INTRODUCTORY 37 



only investigation on this subject was the classical work of Hopkins 

 (70) which appeared in 1912, demonstrating that the addition of a 

 small quantity of milk to an artificial diet induced growth in rats. 

 The amount of milk was so small as to be negligible, as far as its 

 energy factor was concerned. In these experiments, there was no 

 attempt to differentiate between the various vitamines, since milk 

 contains all that is necessary to life. While the vitamine conception 

 had at that tune attained a definite standing in England (not without 

 vigorous effort), the dissemination of these ideas in other countries 

 met with but poor success. For instance, Abderhalden and Lampe 

 denied the existence of vitamines in 1913, and Rohmann even in 

 1916. In the United States there appeared simultaneously with the 

 beginning of vitamine research the very important work of Osborne and 

 Mendel (71) on the artificial feeding of rats, which greatly advanced 

 our knowledge of the food value of the various kinds of proteins. 

 These investigations were indeed the first to be carried out with 

 such carefully purified proteins and over such a long period of time 

 (more than one year, one-third of the lifetime of a rat). These 

 experiments served two purposes, first, to determine the nutritional 

 value of various proteins, and secondly, to determine how long rats 

 can live on an artificial diet. At that tune, we emphasized the fact 

 that for such investigations it is particularly important to provide 

 the animals with vitamines, if clear results are to be obtained. 

 Besides this, we showed that in many nutrition experiments the diet 

 was in some unknown manner contaminated by vitamines which 

 were responsible for the length of time the animals survived. This 

 was later shown to be true; for example, we could demonstrate the 

 presence of some nitrogen -containing impurity in milk sugar; the 

 same is true of other products derived from milk. In the experi- 

 ments of Osborne and Mendel, the diets, taken as a whole, were 

 obviously lacking in vitamines, since many of the animals died 

 suddenly, or they would have died if the diet had not been changed 

 quickly. Young animals lived for a certain time, but mostly failed 

 to grow. 



In another investigation, Osborne and Mendel (72) described 

 experiments in which rats were fed on a mixture that could have 

 been thought of as fat-free. It contained, among other things, 

 protein-free powdered milk extracted with ether. In this case also, 

 normal growth was obtained. The same authors then analyzed the 



