294 DIVERSIONS OF A NATURALIST 



former occasion (" Science from an Easy Chair," Second 

 Series, 191 3, p. 171, "Food and Cookery"). 



Let us take an example. A distinguished medical 

 chemist, Mr. Gowland Hopkins, has recently published 

 an account of some experiments in which he fed young 

 rats on a purely chemical, or " artificial " diet. He gave 

 them, in proper proportions, chemically purified casein 

 or curd, starch, sugar, lard, and salts, mixed into a thin 

 paste with water, of which they had an abundant 

 separate supply. Young rats fed with abundant natural 

 foods of mixed substances, such as cheese, bread, egg, 

 bits of meat and vegetable, and water, grow rapidly ; 

 they double their weight in twenty days. The young 

 rats fed by Mr. Hopkins upon the artificial pure food — 

 though supplied with it and taking it in abundance — did 

 not increase in weight, and most of them died before the 

 twentieth day ! The curious and important fact was 

 established (by careful and repeated experiment) that 

 if a teaspoonful of milk was added to the artificial food 

 (less . than one twenty-fifth of the solid matter of their 

 daily food) the young rats did as well, as on " natural " 

 food, doubled their weight in twenty days, and grew 

 up to be strong and healthy rats. It was made clear 

 that something was obtained by the rats from the small 

 quantity of milk — something necessary for carrying on 

 their nutrition, something the importance of which was 

 not its quantity but its peculiar quality, which was 

 absent in the artificial diet, but present in the mixed 

 diet of varied materials which a young rat naturally 

 gets. It seems that some highly elaborated proteid 

 is necessary, if only in minute quantity, to set nutrition 

 going, and that this is furnished by the teaspoonful of 

 milk. Here, then, we have a case in which the simple 

 rough conclusions as to all that is necessary in diet 



