30 THE SCIENTIFIC PAPERS OF 



is regarded as the undulatoiy motion of a supposed eetherial 

 fluid pervading all nature. 



The supporters of the material theory explain the different 

 phenomena of sensible, radiant, and latent heat, by the free or 

 combined state of their supposed fluids ; the " specific heat of 

 bodies by their different degrees of affinity for that fluid." The 

 affinity of materials for heat is supposed to be invariably increased 

 by increase of volume, and the evolution of heat, by friction be- 

 tween solids, is supposed to arise from permanent compression of 

 their particles. 



The latter supposition has been disproved by Sir Humphry 

 Davy, who showed, that heat was evolved by friction between two 

 pieces of ice, which caused them to melt, and could not, therefore, 

 arise from permanent compression of the solid particles. 



Dulong proved, moreover, that the specific heat of gases is the 

 same before and after compression, showing that the heat lost in 

 their expansion is not absorbed into the gas, and cannot be 

 accounted for according to the " material theory." Joule, of Man- 

 chester, produced heat by agitating water in a closed vessel, and 

 also by an electric current, which, in its turn, was produced by 

 power, in turning the handle of a magneto-electric machine. 



The latter experiments are not only proofs against the suppo- 

 sition that heat is material ; but their greater value consists in 

 showing an intimate connection between heat and the mechanical 

 force by which it was produced, and they are the foundation of 

 the " dynamical theory of heat." 



According to this theory, in its general form, heat, mechanical 

 force, electricity, chemical affinity, light, and sound, are but 

 different manifestations of one great and infinite cause 

 " motion." 



Eecent discoveries and experimental researches all accord with 

 this great principle, which seems destined to open a new era of 

 natural sciences.* Dulong and Gay-Lussac have proved, by their 

 experiments on sound, that the greater the specific heat of a gas 

 the more rapid are its atomic vibrations. Elevation of tempera- 

 ture does not alter the rapidity, but increases the length of those 

 vibrations, and in consequence produces " expansion " of the 

 body. 



* Vide Grove "On the Co-relation of Physical Forces," 1842 and 1850. 



