Page:Popular Science Monthly Volume 12.djvu/221

Rh advocated, it is thought, by Boerhaave and Lémery, it received in 1787 the unrestricted name "caloric" from the French Academy.

According to these hypothetic notions, singularly cramped and superficial, as compared with the more fruitful ideas of Bacon, caloric, or the matter of heat, was thought to be a highly-elastic, imponderable fluid; which, distributed among the constituent molecules of bodies, in quantities varying with the temperature in the same, or the "capacity" in different kinds of substance, occasioned all the known phenomena of heat: the sensation, through an occult property of its own; expansion and repulsion, by the entrance of its own substance among the molecules of the bodies heated; a change of state whenever the effective action of any particular set of molecular forces should thus happen to be overcome; and in radiation passing from one body to another with vast swiftness. Being, moreover, an unchangeable material, a definite created quantity of it was considered to exist at all times in the universe.

The idea of a substance unaffected by the force of gravity did not appear so very improbable in those days, while the then frequent separation of some new or more elementary gas, and the astonishing effects directly traceable to their action, quite naturally suggested an analogous causation in thermal phenomena.

The discovery by Black, of latent heat, seemed also to supply the necessary induction for its quantitative treatment; so that toward the beginning of the present century, and upon chemical considerations merely, the hypothesis of caloric had succeeded in supplanting quite effectually the ideas of Bacon.

The explanations which it gave of the mechanical excitation of heat were not so plausible, however; certain phenomena appearing utterly incongruous with the idea of an unalterable material supply of heat-substance, and its continued production of friction a phenomenon which has been since said to have furnished the key to the whole science of thermo-dynamics—serving eventually to completely overturn it. In explaining such phenomena, therefore, those who still chose the material hypothesis were compelled to overlook some very significant objections; while, still supposing it to be a vibratory