140 ANNUAL OF SCIENTIFIC DISCOVERY. 



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 

 undricd air be sent into the tube that is, one-sixth of the amount 

 which it can contain. These five inches produce their proportionate 

 absorption. The driest day on the driest portion of the earth's sur- 

 face would make no approach to the dryness of our cylinder when it 

 contains only five inches of air. Make it ten, fifteen, twenty, twenty- 

 five, thirty inches : you obtain an absorption exactly proportional to 

 the quantity of vapor 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 

 rock-salt 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 vapor 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 radia- 

 tion 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 vapors must exert on 

 the phenomena of meteorology. 



* This aqueous vapor is a blanket more necessary to vegetable life 

 than clothing is to man. Remove ipr a single summer-night the 

 aqueous vapor 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 a land 

 held fast in the iron grip of frost. The aqueous vapor 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 vapors of the equatorial ocean ; 

 they rise, but for a time a vapor 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 penetrated the 

 vapor 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 vapor is 1G,000 times that of air. Now the power to 

 absorb and the power to radiate are perfectly reciprocal and propor- 

 tional. The atom of aqueous vapor 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 torrents are the consequence. The expansion of the air, 

 no doubt, also refrigerates it : but in accounting for those deluges, the 

 chilling of the vapor by its own radiation must play a most important 

 part. The rain quits the ocean as vapor ; it returns to it as water. 

 How are the vast stores of heat set free by the change from the vapor- 

 ous to the liquid condition disposed of? Doubtless, in great part they 

 are wasted by radiation into space. Similar remarks apply to the 



