46 WRITINGS OF JOSEPH HENRY. [1855- 



The explanation of the result thus produced is not difficult 

 when we understand that a body heated to different degrees 

 of intensity, gives off rays of different quality. Thus if an 

 iron ball be suspended in free space and heated to the tem- 

 perature of boiling water it emits rays of dark heat, of little 

 penetrating power, which are entirely intercepted by glass. 

 As the body is heated to a higher degree, the penetrating 

 power of the rays increases; and finally when the tempera- 

 ture of the ball reaches that of a glowing or white heat, it 

 emits rays which readily penetrate glass and other trans- 

 parent substances. The heat which comes from the sun 

 consists principally of rays of high intensity and great pene- 

 trating power. They readily pass through glass, are absorbed 

 by the blackened surface of the cork, and as this substance 

 is a bad conductor of heat, its temperature is soon elevated, 

 and it in turn radiates heat ; but the rays which it gives off 

 are of a different character from those which it receives. 

 They are non-luminous, and have little penetrating power; 

 they cannot pass through the glass, are retained within the 

 box, and thus give rise to the accumulation of heat. The 

 limit of the increase of temperature will be obtained when 

 the radiation from the cork is of such an intensity that it 

 can pass through the glass, and the cooling from this source 

 becomes just equal to the heating from the sun. The atmos- 

 phere surrounding the earth produces a similar effect. It 

 transmits the rays of the sun which heat the earth beneath; 

 but this in turn emits rays which do not readily penetrate 

 the air, thus effecting an accumulation of heat at the surface. 

 The resistance of the transmission of heat of low intensity 

 depends upon the quantity of vapor contained in the atmos- 

 phere, and perhaps also on the density of the air. The 

 radiation of the earth therefore difiers very much on different 

 nights and in different localities. In very dry places, as for 

 example in the African deserts and our own western plains, 

 the heat of the day is excessive, and the night commen- 

 surably cool. Colonel Emory states in his Report of the 

 Mexican Boundary Survey that in some cases on the arid 

 plains there was a difference of 60° between the temperature 

 of the day and that of the night. Indeed the air in this re- 



