500 
POPULAR SCIENCE REVIEW. 
penetrated the vapour-screen, which lies close to the earth's 
surface, what must occur ? 
It has been said that, compared atom for atom, the 
absorption of an atom of aqueous vapour is 16,000 times that 
of air. Now, the power to absorb and the power to radiate 
are perfectly reciprocal and proportional. The atoms of 
aqueous vapour will therefore radiate with 16,000 times the 
energy of an atom of air. Imagine, then, this powerful radiant 
in the presence of space, and with no screen above it to check 
its radiation. Into space it pours its heat, chills itself, con- 
denses, and the tropical torrents are the consequence. The 
expansion of the air, no doubt, also refrigerates it ; but, in 
accounting for those deluges, the chilling of the vapour by 
its own radiation must play a most important part. The rain 
quits the ocean as vapour, it returns to it as water. How 
are the fast stores of heat, set free by the change from the 
vaporous to the liquid condition, disposed of ? Doubtless, in 
great part, they are wasted by radiation into space. Similar 
remarks apply to the cumuli of our latitudes. The warmed 
air, charged with vapour, rises in columns, so as to penetrate 
the vapour-screen which hugs the earth ; in the presence of 
space, the head of each pillar wastes its heat by radiation, 
condenses to a cumulus, which constitutes the visible capital 
of an invisible column of saturated air. 
Numberless other meteorological phenomena receive their 
solution by reference to the radiant and absorbent properties 
of aqueous vapours. It is the absence of this screen, and the 
consequent copious waste of heat, that causes mountains to be 
so much chilled when the sun is withdrawn. Its absence in 
Central Asia renders the winter there almost unendurable. 
In Sahara, the dryness of the air is sometimes such, that 
though, during the day, “the soil is fire, and the wind is 
flame/'’ the chill at night is painful to bear. In Australia, 
also, the thermometric range is enormous, on account of the 
absence of this qualifying agent. A clear day and a dry day, 
moreover, are very different things. The atmosphere may 
possess great visual clearness whilst it is charged with aqueous 
vapour, and on such occasions great chilling cannot occur by 
terrestrial radiation. Sir John Leslie and others have been 
perplexed by the varying indications of their instruments on 
days equally bright, but all these anomalies are completely 
accounted for by reference to this newly-discovered property 
of transparent aqueous vapour. Its presence would check the 
earth's loss ; its absence, without sensibly altering the trans- 
parency of the air, would open wide a door for the escape of 
the earth's heat into infinitude. 
