VITAMINES 37 



or catalysts with dynamic functions in the tissues. 

 In any case they are nutritive essentials not directly 

 concerned with the supply of potential energy. 



The question will be immediately asked : what, 

 then, is the actual nature of such substances ? It 

 must be confessed that we do not yet know. 

 Patient efforts have already been made to effect 

 their isolation, but success has not yet come. But 

 their nature is perfectly objective. We can extract 

 them, precipitate them, redissolve them : but we 

 cannot yet get them completely separated from 

 other substances, and until we can do so we can- 

 not determine their chemical nature. But many 

 facts have been garnered concerning their general 

 character and behaviour, and what in particular 

 we have learned in the last four or five years is to 

 be sure that there is more than one of these sub- 

 stances. There are, indeed, at least three, each one 

 of them having its own function in nutrition. 



Consider first very briefly the evidence for 

 the existence of two of them. Suppose an animal 

 to be upon artificial diet such as I have already 

 described. Normal growth will fail, as we have 

 seen. But suppose the fat supplied in the artificial 

 mixture to have been butter or almost any other 

 animal fat. Then growth will occur if we merely 

 add to the mixture small quantities of watery ex- 

 tracts made from various plant or animal tissues. 

 A watery extract of the wheat grain, for instance, 



