on the Hypothesis of Undulations, 473 



nation of the development of heat caused by sudden compression 

 of the air was, the simple fact that the compression brings sud- 

 denly fresh atoms into the space through which it extends, and 

 that these instantly become additional centres of secondary heat- 

 undulations, as explained in my Mathematical Theory of Heat 

 contained in the Philosophical Magazine for March 1859. The 

 primary waves from which the secondary are produced, are the 

 heat-bearing wave3 traversing the space under the ordinary cir- 

 cumstances of steady temperature. The new waves destroy for a 

 time the calorific equilibrium, and by propagation from their 

 cent/es are expended in raising the temperature of all neighbour- 

 ing bodies more or less distant. This is the kind of radiation 

 I had in view, which, as it seems to me, corresponds exactly to 

 that which on experimental grounds Prof. Tyndail calls " Dyna- 

 mic Radiation." The surprise felt by the experimenter on wit- 

 nessing this effect, sufficiently indicates that he had discovered 

 a class of facts not previously known. It is clear that the sud- 

 den abstraction of a portion of air would suddenly diminish the 

 number of centres of secondary waves contributing to the main- 

 tenance of the calorific equilibrium, and thus would have the 

 opposite effect of absorbing heat and causing a depression of the 

 temperature of surrounding substances. Since the experiments 

 show that for the same change of density dynamic absorption is 

 equal to dynamic radiation, little or no calorific effect can be 

 attributed to the motion through the sether of the introduced or 

 abstracted atoms, as this motion would in both cases tend to 

 raise the temperature. The experiments were so conducted as 

 to determine with precision the different degrees in which the 

 air and different gases contained in the space into which the air 

 flowed were heated by the developed caloric ; and it is very much 

 to the present purpose to remark that, of all the gases, not ex- 

 cepting oxygen and hydrogen, the air itself was least heated. 

 Olefiant gas was heated nine times as much. Like results were 

 obtained with respect to the chilling effects. These facts di- 

 stinctly indicate the small susceptibility of air to acquire, or part 

 with, temperature. I readily concede to Prof. Tyndail that his 

 experiments fully establish this law. But in asserting that the 

 experiments decide against my views, he surely could not be 

 aware that the difficulty I have always felt, and expressed as 

 clearly as I could, in regard to Laplace's theory is, that it de- 

 mands an extremely great susceptibility of the air to acquire tem- 

 perature in the condensed parts of a wave, and to part with tem- 

 perature in the rarefied parts. Now, even when the free caloric 

 developed by compression is prevented from escaping by confining 

 the compressed air in a closed space, the acquired temperature 

 falls short of the amount which that theory requires, and is, more- 

 Phil. Mag, S. 4. Vol. 24. No. 163. Dec, 18G2. 2 I 



