there has been no out- 

 break of it specially brought on by war conditions. 



The school which has studied the growth factors is 

 mainly located in America, but the foundation-stone 

 was laid by Hopkins in this country about 1906. 

 Two growth factors are recognized, to which the 

 name vitamine has not been given, but they are 

 designated by their discoverers the "fat-soluble A 

 factor" and " the water-soluble B factor". Rather 

 unfortunately, the different points of attack have made 

 it appear as if the class containing the growth factors 

 and the class containing the anti-beriberi and the 

 anti-scurvy factors were separate and distinct, whereas 

 there is pretty good prima facie evidence that " water- 

 soluble B " is the same substance as the anti-beriberi 

 vitamine. The " fat -soluble A factor' is not the 

 same as the anti-scorbutic vitamine, but facts are not 

 wholly inconsistent with the hypothesis that it may 

 be identical with the substance which prevents rickets, 

 and which is occasionally spoken of as the anti- 

 rachitic vitamine. 



Since none of these accessory food-factors has ever 

 been isolated as a definite chemical compound, they 

 can only be studied indirectly by their biological 

 action in feeding-experiments with animals. Every 

 method of attack has thus only the one very slow 

 method of test. 



The accessory food -factors are characterized from 

 one another by their different distribution among 

 food-stuffs, by their varying stability to heat, by their 

 behaviour to certain other physical and a few chemical 

 tests, and, lastly, by the pathological effects pro- 

 duced in man and animals when they are omitted 

 from the diet. In the case of each factor the experi- 

 mental animal has been different, so that at present 

 the work does not fall very well into line, but clinch- 



