Other physical facts going to make up an elementary science of mechanics, that were demonstratively known to prehistoric man, were such as these: the rigidity of solids and the mobility of liquids; the fact that changes of temperature transform solids to liquids and vice versa--that heat, for example, melts copper and even iron, and that cold congeals water; and the fact that friction, as illustrated in the rubbing together of two sticks, may produce heat enough to cause a fire. The rationale of this last experiment did not receive an explanation until about the beginning of the nineteenth century of our own era. But the experimental fact was so well known to prehistoric man that he employed this method, as various savage tribes employ it to this day, for the altogether practical purpose of making a fire; just as he employed his practical knowledge of the mutability of solids and liquids in smelting ores, in alloying copper with tin to make bronze, and in casting this alloy in molds to make various implements and weapons. Here, then, were the germs of an elementary science of physics. Meanwhile such observations as that of the solution of salt in water may be considered as giving a first lesson in chemistry, but beyond such altogether rudimentary conceptions chemical knowledge could not have gone--unless, indeed, the practical observation of the effects of fire be included; nor can this well be overlooked, since scarcely another single line of practical observation had a more direct influence in promoting the progress of man towards the heights of civilization.