Sir Humphrey Davy—referring to the fact that water at a freezing temperature has “more heat in it” (as it was believed) than ice at the same temperature—said: “If I, by friction, liquify ice, a substance will be produced which contains a far greater absolute amount of heat than ice. In this case it cannot reasonably be affirmed that I merely render _sensible_ heat which had been previously _insensible_ in the frozen mass. Liquification will conclusively prove the _generation_ of heat. This reasoning could not be doubted. Sir Humphrey Davy made the experiment. He rubbed together two pieces of ice in the air, and in a vacuum surrounded by a freezing mixture. The ice became liquified, and so the generation of heat by “mechanical means” was proved. Its immateriality was demonstrated, but the Material theory was not even then abandoned by its adherents. So things continued, until in 1842-3, Doctor Julius Meyer, of Heilbronn, and Doctor Joule, of Manchester, separately, and by different means, arrived at the conclusion that a certain definite amount of mechanical work corresponds to a certain definite amount of Heat, and _vice versâ_. Thus was a great support afforded to the Dynamic theory. This fact Doctor Joule communicated to the _Philosophical Magazine_ in 1843, and the conclusions he came to were— 1. “That the quantity of heat produced by the friction of bodies, whether solid or liquid, is always in proportion to the force expended; 2. “That the quantity of heat capable of increasing the temperature of a pound of water (weighed _in vacuo_ and taken at between 55° and 60° Fahr.) by 1° Fahr., requires for its evolution the expenditure of a mechanical force represented by the fall of 772 lbs. through the space of one foot.” [Illustration: Fig. 76.—Melting a piece of tin on a card.] This is the “mechanical equivalent of heat.” The first paper written by Mr. Joule demonstrated that the temperature of water rises when forced through narrow tubes; and to heat it one degree, the force of 770 foot pounds was necessary, which means that the 1 lb. of water falling 770 feet, got hotter by one degree when it reached the earth. He subsequently arrived at the more exact conclusions quoted above. So heat is now known to be a series of vibrations, or vibratory motions, as sound vibrations, which we cannot hear nor see, but the effects of which are known to us as light and heat. In considering heat we must put aside the idea of warmth and cold, for they are only different degrees of heat, not the absence of it.