RADIATION. 197 



Australia — wherever drought reigns, Ave have the heat of 

 day forcibly contrasted with the chill of night. In the Sa- 

 hara itself, when the sun's 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 sur- 

 face of the earth, and as only the superficial portions ra- 

 diate, the act of condensation makes water the radiating 

 body. Now experiment proves that to the rays emitted by 

 water, aqueous vapor is especially opaque. Hence the very 

 act of condensation, consequent on terrestrial cooling, be* 

 comes 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 self-same covering which pro- 

 tects the earth from chill must also shut out the solar ra- 

 diation. 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 thick- 

 ness, the sun's rays are transmitted with comparative free- 

 dom ; but through a layer half this thickness, as Melloni has 

 proved, no single ray from the wanned earth could pass. 

 In like manner, the sun's rays pass with comparative free- 

 dom through the aqueous vapor of the air ; the absorbing 

 power of this substance being mainly 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. 



