298 Professor Tyndall on Radiant Heat, [Jan. 18, 



of nitrous oxide is 250 times that of air ; a fact which perhaps furnishes 

 a stronger presumption than any previously existing, that air is a 

 mixture, and not a compound. Carbonic oxide is about 100 times 

 as powerful as its constituent oxygen ; carbonic acid is 150 times as 

 powerful, while olefiant gas, as already remarked, is 1000 times as 

 powerful as its constituent hydrogen. In the case of the hydro-car- 

 bon vapours, where the atomic groups attain a higher degree of com- 

 plexity, the action is even greater than that of olefiant gas. 



The speaker also referred to the experiments and observations of 

 Niepce, Angstrom, and Foucault ; but more especially to the admira- 

 ble researches of Kirchhoff andBunsen, as regards the influence of the 

 period of oscillation on the rate of absorption. He pointed 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 experi- 

 ments. 



With regard to conduction, the speaker 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 molecyles, instead of being expended on 

 the ether between them, and then comnmnicated in part to the ether 

 external to the mass, is transferred freely from particle to particle ; or 

 in other words, is freely conducted. This a priori conclusion is com- 

 pletely verified by the author's experiments, 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, and for this very 

 reason finds diflSculty 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 pro- 

 nounced a bad conductor. These considerations seem to reduce the 

 phenomena of absorption, radiation, and conduction to the simplest 

 mechanical principles. 



[J. T.] 



