RADIATION. 45 



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 transmitted with comparative 

 freedom; but through a layer half this thickness, as Mel- 

 loni has proved, no single ray from the warmed earth 

 could pass. In like manner, the sun's rays pass with com- 

 parative freedom through the aqueous vapor of the air; 

 the absorbing power of this substance being mainly exerted 

 upon the invisible 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. 



13. Liquids and their Vapors in relation to Radiant 

 Heat. 



The deportment here assigned to atmospheric vapor has 

 been established by direct 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 after- 

 ward rendered artificially humid by pure distilled water. 

 It has also been established in the following 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 determined. 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. Com- 

 mencing with the substance which exerted the least absorp- 



tive power, and proceeding onward to the most energetic, 

 the following order of absorption was observed: 



Liquids. Vapors. 



Bisulphide of carbon. Bisulphide of carbon. 



Chloroform. Chloroform. 



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 



