RADIATION THROUGH THE EARTH’S ATMOSPHERE. 
£8 
The cylinder which contained the air through which the calorific rays passed was 
polished within, and the rays which struck the interior surface were reflected from it 
to the thermo-electric pile which measured the radiation. The following objection 
was raised :—You permit moist air to enter your cylinder ; a portion of this moisture 
is condensed as a liquid film upon the interior surface of your tube; its reflective 
power is thereby diminished ; less heat therefore reaches the pile, and you incorrectly 
ascribe to the absorption of aqueous vapour an effect which is really due to diminished 
reflection of the interior surface of your cylinder. 
But why should the aqueous vapour so condense ? The tube within is warmer 
than the air without, and against its inner surface the rays of heat are impinging. 
There can be no tendency to condensation under such circumstances. Further, let 
five inches of undried air be sent into the tube,—that is, one-sixth of the amount 
winch it can contain. These five inches produce their proportionate absorption. The 
driest day, on the driest portion of the earth’s surface, would make no approach to 
the dryness of our cylinder when it contains only five inches of air. Make it 10, 15, 
20, 25, 30 inches : you obtain an absorption exactly proportional to the quantity of 
vapour present. It is next to a physical impossibility that this could be the case if 
the effect were due to condensation. But lest a doubt should linger in the mind, not 
only were the plates of rocksalt abolished, but the cylinder itself was dispensed with. 
Humid air was displaced by dry, and dry air by humid in the free atmosphere; the 
absorption of the aqueous vapour was here manifest, as in all the other cases. 
No doubt, therefore, can exist of the extraordinary opacity of this substance to the 
rays of obscure heat; and particularly such rays as are emitted by the earth after it 
has been warmed by the sun. It is perfectly certain that more than ten 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 vapours 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 man. 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 at 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 wdien, by their levity, they have 
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 atom 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, condenses, and the tropical tor¬ 
rents 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 vast 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 radia¬ 
tion 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 vapour. 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 
