FEEDING EXPERIMENTS WITH ISOLATED FOOD SUBSTANCES. 



39 



ISO 



130 



0.05 



Chart XIII. Rats xxxvir, xl. and xlii fed on pure Casein as the only protein and Salt Mixture n 

 for 42 days. Numbers on Body-weight lines indicate time at which each period began. 



DISCUSSION. 



The essential differences in these series lasting 42 days involve 

 the inorganic constituents of the diet. All of the animals selected, 

 with two exceptions (xl and xlii)* gave a fairly satisfactory record 

 during a three weeks preliminary trial, whereupon they were selected 

 for the casein feeding. It will be seen that the most promising 

 nutritive conditions were afforded by mixture 1, fed to rats xxxiii, 

 xxxiv, and xxxv, in which the inorganic constituents were closely 

 copied after Rohmann's successful ash mixture, with the addition 

 of a little iron. During the first two weeks of casein feeding these 

 three rats lost weight and nitrogen. This was caused by diarrhoea 

 due to too great a proportion of inorganic salts in the food mixture. 

 When this was reduced from 4.8 to 2.5 per cent a very rapid gain 

 in weight and nitrogen at once took place. The superiority of the 

 food mixture given to these three rats in contrast with the other two 

 mixtures given to the other rats is plainly evident from comparison 

 of these data. These experiments lasted 42 days and showed distinct 

 gains in the nitrogen balance and also in body-weight during the 

 period mentioned. f 



*These two animals were not pure white rats, but partly colored. They ate very 

 poorly. We have gained the impression from observations on a large number of rats 

 that these hybrid forms are not suited to our experimental needs and therefore have 

 lately employed only the pure white races. 



fit may be noted that the apparently poorer utilization of nitrogen during some of 

 the periods when little food was eaten is in part attributable to the fact that output of 

 nitrogenous alimentary secretions continues despite the smaller intake of nitrogen. 



