552 THE POPULAR SCIENCE MONTHLY 



communicated his ideas to friends, and sought to convince them. The 

 mathematicians among them were so affected by the faulty form in 

 which Mayer put his theory, that they would not listen and paid him 

 no attention, even after he had corrected the error. The professors 

 proved no more amenable. One of them remarked, sarcastically, that 

 if that were so, a bottle of water violently shaken ought to become 

 warmer. Mayer, undeterred by the sarcasm, tried the experiment, the 

 result of which is now so familiar to us as to seem almost axiomatic. 



We see clearly that in doing mechanical work the losses of work, 

 due to friction, result in heat. In a word, friction is a means of con- 

 verting work into heat, just as a steam engine is a means of converting 

 heat into work. Incomplete machines are those that transmute part 

 of the work into heat, and the smaller this part is, the more perfect we 

 call the machine. 



Heat is, however, not the only thing into which work may be con- 

 verted. It may reappear in the form of electricity, or of light, or of 

 chemical change. All these tilings, out of which work may arise or into 

 which it may be changed, we now call energy. While the law of the 

 conservation of work has only a limited and ideal application, that of 

 the conservation of energy is a natural law of universal application, 

 without any limitation or exception. If we call those amounts of 

 energy equal which arise from one another, we can, as the result of all 

 experience, express the conservation law in the following terms: 



Within a closed system, through whose walls no energy can pass in 

 or go out, the total amount of energy remains the same, regardless of 

 what happens inside the system. 



In what light are we to consider this energy? Is it an actuality 

 or a mere figment of thought? The reply can scarcely be doubtful. 

 That which can not be created by any power in the universe, which 

 maintains itself unaltered in amount in spite of all the unnumbered 

 and protean changes to which it is subjected through all the ages, must 

 be the most real thing we can conceive of. All efforts of the adherents 

 of the older view to discredit the reality of energy have been fruitless, 

 so that it is now thought of not merely as a formal thing, but as an 

 essential entity, as a commodity which can be measured, stored, bought 

 and sold. When you have the storage battery of your electric car 

 charged up, and pay for it, what are you getting for your money ? The 

 battery is not heavier by the smallest fraction of an ounce. You have 

 not purchased a figment of the mind, nor any mere abstraction, but an 

 absolutely real thing, so many units of energy. You may think of it 

 perhaps as electricity, but this is erroneous, since the charged battery 

 contains its energy in chemical, not in electrical form. 



Energy is more than a reality. It is the reality. No phenomenon, 

 no effect, is anything but a (more or less transitory) manifestation of 

 energy and, as such, is subject to the energy law. 



