60 FRAGMENTS OF SCIENCE. 



on the burning soil, the temperature runs rapidly down 

 to freezing, because there is no vapour 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 vapour 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 that to the rays emitted by water, 

 aqueous vapour is especially opaque. Hence the very 

 act of condensation, consequent on terrestrial cooling, 

 becomes a safeguard to the earth, imparting to its 

 radiation that particular character which 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 selfsame covering 

 which protects the earth from chill 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 rays, and it does not at all follow that the 

 substance which absorbs the one must necessarily absorb 

 the other. Through a layer of water, for example, one 

 tenth of an inch in thickness, the sun's rays are trans- 

 mitted with comparative freedom ; but through a layer 

 half this thickness, 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 vapour of the air : the absorbing power of 

 this substance being mainly exerted upon the invisible 

 heat that endeavours 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. 



