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XLVIT. Remarks on Radiation and Absorption. 

 To Sir John F. W. Herschel, Bart. fyc. fyc. §c. 

 Dear Sir John, 



I AM anxious to address this note to you upon a subject 

 which you have in great part made your own, because I fear 

 that neicher in my book upon the Alps, nor in my recently 

 published papers, have I made due reference to your estimable 

 researches on Solar Radiation. I have been for some time expe- 

 rimenting on the permeability of our atmosphere to radiant heat, 

 and have arrived at the conclusion that true air, that is to say, 

 the mixture of oxygen and nitrogen which forms the body of our 

 atmosphere, is, as regards the transmission of radiant heat, a 

 practical vacuum. The results from which the opacity of air 

 has been inferred are all to be ascribed to the extraneous matters 

 diffused in the atmosphere, and mainly to the aqueous vapour. 

 The negative results recently obtained by that eminent experi- 

 menter, Professor Magnus of Berlin, have induced me to rein- 

 vestigate this point; and the experiments which I have made 

 not only establish the action of aqueous vapour, but prove this 

 action to be comparatively enormous. Here is a typical case : — 

 On the 10th of this month I found the absorptive action of the 

 common air of our laboratory to be made up of three components, 

 the first of which, due to the pure air, was represented in mag- 

 nitude by the number 1 ; the second, due to the transparent 

 aqueous vapour, was represented by the number 40 ; while the 

 third, due to the effluvia of the locality and the carbonic acid of 

 the air, was represented by the number 27. The total action of 

 its foreign constituents on the day in question was certainly 

 sixty-seven times that of the atmosphere itself; while the aqueous 

 vapour alone exerted an action at least forty times that of the air. 



I have also to communicate to you some results of lunar 

 radiation which connect themselves with your speculations. On 

 Friday the 18th of this month, I made a series of observations 

 on the moon from the roof of the Royal Institution. From six 

 concurrent experiments, I was compelled to infer that my thermo- 

 electric pile lost more heat when presented to the moon than 

 when turned to any other portion of the heavens of the same 

 altitude. The effect was equivalent to a radiation of cold from 

 our satellite. I was quite unprepared for this result, which, 

 however, you will at once perceive, may be an immediate conse- 

 quence of the moon's heat. On the evening in question a faint 

 halo which surrounded the moon, and which was only visible 

 when sought for, showed that a small quantity of precipitated 

 vapour was afloat in the atmosphere. Such precipitated par- 

 ticles, in virtue of their multitudinous reflexions, constitute a 



Phil. Mag, S. 4. Vol. 22. No. 148. Nov. 1861 . 2 C 



