ENERGY IN GENERAL. 6l 



mechanics which represents exactly all the pheno- 

 mena of light, and furnishes us with a perfect objective 

 image of it Heat, in its turn, has been reduced to a 

 mode of motion, and thermodynamics claims to em- 

 brace all its manifestations. As early as 1812, Sir 

 Humphry Davy wrote as follows: "The immediate 

 cause of heat is motion, and the laws of transmission 

 are precisely the same as those of the transmission 

 of motion." From that time forth, this conception 

 developed into what is really a science. The constitu- 

 tion of gases has been conceived by means of two 

 elements particles, and the motions of these particles, 

 determined in the strictest detail And finally, in spite 

 of the difficulties of the representation of electrical and 

 magnetic phenomena after Ampere and before Maxwell 

 and Hertz, physicists have been able to announce in the 

 second half of the nineteenth century the unity of the 

 physical forces realized in and by mechanics. From 

 that time forth, all phenomena have been conceived 

 as motion or modes of motion, only differing essen- 

 tially one from the other in so far as motions may 

 differ that is to say, in the masses of the moving 

 particles, their velocities, and their trajectories. The 

 external world has appeared essentially homogeneous ; 

 it has fallen a prize to mechanics. Above all, there 

 is heterogeneity in ourselves. It is in the brain, 

 which responds to the nervous influx engendered by 

 the longitudinal vibration of the air, by the specific 

 sensation of sound, which responds to the transverse 

 vibration of the ether by a luminous sensation, and in 

 general to each form of motion by an irreducible 

 specific sensation. 



Forty years have passed since the mechanical ex- 

 planation of the universe reached its definite and 



