Man has been called a reasoning animal, a laughing animal, etc., according to the momentary whim or humour of the classifier; but he is perhaps still more definitely separated from all other animals when specified as _the_ ‘cooking animal.’ Now, it has always appeared to us as something little short of marvellous that, even for the high purpose of cooking his food, or of inflicting exquisite torture on a vanquished foe, savage man should ever have hit upon the process of procuring fire by friction. Considering his condition, and comparing his opportunities and his success with those of even our greatest modern physicists, we cannot but look upon this as one of the very greatest and most notable discoveries ever made in physics. All the more notable, too, from the fact that a man like Newton, though of course aware of it, absolutely missed its significance even at the very moment when it alone was wanted to fill a serious _lacuna_ in one of his grandest and most important practical generalisations. The missing link was all but supplied by Rumford and Davy at the very end of last century. Rumford’s boiling of water by the heat generated in the boring of a cannon, and Davy’s melting of ice by friction _in vacuo_, were each conclusively demonstrative alike of the non-materiality of heat and of the ultimate fate of work spent in friction, which is thus seen to be converted into heat; or at least these experiments could easily have been made demonstrative by very slight additions to, or modifications of, their author’s methods or reasoning. But the exact and formal enuntiation of the equivalence of heat and work required to fill the _lacuna_ in Newton’s statement was first given by Davy in 1812. 102. Let us here pause for a moment and contemplate the position to which the solution of our problem had even then attained. Visible kinetic energy, such as that of a cannon-ball shot upwards, is transformed as it rises into visible potential energy. As the ball descends its energy is retransformed from the potential into the kinetic variety until, when it is about to strike the earth, it has, or rather would have if there were no atmosphere, as much kinetic energy as it had when it was first shot upwards. When the ball has once struck the earth its kinetic energy of visible motion is changed by impact into that kinetic energy of invisible motion of its particles which is called heat; and, generally speaking, in all cases of friction, percussion, and atmospheric resistance we have a change of visible energy into heat, as for instance when a railway train is stopped by the action of the brake, when a blacksmith strikes the anvil with his hammer, when a cannon-ball moves through and heats the air, or when a meteorite or falling-star is rendered incandescent by the resistance it meets with even in the higher and rarer strata of the atmosphere.