Page:Popular Science Monthly Volume 20.djvu/692

672 up the weight until you impart to the clock energy in this potential form of sufficient power to keep a clock going for a fortnight. In gunpowder there exists a splendid example of the storage of energy. It remains quiescent until heat is applied, when its potential form of energy is at once transformed into its active or kinetic form, impelling a cannon-ball or bullet with tremendous force. Coal, again, is a grand store of energy. All our steam-engines employed in manufactures are set going by the energy that is stored up in coal. This energy has remained stored up in coal for millions of years in the bowels of the earth, it is brought to the surface, and is there used to produce power in the innumerable ways with which you are acquainted. Take food—I had a very good chop just now that supplied me with a store of energy which will enable me, for one hour at least, to talk to you; and, had I not had that chop, I do not think I could have talked to you for more than half an hour, instead of the three quarters more that I hope I may yet be able to get through. I have given instances of energy in its potential form. We will now take one or two instances of it in its kinetic form—that is, when it is in a state of motion. A moving cricket-ball is a good example of kinetic energy. In its passage through the air it possesses kinetic energy, and should the ball catch your head you know pretty well what is the effect. A steamship moving and a running railway-train are kept in motion by the energy imparted to them from coal, and we know what tremendous energy a steamship or a railway-train has, as when a collision occurs there is a terrific smash-up. Our earth is another magnificent store of energy in its rotation every twenty-four hours; a portion of this energy it gives up in the shape of tides. Our tides abstract energy from the earth, which is gradually robbing the earth of its motion, and the result is that our day is getting longer and longer. If any of you live a million years, you will probably find that the day will be a minute longer than it is now!

Such being the two general forms of energy, I want next to give you two or three ideas of the forms in which this mysterious energy appears. I had intended to bring a familiar toy, a humming-top, before you, to show you the energy it contained when spinning: but we have other illustrations of various kinds. Take a piece of a very dangerous material called sodium, a beautiful white metal, of which I have a quantity in a bottle here, put it on a piece of blotting-paper, and then place it on top of the water contained in this glass vase: you will see that the sodium possesses energy in itself in a potential form. The moment it is brought in contact with the water, the oxygen of the water unites with the sodium and produces the effects of heat, light, and sound, the result of the chemical energy of the sodium in its potential form being brought into the active or kinetic form by mere contact with the water. Another instance: Here is the dome of a bell, which I strike with a mallet or hammer, and thereby impart