ON THE ABSORPTION AND RADIATION OP HEAT. 
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molecules of aqueous vapour. Thinly scattered as these latter 
are, we might naturally think meanly of them as barriers to 
the waves of heat. We might imagine that the wide spaces 
between the vapour molecules would be an open door for the 
passage of the undulations, and that, if those waves were at 
all intercepted, it would be by the substances which form 99J 
per cent, of the whole atmosphere. Three or four years ago, 
however, it was found that this small modicum of aqueous 
vapour intercepted fifteen times the quantity of heat stopped 
by the whole of the air in which it was diffused. It was after- 
wards found that the dry air then experimented with was not 
perfectly pure, and that the purer the air became, the more it 
approached the character of a vacuum, and the greater, by 
comparison, became the action of the aqueous vapour. The 
vapour was found to act with 80, 40, 50, 60, 70 times the 
energy of the air in which it was diffused, and no doubt was 
entertained that the aqueous vapour of the air which filled 
the Royal Institution theatre absorbed 90 or 100 times the 
quantity of radiant heat which was absorbed by the main 
body of the air of the room. 
Looking at the single atoms, for every 200 of oxygen and 
nitrogen there is about 1 of aqueous vapour. This 1, then, is 
80 times more powerful than the 200 ; and hence, comparing 
a single atom of oxygen or nitrogen with a single molecule of 
aqueous vapour, we may infer that the action of the latter is 
16,000 times that of the former. It is perfectly certain that 
more than 10 per cent, of the terrestrial radiation from the 
soil of England is stopped within ten feet of the surface of 
the soil. This one fact is sufficient to show the immense 
influence which this newly- discovered property of aqueous 
vapour must exert on the phenomena of meteorology. 
This aqueous vapour is a blanket more necessary to the 
vegetable life of England than clothing is to men. Remove 
for a single summer night the aqueous vapour from the air 
which overspreads this country, and you would assuredly 
destroy every plant capable of being destroyed by a freezing 
temperature. The warmth of our fields and gardens would 
pour itself unrequited into space, and the sun would rise upon 
an island held fast in the iron grip of frost. The aqueous 
vapour constitutes a local dam, by which the temperature of 
the earth's surface is deepened; the dam, however, finally 
overflows, and we give to space all that we receive from 
the sun. 
The sun raises the vapours of the equatorial ocean; they 
rise, but for a time a vapour-screen spreads above and around 
them. But the higher they rise the more they come into the 
presence of pure space, and when, by their levity, they have 
