NUTRITION. 303 



determine the possible influence of these substances on growth. 

 Since our previous experience made it very improbable that the 

 removal of the protein was the cause of the failure to continue to 

 grow, we tried the effect of diets in which a part of the lard of the 

 ''protein-free milk" food was replaced by a quantity of butter equal 

 to that contained in the milk food. The outcome of a large number 

 of experiments, in which this ''butter food" was given to rats declin- 

 ing after earlier growth on "protein-free milk," was exactly the same 

 as when the milk food was supplied. 



The fact that rats can make good growth over comparatively long 

 periods on diets free from fat makes it improbable that the glycerides 

 of the fatty acids, of which butter is chiefly composed, cause the 

 resumption of growth. As it was possible that the small amount of 

 cellular matter contained in the suspended solids of the butter might 

 have caused the renewed growth, a series of experiments was made 

 with pure butter fat, obtained by centrifugating butter which had 

 been melted at 45°. This perfectly clear fat was quite as effective as 

 the butter in promoting resumption and continuation of growth. It 

 is evident that the butter-fat fraction of milk contains something 

 essential for long-continued, normal growth; but we are not yet in a 

 position to suggest what this substance may be. 



A continuation of our experime'nts with "artificial protein-free 

 milk" indicates that the success of our first series of experiments was 

 probably due to minute amounts of impurities contained in the 

 chemicals used in making the preparations of this material fed at 

 that time. New preparations made with carefullj^ purified chemicals 

 failed to promote growth in any degree comparable with that obtained 

 in the first series of experiments. We were therefore led to try 

 adding minute quantities of iodine, fluorine, manganese, and alumi- 

 num, all of which have been found in traces in the tissues of various 

 species of animals. The results thus far obtained with this prepara- 

 tion of "artificial protein-free milk" have been decidedly superior to 

 those obtained without the addition of traces of these inorganic 

 elements, but we are not at present prepared to draw final conclusions 

 in respect to the inorganic constituents of the diet essential for the 

 growth of the rat. 



Whether or not, in addition to inorganic substances, some organic 

 constituent of the natural "protein-free milk" has an important 

 influence on growth has received our attention, and much time has 

 been spent in attempting to extract such a substance from the "pro- 

 tein-free milk." The effect of such extracts has been studied, but the 

 experiments now in progress do not yet justify final conclusions. 



The progress of our work during the summer of 1912 was greatly 

 impeded by the development of intestinal disturbances in a large 

 proportion of our young experimental rats, in consequence of which 



