WHY DO WE EAT OUR DINNER? 801 



of tliemselves, apart from work done, than there is for a similar wasting 

 away in the case of a mineral body such as a stone. When an animal 

 does practically no work, as in the instance of our desert-snail, his body 

 actually does not waste, but remains throughout just as big as ever. 

 So we must look a good deal more closely into the problem if we want 

 to understand it, and not rest content with vague generalities about 

 food and fuel. Such half knowledge is really worse than no knowledge 

 at all, because it deludes us into a specious self-deception, and makes us 

 imagine that we comprehend what in fact we have not taken the least 

 trouble to examine for ourselves. 



Let us begin, then, by clearly realizing what is the use of fuel to the 

 steam-engine. Obviously, you say, to set up motion. But where does 

 the motion come from ? '* From the coal," answers the practical man, 

 unhesitatingly. " Well, not exactly," says the physicist, " but from the 

 coal and the air together." All energy or moving power, as we now 

 know, is derived from the union of two bodies which have affinities or 

 attractions for one another. Thus, if I wind up a clock, moved by a 

 weight, I separate the mass of lead in the weight from the earth, for 

 which it has the kind of affinity or attraction known as gravitation. 

 This attraction then draws together the weight and the earth ; and, in 

 doing so, the energy I put into it is given out as motion of the clock. 

 Similarly with coal and air : the hydrogen and carbon of the coal have 

 affinities or attractions toward the oxygen of the air, and when I bring 

 them together at a high temperature (of which more hereafter) they 

 rush into one another's embrace to form carbonic acid and water, while 

 their energy is given off as heat or motion of the surrounding bodies. 

 We might have whole minefuls of coal at our disposal ; but if we had 

 no oxygen to unite with it, the coal would be of no more use than so 

 much earth or stone. In ordinary life, however, the supply of oxygen 

 is universal and abundant, while the supply of coal is limited ; and so, 

 as we have to lay in coals, while we find the oxygen laid in for us, we 

 always quite disregard the latter factor in our fires, and speak as though 

 the fuel were the only important element concerned. Yet one can easily 

 imagine a state of things in which oxygen might be deficient ; and in a 

 world so constituted it would have to be regularly laid on in pipes, like 

 gas or water, if the people wished to have any fires. 



All energy, then, is derived from the separation of two or more 

 bodies having affinities for one another. So long as the bodies remain 

 separate, the energy is said, in the technical slang of physics, to be 

 potential y as soon as the bodies unite, and the energy is manifested 

 as motion, it is said to be Jcinetic. But these words are rather mystify- 

 ing to ordinary readers, and frighten us by their bigness and their ab- 

 stract sound ; so I shall take the liberty of altering them for our present 

 purpose to dormant and active respectively, which are terms quite as 

 well adapted to express the meaning intended, and not half so likely to 

 land us in an intellectual cul-de-sac, or to envelop us in a logical fog. 



