302 THE HUMAN BODY. 



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 interconvertible, 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 pro- 

 duced in the furnace; 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 between the heat generated in the furnace and that 

 leaving the machine. If, however, we used the work for rub- 

 bing two rough surfaces together we could get the heat back 

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

 could avoid all friction in the moving parts of the machine, 

 the quantity thus restored would be exactly equal to the 

 excess of the heat generated in the furnace over that leaving 

 the engine. Having turned some of the heat into mechanical 

 work we could thus turn the work back into heat again, and 

 find it yield exactly the amount which seemed lost. Or we 

 might use the engine to drive an electro-magnetic machine 

 and so turn part of the heat liberated in its furnace first into 

 mechanical work and that into electricity; and if we chose, to 

 use the latter with the proper apparatus, we could turn more 

 or less of it into light, and so have a great part of the energy 

 which first became conspicuous as heat in the engine furnace, 

 now manifested in the form of light at some distant point. 

 In fact, starting with a given quantity of one kind of energy, 

 we may by proper contrivances turn all or some of it into 

 one or more other forms; and if we collected all the final 

 forms and retransformed them into the first, we should have 

 exactly the amount of it which had disappeared when the 

 other kinds appeared. This law, that eneryv can 



