204 Prof. Tyndall on Radiation 



tive powers exercised by the transparent and impalpable aqueous 

 vapour diffused in the air. 



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 reflexion 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 which 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 absorp- 

 tion 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 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 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 meteoro- 

 logy. 



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 unre- 



