8o2 THE POPULAR SCIENCE MONTHLY. 



When we take a piece of coal and a lot of free oxygen, we possess 

 energy in the dormant state. But though the oxygen has strong at- 

 tractions for the carbon and hydrogen, they can not unite, because their 

 atoms do not come into close contact with one another, and because the 

 two last-named substances are bound up in the solid form of the coal. 

 We might compare their condition to that of a weight suspended by a 

 string, which has strong attractions toward the earth, but can not unite 

 with it till we cut the string. Just analogous is our action when we 

 apply a match to the coal. The heat first disintegrates or disunites 

 little atoms of the hydrocarbons which make it up, and sets them in a 

 state of rapid vibration among themselves. This vibration brings them 

 into contact with the atoms of oxygen, which at once unite with them, 

 causing a fresh development of heat, and a liberation of all the dormant 

 energy, which immediately assumes the active form. The carbonic acid 

 and water (or steam) thus produced fly up the chimney, carrying with 

 them the little bits of unburned coal which we call smoke ; and a current 

 of fresh oxygen rushes in to unite with the fresh atoms of hydrogen and 

 carbon which have been disengaged by the energy liberated from their 

 fellows. So the process continues, till all the coal has been converted 

 into carbonic acid and water — of course by the aid of a corresponding 

 quantity of oxygen — and all the energy has been turned loose as heat 

 upon the room in which we sit and upon the air outside. 



In the case of an ordinary fire, where warmth is the single object we 

 have in view, we only think of the heat, and disregard the other aspects 

 of the process. But it is clear that an enormous amount of motion has 

 also been set up by the energy of the free coal and oxygen, as exempli- 

 fied by the draught up the chimney, and the numerous currents of air 

 produced by its action within and without the room. Now, in a steam- 

 engine we deliberately make use of this motion for our own purposes 

 by a specially devised mechanism. We allow the fire to heat and ex- 

 pand the water in the boiler, thus transferring to its molecules the 

 separation which formerly existed between the atoms of the coal and 

 the oxygen. Then we make the expanded water or steam push up the 

 piston, and we connect the piston in turn with a crank which sets in 

 motion the wheels, and so passes on the active energy to the mill, train, 

 or ship which we desire to move, as the case may be. Thus the dormant 

 energy of the coals and oxygen is liberated in the active state by their 

 union, and is finally employed to effect movement in external bodies by 

 the intermediation of the boiler. Even then the energy does not dis- 

 appear : for energy, like matter, is indestructible ; but it merely passes 

 by friction as heat to that wonderful surrounding medium which we call 

 ether, and is dissipated into the vast void of space, no longer recover- 

 able by us, though quite as really existent as ever. 



In what way, however, has all this to do with the reason for eating 

 our dinners ? Simply this : Men and other animals may be regarded 

 from the purely physical point of view as a kind of conscious locomo- 



