78 FIRST LESSONS IN ZOOLOGY 



while in even a very large shop there may be but one 

 engine generating energy to run all the different machines, 

 in the body ever\' muscle is a separate engine, and one 

 bone may be connected with a number of them. Never- 

 theless the essential facts are the same in both cases. 

 The muscle-engine, like the steam-engine, produces a form 

 of energy and applies it to machines so as to lift weights 

 or to move things from place to place. But we learn in 

 physics that we can get any form of energy only by 

 changing some other form into the one desired. The 

 forms of energy are heat, light, electricity, chemical en- 

 ergy, and that of a body in motion. Now the only way 

 to get heat, for example, is by a change from one of the 

 others. VVe can make a piece of iron hot by striking it 

 with a hammer; here the energy of a moving body is con- 

 verted into heat. Or the energy of the electric current 

 may be converted into heat or motion. Man's most com- 

 mon wa)' of getting heat is to take coal, wood, or oil, and 

 apply some heat to start with, when the ox\'gen of the air 

 will unite with carbon and hj'drogen, substances in the coal, 

 wood, or oil, to make two new substances, one of these 

 being carbon dioxide, the other water. This is chemical 

 action ; it results in changing chemical energy into heat. 

 In ordinary language this union of oxygen with carbon or 

 hydrogen is spoken of as " burning " or " combustion." 

 An animal cannot make the least motion without using 

 a certain amount of energy. And it has been shown by 

 investigation that the energy possessed by an animal 

 is derived from the chemical energy resulting from the 

 union of oxygen with the carbon, hydrogen, and nitro- 

 gen in other sub.stances. The muscles are the engines 

 in which this energy is made use of for motion. This 

 brings us now to see how essential it is that the animal 

 should have in its body oxygen and substances for the 

 oxygen to combine with. 



