280 THE HUMAN BODY. 



longer suppose, with, the older physiologists, that it works 

 by means of a mysterious vital force existing in or created 

 by it; but that getting energy from the outside it utilizes 

 it for its purposes for the performance of its nutritive and 

 other living work and then returns it to the exterior 

 in what the physicists know as a degraded state; that is in 

 a less utilizable condition. While energy like matter is in- 

 destructible it is, unlike matter, transmy.table; iron is always 

 iron and gold always gold; neither can by any means which 

 we possess be converted into any other form of matter; and 

 so the Body, needing carbon, hydrogen, oxygen, and nitro- 

 gen to build it and to cover its daily losses, must be sup- 

 plied with those very substances. As regards energy this 

 is not the case. While the total amount of it in the uni- 

 verse is constant, its form is constantly subject to change 

 and that one in which it enters the Body need not be that 

 in which it exists while in it, nor that in which it leaves it. 

 Daily losing heat and mechanical work the Body does not 

 need, could not in fact much utilize energy, supplied to it 

 in these forms; but it does need energy of some form and 

 in amount equivalent to that which it loses. 



The Conservation of Energy. The forms of energy 

 yet discovered are not nearly so numerous as the kinds of 

 matter. Still we all know several of them; such as light, 

 heat, sound, electricity, and mechanical work; and most 

 people nowadays know that some of these forms are inter- 

 convertible, so that directly or indirectly we can turn one 

 into another. In such changes it is found that a definite 

 amount of one kind always disappears to give rise to a 

 certain quantity of the other; or, in other words, that so 

 much of the first form is equivalent to so much of the 

 second. In a steam-engine, heat is produced in the fur- 

 nace; when the engine is at work all of this energy does 

 not leave it as heat; some goes as mechanical work, and the 

 more work the engine does the greater is the difference be- 

 tween the heat generated in the furnace and that leaving 

 the machine. If, however, we used the work for rubbing 

 two rough surfaces together we could get the heat back 

 again, and if (which of course is impossible in practice) 



