306 RADIATION. 



rays cease to impinge on the burning soil, the temperature runs rapidly down to 

 freezing, because there is no vapor overhead to check the calorific drain. And 

 here another instance might be added to the numbers already known, in which 

 nature tends, as it were, to check her own excess. By nocturnal refrigeration 

 the aqueous vapor of the air is condensed to water on the surface of the earth, 

 and, as only the superficial portions radiate, the act of condensation makes water 

 the radiating body. Now experiment proves tliat to the rays emitted by water, 

 aqueous vapor is especially opaque. Hence the very act of condensation, con- 

 sequent on terrestrial cooling, becomes a safeguard to the earth, imparting to its 

 radiation that particular character wliicli renders it most liable to be prevented 

 from escaping into space. 



It might, however, be urged that, inasmuch as we derive all our heat from the 

 sun, the self-same covering which pi'otects the earth from cliill must also shut out 

 the solar radiation. This is partially true, but only partially; the sun's rays are 

 different in quality from the earth's raj's, and it does not at all follow that the 

 substance which absorbs the one must necessarily absorb the other. Through 

 a laver of water, for example, one-tenth of an inch in thickness, the sun's ra3"S 

 are transmitted with comparative freedom ; but through a layer half this thick- 

 ness, as Melloni has proved, no single ray from the warmed earth could pass. In 

 like manner, the sun's rays pass with comparative freedom through the aqueous 

 vapor of the air, the absorbing power of this substance being nuunly exerted 

 upon the heat that endeavors to escape from the earth. In consequence of this 

 differential action upon solar and terrestrial heat, the mean temperature of our 

 planet is higher than is due to its distance from the sun. 



XIII. — LIQUIDS AND THEIR VAPOES IN RELATION TO RADIANT HEAT. 



The deportment here assigned to atmospheric vapor has l>een established by 

 dir<!Ct experiments on air taken from the streets and parks of London, from the 

 downs of Epsom, from the hills and sea-beach of the Isle of Wight, and also 

 by experiments on air in the first instance dried and afterwards rendered arti- 

 ficially humid by ptn-e distilled water. It has also been established in the fol- 

 lowing way : Ten volatile liquids were taken at random, and the power of these 

 liquids, at a common thickness, to intercept the waves of heat was carefully deter- 

 nrined. The vapors of the liquids were next taken, in quantities proportional to 

 the quantities of liquid, and the power of the vapors to intercept the waves of 

 heat was also determined. Conmiencing with the substance which exerted the 

 least absorptive power, and proceeding upward to the most energetic, the follow- 

 ing order of absorption was observed : 



Liquids. Vapors. 



Bisulphide of carbon. Bisulphide of carbon. 



Chloroform. C hloroform. 



Iodide of methyl. Iodide of methyl. 



Iodide of ethyl. Iodide of ethyl. 



Benzol. Benzol. 



Amylene. Amylene. 



Sulphuric ether. Sulphuric ether. 



Acetic ether. Acetic ether. 



Formic ether. Formic ether. 



Alcohol. Alcohol. 

 Water. 



We here find the order of absorption in both cases to be the same. We have 

 liberated the m(decules from the bonds which trammel them more or less in a 

 liquid condition ; but this change in their state of aggregation does not change 

 tlieir relative powers of absorption. Nothing could nu^rc clearly prove that tlio 



