GENERAL INTRODUCTION TO VITAMIN THEORY 19 



supplied, the animal still cannot flourish. The animal body is adjusted 

 to live either upon plant tissues or other animals and these contain 

 countless substances other than the proteins, carbohydrates, and fats." 

 In his experiments, using rats as subjects, Hopkins found that the 

 addition of small amounts of milk to diets otherwise composed of 

 purified foodstuffs resulted in growth (Figs. 1 and 2), and that this 

 was due to an alcohol-soluble organic substance or substances in the 



Fig. 2. — Growth curves of rats with and without small amounts of milk as 

 source of vitamins. The lower curve up to the eighteenth day represents rats on 

 purified food ; the upper curve similar rats having milk each day in addition to 

 this food. On the eighteenth day marked by the vertical dotted line the milk was 

 transferred from one set to the other. Abscissae — time in days ; ordinates — average 

 weight in grams. (Courtesy of Professor Hopkins and the Medical Research 

 Committee of Great Britain.) 



milk and not to any of its previously known constituents. Certain vege- 

 tables had the same property in lesser degree than milk. As described 

 in the following chapter, Osborne and Mendel demonstrated that a 

 similar growth-promoting property was possessed by their "protein- 

 free milk," a powder prepared by removing the fat, casein, and 

 albumin from cows' milk and evaporating the filtrate to dryness. A 

 little later it was found both by McCollum and Davis (1913) and by 

 Osborne and Mendel (1913a) that the fat of milk also possesses a 



