"Dr. Joule, of Manchester, England, has discovered the ratio between heat and force, that is, the amount of force which by transmutation produces any given amount of heat. The force of a one-pound weight which has fallen one foot is taken as the unit of force, and the amount of heat which is required to raise one pound of water one degree is taken as the unit of heat. By many and various careful experiments, Dr. Joule demonstrated that 772 units of force are the equivalent of one unit of heat. A pound weight falling 772 feet, or 772 pounds falling one foot, and then arrested, produces heat sufficient to raise one pound of water one degree. The result is the same whatever the method by which the force is expended. If water be agitated or shaken, if sticks of wood or iron plates be rubbed together, if an anvil be struck with a hammer, or if a bar of iron or copper be moved back and forth between the poles of an electromagnet, the force expended is changed to heat. You must remember, however, that force becomes heat only so far as the force is actually expended, or used up so that it no longer exists as force. "These conclusions are supported by other beautiful experiments. 'An electric current which, by resistance in passing through an imperfect conductor, produces heat sufficient to raise one pound of water one degree, sets free an amount of hydrogen which, when burned, raises exactly one pound of water one degree. Again, the same amount of electricity will produce an attractive magnetic force by which a weight of 772 pounds may be raised one foot high.'--_Youmans._ We conclude from experiments like these that heat, mechanical force, and electricity are interchangeable forces; they may be transmuted the one into another. "By this principle of the transmutation of heat and mechanical force we explain the production of heat by compression and the loss of heat by expansion. Samuel, you may state the fact upon this point." "If any substance be suddenly compressed," answered Samuel, "heat appears; if it be expanded, cold is produced. Since gases expand or yield to pressure so readily, they furnish the best illustration of this principle." "The suddenness of the compression or expansion," said Mr. Wilton, "is a matter of no consequence. The effect is the same whether the operation be sudden or slow, but if the compression or expansion be slow, the heat or cold generated is less apparent; the heat is dissipated as fast as produced and the colder gas is warmed by the vessel which contains it. Ansel, how shall we explain this?" "I cannot explain it, sir."