RADIATION. 305 



Tims tlip absoi-jition l)y a tube full of dry air heinir one, that of tlio oilor of 

 patchouli (liHused in it is 30, that of hivendci GO, that of rosemary 74, wliilothat 

 of aniseed amounts to 372. It would be idle to speculate on the quantities of 

 matter concerned in these actions. 



Xll. AQUEOUS VAPOK IN KELATIOX TO TERKESTRIAL TEMPERATURES. 



"Wc arc now fully prepared for a result which, without such preparation, might 

 appear incredible. AVater is, to some extent, a volatile body, and oiu- atmo- 

 sphere, resting as it does upon the surface of the ocean, receives from it a con- 

 tinuous supply of aqueous vapor. It would be an error to confound clouds of 

 fog or any visible mist with the vapor of water; this vapor is a perfectly inipal- 

 pai)le gas, diffused, even on the clearest da^'S, throughout the atmosphere. Com- 

 ])ared with the great body of the air, the aquet)us vapor it contains is of almost 

 inlinitesimal amount, 99A out of every 100 parts of the .atuKv^phere being com- 

 })osed of oxygen and nitrogen. In the al>sence of experiment we should never 

 think of ascri))ing to this scant and varying constituent any important inlliience 

 on terrestrial radiation ; and yet its influence is far more jiotent than tliat of the 

 great liody of the air. To say that on a day of average humidity in England 

 the atmospheric vapor exerts ioO times the action of the air itself, would certainly 

 be an understatement of the I'act. The peculiar (pialities of this vapor, and the 

 circumstance that at ordinary temperatures it is very near its jioint of cojidensa- 

 tion, render the results which it yields in the apparatus already described less 

 than the truth; and I am not prepared to say that the absorption by this sub- 

 stance is not 200 times that of tlie air in which it is dili'used. Comparing a 

 single molecule of aqueous vapor with an atom of either of the main constitu- 

 ents i)f our atmosphere, 1 am not prepared to say how many thousand times the 

 action of the former exceeds that of the latter. 



These large numbers depend in ]iart upon the extreme feebleness of the air; 

 the power of aquecms vapor seems vast, liccause that of the air with which it is 

 compared is infinitesimal. Absolutely considered, however, this substance exer- 

 cises a very potent action. Probably a column of ordinary air 10 feet long 

 would intercept from 10 to 15 ))er cent, of the heat radiated from an obscure 

 source, and I think it certain that tlie larger of these numbers fails to exi)ress the 

 altsorption of the terrestrial rays effecteii within 10 feet of the earth's surface. 

 This is of the utmost consequence to the life of the world. Imagine the super- 

 ficial molecules of the earth trembling with the motion of heat, and inqiarting it 

 to the surrounding ether ; this tuotion would be carried rapidly away and lost 

 forever to our })lanet if the waves of ether had nothing but the air to contend 

 with in their outward course. But the acpieous vajior takes up the motion of the 

 ethereal waves and becomes thereby heated, thus wrapjiing the earth like a warm 

 garment, and protecting its surface from the deadly chill which it would other- 

 wise sustain. Various philosophers have speculated on the influence of an 

 atmosi)heric envelope. De Saussure, Fourier, M. Pouillet, and Mr. IIoj)kins 

 liave, one and all, enriched scientific literature with contributions on this subject, 

 but the considerations which these emipent men have ajiplied to atmospheric air 

 have now to be transferred to acpieous va])or. 



The (jbservations of meteorologists furnish important, though hitherto un(()n- 

 Rcious, evidence of the influence of this agent. AVherever the air is dry we are 

 liable to daily extrenu-s of ten)perature. J'y day, in such places, the sun's heat 

 reaches the earth unimpeded and renders the maximum high ; by night, on the 

 other hand, the earth's heat esca})es unhindered into space and renders the nn'ni- 

 mum low. Hence the difference betweoi the maxinnim an<l mininnnn is greatest 

 where the air is driest. In the plainsof India, on the heightsof the Himalaya, 

 in central Asia, in Australia, wherever drought reigns we have the heat oJ" (hiy 

 forcibly contrasted with the chill of night. In the .Sahara itself, when the sun's 

 20 s 



