496 POPULAR SCIENCE MONTHLY. 



from the fire box of the engine. That the body fat and muscular tissue 

 are also burned, producing heat, is literally true. A hibernating ani- 

 mal keeps his body warm all winter by burning up his autumnal store 

 of body fat. Even a well-fed body is constantly wearing away, or 

 burning away, and hence requires constant repair. Thus we see two 

 distinct functions for food, which should be carefully distinguished. 



In the first place, as already indicated, food repairs waste and builds 

 up the body. It makes blood, bone and muscular tissue. Herein we 

 see a departure from the parallel with the steam engine. A locomotive 

 is a machine which runs in a way determined by its builder. But it 

 cannot grow nor repair wear and tear. It requires a whole machine 

 shop phis skilful mechanics to do that. The body, on the other hand, 

 not only runs like a complex mechanism when supplied with energy, 

 but also builds itself up and repairs waste. We express this by saying 

 that it possesses vital force or life, but in just what vital force consists 

 is a matter of speculation and controversy. The raw material which is 

 employed in this work of repairing and building up is found in the 

 food. But not all food can be so utilized. Only those materials which 

 contain nitrogen, the so-called proteids, as lean meat, the casein of milk 

 and gluten of wheat, can be made use of in this most important work 

 of growth and repair. 



In the second place, food is the fuel of the body and is just as truly 

 burned as is coal in a furnace. Moreover, the quantity of heat which a 

 piece of meat or a slice of bread yields when burned in the body is just 

 the same as if it had been burned in a stove. Complete combustion 

 yields a definite amount of heat wherever and whatever may be the 

 place and manner of burning. Any kind of food may serve as fuel for 

 the body, but those which consist mainly of sugar, starch and fat, which 

 contain no nitrogen and so cannot build up the body, are used chiefly 

 as fuel. These fuel foods form the bulk of our daily ration, compara- 

 tively little being required for purposes of growth and repair. 



We are hearing a good deal recently about alcohol as a food. When 

 it is remembered that alcohol contains no nitrogen it will be seen that 

 it cannot serve the first function of food, namely, the purpose of growth 

 and repair. It can, however, serve as fuel food, for when taken into the 

 body in small quantities it is assimilated and burned up, producing the 

 same amount of heat as if burned in a lamp. In sickness this may be 

 beneficial, at times when the body cannot assimilate other foods. But 

 the injurious effects of alcohol upon the digestive and nervous systems 

 are so important and far-reaching that its value as a fuel food sinks into 

 insignificance in comparison. 



The process of combustion or burning in the fire box of our locomo- 

 tive consists, as has been said, in oxygen of the air uniting with the 

 carbon and hydrogen of the coal, forming carbonic acid and water, and 



