104 



where the atomic groups attain a higher degree of complexity, the 

 action is even greater than that of olefiant gas. 



The author also refers to the experiments and observations of 

 Niepce, Angstrom, Foucault ; hut more especially to the admirable 

 researches of Kirchhoff and Bunsen, as regards the influence of the 

 period of oscillation on the rate of absorption. He points out how 

 the grouping of atoms to systems in a resisting medium must tend 

 to make their periods of oscillation longer, and thus bring them into 

 isochronism with the periods of the obscure radiations made use of 

 in the experiments. 



With regard to conduction, the author would illustrate his views 

 by reference to two substances rock-salt and alum. He was once 

 surprised to observe the great length of time required by a heated 

 mass of rock-salt to cool ; but this was explained by the experiments 

 of Mr. Balfour Stewart, who shows that rock-salt is an exceedingly 

 feeble radiator. The meaning of this is, that the molecules of the 

 salt glide through the ether with small loss of vis viva. But the 

 ease of motion which they are thus proved to enjoy must facilitate 

 their mutual collision. The motion of the molecules, instead of 

 being expended on the ether between them, and then communicated 

 in part to the ether external to the mass, is transferred freely from 

 particle to particle j or, in other words, is freely conducted. This 

 ft priori conclusion is completely verified by the author's experi- 

 ments, which prove rock-salt to be an excellent conductor. It is 

 quite the reverse with alum. Mr. Balfour Stewart's experiments 

 prove it to be an excellent radiator, and the author's experiments 

 show it to be an extremely bad conductor. Thus it imparts with 

 ease its motion to the ether, but finds difficulty in transferring it 

 from particle to particle. Its molecules are, in fact, so constituted, 

 that when one of them approaches its neighbour, a swell is produced 

 in the intervening ether ; this motion is immediately communicated 

 to the ether outside, and is thus lost for the purposes of conduction. 

 The lateral waste prevents the motion from penetrating the alum to 

 any great extent, and hence it is pronounced a bad conductor. 

 These considerations are dwelt upon more fully in the memoir of 

 which this is an abstract ; and they seem to reduce the phenomena 

 of absorption, radiation, and conduction to the simplest mechanical 

 principles. 



