VITAMINS 275 



listed. As a result of feeding mice on food mixtures 

 composed of purified nutrients and meeting all the views 

 expressed in our qualifications for a perfect food, the 

 animals failed to grow. Hopkins expressed his views at 

 the time as follows : 



But further, no animal can live upon a mixture of pure protein, 

 fat, and carbohydrate, and even when the necessary inorganic ma- 

 terial is carefully supplied, the animal still cannot flourish. . . . 

 The field is almost unexplored, only it is certain that there are many 

 minor factors in all diets of which the body takes account. 



In 1912, Hopkins first published the evidence on 

 which he based these prophetic utterances. In this 

 paper he demonstrated that a small quantity of milk 

 contains something other than purified nutrient sub- 

 stances of suitable quality, which is necessary to rat 

 growth. He suggested the name "accessory factor" 

 for this substance. 



In 1911 appeared the classical work of Osborne and 

 Mendel in demonstration of the significance of the 

 amino acids for maintenance and growth. They also 

 used rat-feeding experiments and, like Hopkins, soon 

 found that rats fed on purified substances alone would 

 not grow. They believed that they had found an ac- 

 cessory factor in milk, and by removing the protein 

 from the milk obtained a factor which they called 

 "Protein-free milk," and which, when added to their 

 otherwise adequate food mixtures, actually promoted 

 growth. They thought at first that the mineral con- 

 tent of the milk was the answer, but when they care- 

 fully analyzed their milk and substituted for natural 



