Bevieivs and Notices of BooTxs. 



127 



whicli goes by the name of the tension of vapour, increases in 

 each case in proportion to the advance of its temperature. 



Whenever, therefore, the soil and subsoil are not entirely- 

 destitute of humidity, the atmosphere above must contain a certain 

 amount of water, not as vesicular vapour, but in a perfectly aeri- 

 form condition. 



" Now the effects produced upon living beings by the presence of 

 moisture in the atmosphere are entirely different, according as it 

 exists in one or other of these states. 



" Vesicular vapour, manifesting itself in the form of fog or 

 mist, causes, as every one knows, a sensation of chill, owing to the 

 abstraction of heat from our persons, caused by the moisture which 

 attaches itself to them, and likewise, for the same reason, inter- 

 feres with the healthy functions of the skin, and even of the 

 lungs. 



"But in an aeriform condition the very opposite effect takes 

 place. 



*' Professor Tyndall has lately pointed out that humid air, or 

 air containing much moisture in a transparent or an aeriform con- 

 dition, exerts a remarkable influence both in absorbing and in 

 radiating heat. Owing to the former property, aqueous vapour 

 acts as a kind of blanket upon the ground, and contributes in a 

 very striking manner to the retention of its heat. 



" Hence when the air is perfectly divested of moisture, as in 

 the sandy deserts of Africa, in Siberia, and even in Australia, the 

 cold at night is almost insupportable, owing to the absence of 

 that protection which is afforded by aqueous vapour when present 

 in the atmosphere ; whilst during the day the rapid abstraction 

 of moisture from the surface of plants and animals, caused by the 

 dryness, is equally deleterious to both. 



And as the radiation of heat from a body is always equal to 

 its power of absorbing it, it follows that air containing much mois- 

 ture will, when it rises into the higher regions, sink rapidly in 

 temperature, in consequence of the heat it sends forth into space ; 

 and, indeed, according to Tyndall, the amount radiated from air 

 saturated with moisture is 16,000 times as great as that of air 

 perfectly dry. 



" One cause, therefore, of the profuse rains that occur in the 

 tropics may be the cooling of the heated air, which rises from 

 the earth into the higher regions of the atmosphere, and which, 

 when it arrives there, radiates its heat freely into space, and thus 

 has its capacity for moisture reduced. 



" Professor Tyndal calculates, that 10 per cent, of the heat 

 radiated from the earth in this country is stopped by 10 feet of 

 the air which lies nearest to the ground. 



" It would appear from the recent investigations of M. Duchartre 



