FOOD 203 



verted into electrical force or light, or again become heat. 

 A mechanic's blow upon an anvil converts muscular force 

 into motion, heat, and light. Any form of energy must 

 result from some other form of energy. It cannot come 

 from nothing. 



286. Conservation of Energy and Correlation of Forces. 

 These facts, in works upon the science of physics, are 

 stated as the laws of conservation of energy and of the 

 correlation of forces. The human body, like all other 

 matter, is subject to these laws. 



As the power for a large proportion of our machines 

 comes from the heat of combustion, that is, the union 

 of oxygen with other matter, so does the energy of the 

 body come mainly from the same chemical action, and the 

 products of the slow combustion of food which takes place 

 within the body are much the same as they would be if 

 the food were burned in a furnace by swift combustion. 

 Those products are carbon dioxide, water, and a nitrogen- 

 containing substance which is discharged from the body 

 as urea, besides certain salts not oxidizable which would 

 appear in the furnace as ashes. 



287. Food Elements. The valuable parts of the matter 

 which we call food enter in varying proportions into many 

 different articles which we eat. Chemists divide these 

 elements, which are essential to the maintenance of the 

 body in health, into five classes, as follows : 



1. Proteids (Nitrogenous foods) 



8 F^ ^^ 68 [ (NonnitrogenoiiB) 



4. Water ) 



5. Salts f Inorganic 



Milk and eggs are examples of food containing all 

 these materials in proportions suitable for young, grow- 



