14 



FEEDING EXPERIMENTS WITH ISOLATED FOOD SUBSTANCES. 



complete and rapid mixing of the ingredients of the paste. This 

 method of mixing was used for all of the foods described in this 

 paper. 



A series of rats was fed with this food at the same time that 

 other experimental diets were being investigated. In this way all 

 the animals weie exposed to the same indeterminable variables of 

 climate and environment which might perchance have exerted an 

 unsuspected deleterious influence and which would exhibit them- 

 selves in the control animals as well as those under special observa- 



60 60 100 120 140 160 ISO 200 220 240 260 280 300 320 >40 360 330 400 



Days 

 Chart I. Average Normal Rate of Growth of Male White Rat, according to Donaldson. 



tion. Like most of our trials during the first year of this work, they 

 served chiefly for the purpose of orientation in respect to future 

 procedure. It is scarcely necessary to record here the numerous 

 individual experiments which resulted in a failure to maintain the 

 rats in health and nutritive equilibrium. Failures, unless they are 

 invariable in their occurrence, may well be due to accidents or inci- 

 dents in no wise associated directly with the nutritive functions. 

 Intercurrent parasitic diseases, incipient senility, hereditary defects, 

 and other incidental features may be present or arise to interfere 

 with the normal progress of an experiment. We have gradually 

 learned to watch for such undesirable conditions and to exclude such 

 animals as unsuitable for these studies, since proper allowance can 



