A lead bullet may be made hot enough to burn the hand, by striking it with a hammer, or by rubbing it against a board; a clever blacksmith can make a nail red-hot by hammering it; Count Rumford boiled water by the heat developed in the boring of cannon, and inferred from the experiment that heat was not what it was generally supposed to be, an imponderable fluid, but a kind of motion generated by the friction. Now Mr. Joule's experiments enable us to state the exact amount of heat which a definite expenditure of mechanical force can originate. I say _originate_, not drag from any hiding-place in which it had concealed itself, but actually bring into existence, so that the total amount of heat in the universe is thereby augmented. If a mass of iron fall from a tower 770 feet in height, we can state the precise amount of heat developed by its collision with the earth. Supposing all the heat thus generated to be concentrated in the iron itself, its temperature would thereby be raised nearly 10 deg. Fahr. Gravity in this case has expended a certain amount of force in pulling the iron to the earth, and this force is the _mechanical equivalent_ of the heat generated. Furthermore, if we had a machine so perfect as to enable us to apply all the heat thus produced to the raising of a weight, we should be able, by it, to lift the mass of iron to the precise point from which it fell. But the heat cannot lift the weight and still continue heat; this is the peculiarity of the modern view of the matter. The heat is consumed, used up, it is no longer heat; but instead of it we have a certain amount of gravitating force stored up, which is ready to act again, and to regenerate the heat when the weight is let loose. In fact, when the falling weight is stopped by the earth, the motion of its mass is converted into a motion of its molecules; when the weight is lifted by heat, molecular motion is converted into ordinary mechanical motion, but for every portion of either of them brought into existence an equivalent portion of the other must be consumed. What is true for masses is also true for atoms. As the earth and the piece of iron mutually attract each other, and produce heat by their collision, so the carbon of a burning candle and the oxygen of the surrounding air mutually attract each other; they rush together, and on collision the arrested motion becomes heat. In the former case we have the conversion of gravity into heat, in the latter the conversion of chemical affinity into heat; but in each case the process consists in the generation of motion by attraction, and the subsequent change of that motion into motion of another kind. Mechanically considered, the attraction of the atoms and its results is precisely the same as the attraction of the earth and weight and _its_ results. [Sidenote: HEAT PRODUCED IF THE EARTH STRUCK THE SUN.]