222 THE VITAMINS 



knowledge of vitamins, the results obtained by Stepp in his effort to 

 identify the indispensable substances removed from the food materials 

 by extraction with alcohol and ether, as reported in this and later studies, 

 can readily be interpreted from the known properties of the water- 

 soluble and fat-soluble vitamins, both soluble in alcohol and the latter 

 in ether, and both consequently removed from the materials used in 

 his experiments. Unable to identify the removed indispensable com- 

 ponent as any of the known lipoids, Stepp (1911, p. 150) called atten- 

 tion to the property of lipoids of influencing the solubility of other 

 materials to such an extent that they also would be removed during 

 the extraction. "So ware es nicht undenkbar dass gemeinschaftlich mit 

 den Lipoiden irgenwelche unbekannte lebenswichtige Stofife in Losung 

 gehen und dass so die Lipoide gewissermassen zu Tragern fiir diese 

 Stoffe wiirden, dass mit anderen Worten bei der Entfernung von 

 Lipoiden die unbekannten Korper mit enfernt und bei Zusatz von 

 Lipoiden mit diesen zugesetzt werden. Ein Hinweis auf eine derartige 

 Moglichkeit erscheint notwendig, solange es nicht gelingt, die Versuche 

 mit chemisch reinen Korpern durchzufiihren." 



The work of Osborne and Mendel during this same period was 

 directed largely to the question as to whether normal nutrition could 

 be secured upon a dietary containing a single individual protein instead 

 of mixtures of proteins such as ordinary dietaries comprise. They soon 

 came to emphasize also the importance of factors other than protein, 

 and discovered that mixtures of purified foodstuffs which were capable 

 of maintaining for some time the life of full-grown rats were unable 

 to support the growth processes in young animals. Favorable experiences 

 with milk and milk powder led to many unsuccessful attempts to 

 modify the inorganic and non-protein ingredients of the diet of puri- 

 fied foodstuffs and finally to the use of the so-called protein-free 

 milk which was prepared by removing as far as possible by precipi- 

 tation and coagulation the proteins from the fat-free milk, evaporating 

 the filtrate and grinding the residue to powder. This material, which 

 was added to the artificial dietaries in liberal amounts (about 28 per 

 cent of the total food mixture), was found to promote rapid growth in 

 rats. 



Shortly after the publication of Osborne and Mendel's work in- 

 volving the use of protein-free milk, Hopkins (1912) published the 

 results of his experiments conducted in 1906 and 1907 and showing 

 a remarkable effect produced in the growth of rats on a diet of purified 

 food substances by the addition of small amounts of milk or the alcohol- 

 soluble portion thereof. That his observations had at an even earlier date 



