visible in the Solar Spectrum. 7 



respecting the nature of heat ; and at the same time the success 

 of such treatment must depend essentially on the truth of the 

 hypotheses assumed, whence it is evident that, in assuming the 

 grounds for such a theory, it is necessary, as much as possible, to 

 interrogate Nature by experiments. A first clue is offered by 

 the natural connexion which exists between radiant and thermo- 

 metric heat. Radiant heat must necessarily consist of trans- 

 verse aethereal vibrations, while, again, those vibrations are 

 dependent on the influence which the molecules of the body 

 itself have upon the particles of the aether. In the mean 

 time experiment shows that the influence of the medium 

 upon the motion of the aether consists in (1) a constant 

 alteration of the wave's length and velocity, (2) a diminution 

 of the amplitudes dependent on the distance. This diminution 

 is what is called absorption. It cannot, however, take place 

 unless the particles of the body be set in motion, and the heat 

 accordingly become thermometric. In other words, experience 

 shows that radiant heat is absorbed when it passes through any 

 more or less athermanous body, and that the body is thereby 

 heated : if now radiant heat consist in certain undulatory mo- 

 tions of the aether, and these motions are checked by the in- 

 fluence of the particles of the body, then, according to the law 

 of vis viva, these particles must themselves acquire a motion. 

 Now, it is true that this does not absolutely prove that these 

 motions are what constitute thermometric heat ; but when we 

 reflect, on the one hand, that these are the only changes one can 

 imagine to take place in consequence of the aether's motion, and, 

 on the other hand, that experiments seem to show that the aether 

 itself cannot be the medium of thermometric phenomena, it ap- 

 pears that such a supposition is necessary. 



We must, in fact, consider the conducting-power as a function 

 of the conducting medium's elasticity, whether that medium be 

 the body itself or the aether, though in the latter case the direc- 

 tions of greatest or least conductibility must coincide with the 

 axes of optical elasticity. This is, however, not the case with 

 monoclinoedrical crystals, in which the different systems of 

 axes of elasticity have another position. In gypsum and fel- 

 spar, it is true that the direction of the greatest conducting- 

 power and that of the greatest elasticity as regards acoustic pro- 

 perties coincide, whereas the bisecting lines of the optic axes 

 make, in the case of gypsum, an angle of 24°, in that of felspar 

 50°, with the said direction. Add to this that thermometric heat 

 always has an influence on the relative position of the particles, 

 by changing the volume and form of aggregation of the mass, and 

 that cause and effect always stand in a mutual relation to each 

 other, whereas radiant heat does not stand in any known con- 



