HISTORY OF DYNAMICAL THEORY OF HEAT. 209 



advocated, it is thought, by Boerhaave ' and Lemery, 2 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, imponder- 

 able 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, 3 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. 



Tlie 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 phe- 

 nomenon 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 mo- 



1 " De Igne, Elementa Chemise," i., 116. 



2 " Sur la Matiere du Feu," " Histoire et Memoires de l'Ac. Par.," 1709, pp. 6, 400. 



3 We know, however, that these discoveries did not fail to be correctly interpreted at 

 the time, for Cavendish, in a foot-note to some " Observations on Mr. Hutchinson's Ex 

 periments," etc., "Philosophical Transactions," 1783, p. 312, remarked: 



" I am informed that Dr. Black explains the above-mentioned phenomena in the same 

 manner; only instead of using the expression, 'heat is generated or produced,' he says, 

 ' latent heat is evolved or set free ; ' but as this expression relates to an hypothesis de- 

 pending on the supposition that the heat of bodies is owing to their containing more or 

 less of a substance called the matter of heat, and as I think Sir Isaac Newton's opinion, 

 that heat consists in the internal motion of the particles of bodies, much the most prob- 

 able, I choose to use the expression ' heat is generated.' " 

 vor,. XII. 14 



