158 Prof. Tyndall, on the Transmission of Heat. [June 10, 



of heat intercepted by the former is enormously greater than that inter- 

 cepted by the latter. 



To test the influence of quality, the following experiment was 

 devised. A powerful lime light was placed at one end of the tube, 

 and the rays from it, concentrated by a convex lens, were sent through 

 the tube, having previously been caused to pass through a thin layer of 

 pure water. The heat of the luminous beam excited a thermo-electric 

 current in the pile at the end of the exhausted tube ; and this current 

 being neutralised by the current from the second pile, coal-gas was 

 admitted. This powerful gas, however, had no sensible effect upon the 

 heat selected from the lime light ; while the same quantity of heat, 

 from an obscure source*, was strongly affected. 



The bearing of this experiment upon the action of planetary 

 atmospheres is obvious. The solar heat possesses, in a far higher 

 degree than that of the lime light, the power of crossing an atmo- 

 sphere ; but, and when the heat is absorbed by the planet, it is so 

 changed in quality that the rays emanating from the planet cannot get 

 with the same freedom back into space. Thus the atmosphere admits 

 of the entrance of the solar heat, but checks its exit ; and the result is 

 a tendency to accumulate heat at the surface of the planet. 



In the admirable paper of M. Pouillet already referred to, this 

 action is regarded as the cause of the lower atmospheric strata being 

 warmer than the higher ones ; and Mr. Hopkins has shown the possible 

 influence of such atmospheres upon the life of a planet situated at 

 a great ^distance from the sun. We have hitherto confined our 

 attention to solar heat ; but were the sun abolished, and did stellar 

 heat alone remain, it is possible that an atmosphere which permits 

 advance, and cuts off retreat, might eventually cause such an accumu- 

 lation of small savings as to render a planet withdrawn entirely from 

 the influence of the sun a warm dwelling-place. But whatever be the 

 fate of the speculation, the experimental fact abides— that gases absorb 

 radiant heat of different qualities in different degrees ; and the action 

 of the atmosphere is merely a particular case of the inquiry in which 

 the speaker was at present engaged. f 



[J. T.] 



* The quantity of heat is measured by the amount of the galvanometric deflec- 

 tion which it produces ; its power of passing through media may be taken as a 

 test of quality. 



t While correcting the proof of this abstract, I learned that Dr. Franz had 

 arrived at the conclusion that an absorption of 3*54 per cent, of the heat passing 

 through a column of air 90 centimeters long takes place ; for coloured gases he 

 finds the absorption greater ; but all colourless gases he assumes show no marked 

 divergence from the atmosphere.— Poggendorff's Annalen, xciv. p. 337. 



