820 THE POPULAR SCIENCE MONTHLY. 



Food is also demanded for this, and the non-nitrogenous food is the 

 most readily combustible, especially the hydrocarbons, or fats ; the 

 carbo-hydrates — starch, sugar, etc. — also, but in lower degree. These* 

 then, were described as fuel-food, or heat-producers. 



This view is strongly confirmed by a multitude of familiar facts. 

 Men, horses, and other animals can not do continuous hard work with- 

 out a supply of nitrogenous food ; the harder the work the more they 

 require, and the greater becomes their craving for it. On the other 

 hand, when such food is eaten in large quantities by idle people, they 

 become victims of inflammatory disease, or their health otherwise suf- 

 fers, according, probably, to whether they assimilate or reject it. 



Man is a cosmopolitan as well as an omnivorous animal, and the 

 variation of his natural demand for food in different climates affords 

 very direct support to Liebig's theory. Enormous quantities of hydro- 

 carbon, in the form of fat, are consumed by the Esquimaux and by Eu- 

 ropeans when they winter in the Arctic regions. They can not live 

 there without it. In hot climates some fuel-food is required, and the 

 milder form of carbo-hydrates is chosen, and found to be most suit- 

 able ; rice, which is mainly composed of starch, is an example. Sugar, 

 also. Offer an Esquimau a tallow-candle and a rice-pudding, he will 

 reject the latter, and eat the former with great relish. 



A multitude of other facts might be stated, all supporting Liebig's 

 theory. 



There is one that just occurs to me as I write, which I will state, 

 as it appears to have been hitherto unnoticed. Some organs which act 

 in such wise that we can see their mode of action are visibly disinte- 

 grated and consumed by their own activity, and may be seen to de- 

 mand the perpetual renewal described by Liebig. There are certain 

 glands of cellular structure which cast off their terminal cells contain- 

 ing the fluid they secrete ; do their work by giving up their own struct- 

 ural substance at their peripheral working surface. 



Where, then, is the quicksand ? It is here. If muscular and men- 

 tal work were done at the expense of the nitrogenous muscular and 

 cerebral tissues, the quantity of nitrogen excreted should vary with 

 the amount of work done. This was formerly stated to be the case 

 without hesitation, as the following passage from Carpenter's " Man- 

 ual of Physiology " (third edition, 1856, page 256) shows : " Every 

 action of the nervous and muscular systems involves the death and 

 decay of a certain amount of the living tissue — as is indicated by the 

 appearance of the products of that decay in the excretions." 



More recent experiments by Fick and Wislicenus, Parkes, Hough- 

 ton, Ranke, Yoit, Flint, and others, contradict this by showing that 

 the waste nitrogen varies with the quantity of nitrogenous food that 

 is eaten, but not with the muscular work done. For the details of 

 these experiments I must refer the reader to standard modern physio- 

 logical treatises, as a description of them would carry me too far away 



