66 FRAGMENTS OF SCIENCE 



Hence the very act of condensation, consequent on ter- 

 restrial cooling, becomes a safeguard to the earth, impart- 

 ing 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 de- 

 rive all our heat from the sun, the self -same 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 ab- 

 sorbs the one must necessarily absorb the other. Through 

 a layer of water, for example, one-tenth of an inch in 

 thickness, the sun T 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 compara- 

 tive 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. 



18. 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 



