THE HUMAN BODY AS AN ENGINE. 493 



we know one cannot be converted into another. They may be united 

 in countless combinations, but each is itself not only indestructible but 

 unchangeable. Why this is so is an interesting subject of speculation. 

 We do not positively know. 



That energy is also something which cannot be created or destroyed 

 is not so generally recognized. Transformations of energy from one 

 form to another are constantly occurring before our very eyes; and yet 

 we seldom stop to think what the conservation of energy means in any 

 given case. Energy itself is often defined as that which has the capac- 

 ity for doing work, and work is done when force or resistance is over- 

 come. A hod carrier does work when, overcoming the force of gravity 

 upon his body and his hod of brick, he climbs to the top of a ladder; 

 and the work done is a measure of the energy expended. Energy stored 

 up in his body has been transferred to the brickin their elevated position, 

 and if they are allowed to fall to the ground their energv is turned into 

 heat, developed by their impact upon the ground. Again, work is done 

 by a windmill in pumping water up into an elevated reservoir, and the 

 so-called 'potential' energy which the water possesses in its elevated 

 position has all been transferred to the water from the wind which drove 

 the mill. If the water be allowed to flow down to the ground again 

 through a water motor the latter could drive machinery and so do 

 work; and the work it could do plus the heat produced by friction would 

 exactly equal the work done in pumping the water up to its elevated 

 position. Thus is the energy conserved, and not destroyed. More or 

 less of it is dissipated by friction, and lost, so far as useful effect may 

 go. But it all remains in existence, somewhere. 



Again, coal is burned under the boiler of a steam engine. Heat is 

 produced, steam is generated, the engine does work. The coal possessed 

 a store of energy, potentially. That is, the coal had the capacity of 

 uniting with the oxygen of the air and setting free a store of energy. 

 This energy, potential or latent in the coal, becomes kinetic and evi- 

 dent in the heat of the boiler and the work of the engine. Moreover, 

 the work done by the engine added to the heat given off by the boiler 

 and engine is exactly equal to the total store of energy possessed by the 

 coal. And if from a store of energy, either in the body of a man or 

 horse, or in a pile of wood or coal, a certain portion is expended in doing 

 work, the amount remaining is exactly the difference between that ex- 

 pended and the original amount. In short, energy can be measured, 

 stored up and expended, just as truly as merchandise or money. 



Thus the conservation of energy means that energy cannot be 

 created or destroyed; but it may be transferred from one body to an- 

 other or transformed from one form to another. Heat may be con- 

 verted into work and work into heat. The chemical energy of a zinc 

 rod may be expended to generate an electric current, and the latter 



