106 On Decrease of direct Solar Heat in the Upper Air. 



is an excellent absorbent with a barometer at 30 inches, may 

 be by no means the best with a barometer at 12 inches. The 

 second theory is equally obvious, and was suggested, if I re- 

 member right, by Mr. I)e la Rue, viz. that the motion we call 

 radiant heat cannot be communicated to solid bodies except 

 through the intervention of a gas ; and that the amount so com- 

 municable depends on the density of the gas. This is a very 

 important conclusion, and needs a fuller statement. It asserts 

 that a form of motion is propagated by the sun, travels through 

 the interplanetary spaces, and strikes on our atmosphere. That 

 there the motion is of such a kind, either as regards its matter or its 

 movement, as not to be communicable to the molecules of a solid 

 body ; but that this motion is communicated to the atmosphere, 

 there of extreme tenuity, and by it might be transmitted to a 

 thermometer. At greater depths in the atmosphere more and 

 more of the original form of motion is being converted into heat, 

 and the directly propagated motion more and more exceeds the 

 diffused motion. 



This suggests various questions of great importance in the 

 theory of heat. For example; Is the original form of motion 

 here spoken of that which we know as light ? Is there a gra- 

 dual conversion of light into heat? A comparison between the 

 lights and heats of the sun and moon will probably indicate a 

 negative answer to this question. But the law of conservation 

 of force requires that there should be some motion so converted 

 into heat. Can we learn to recognize it in any other form ? 



Experimental investigations may solve some of the problems, 

 and clear away some of the uncertainties in this new branch of 

 the subject. Is heat of all qualities capable of propagation, and 

 of being received by absorbent surfaces in vacuo or in a very rare 

 gas ? Do the absorptive powers depend on the pressure ? It 

 would not seem to be difficult to answer these questions by ex- 

 periments. And by observing the effect of different simple and 

 compound gases on radiant heat, a new insight into their rela- 

 tions to the motions of heat and light may be obtained. 



Certainly this observation of Mr. Glaisher's leads to a promi- 

 sing field for inquiry. 



I remain, Gentlemen, 



Your obedient Servant. 



James M. Wilson. 



