252 Prof. S. Arrhenius on the Influence of Carbonic Acid 

 Morning. Noon. Evening, 



D. W. I. I r D. W. T. I x . D. W. I. I r 



Lrae /29-3 0-61 1-424 1-5541 f23'6 0-46 1-692 1715 I \ 266 051 1-417 1-351 1 

 Pine. \21-1 0-84 1458 1-583 J" \ 269 0-59 1-699 1*721 / j 232 0"74 1-428 1-359/ 



Mountain f 23-5 0'088 1-790 1 /22'5 0-182 1-904 1-8731 J 24-5 0205 1701 1-641] 



Camp. [23-5 0-153 1-749 f \24-5 0215 1-890 1-917/ 1 22-5 0-32 1*601 1527/ 



At a very low humidity (Mountain Camp) it is evident that 

 the absorbing power of the aqueous vapour has an influence, 

 for the figures for greater humidity are (with an insignificant 

 exception) inferior to those for less humidity. But for the 

 observations from Lone Pine the contrary seems to be true. 

 It is not permissible to assume that the radiation can be 

 strengthened by its passage through aqueous vapour, but the 

 observed effect must be caused by some secondary circum- 

 stance. Probably the air is in general more pure if there 

 is more water-vapour in it than if there is less. The 

 selective diffusion diminishes in consequence of this greater 

 purity, and this secondary effect more than counterbalances 

 the insignificant absorption that the radiation suffers from the 

 increase of the water-vapour. It is noteworthy that Elster 

 and Geitel have proved that invisible actinic rays of very 

 high refrangibility traverse the air much more easily if it is 

 humid than if it is dry. Lang-ley's figures demonstrate mean- 

 while that the influence of aqueous vapour on the radiation 

 from the sun is insensible as soon as it has exceeded a value 

 of about 0*4. 



Probably the same reasoning will hold good for car- 

 bonic acid, for the absorption spectrum of both gases is of the 

 same general character. Moreover, the absorption by car- 

 bonic acid occurs at considerably greater wave-lengths, and 

 consequently for much less important parts of the sun's 

 spectrum than the absorption by water-vapour*. It is, 

 therefore, justifiable to assume that the radiation from the 

 sun suffers no appreciable diminution if K and W increase 

 from a rather insignificant value (K = l, W = 0'4) to higher 

 ones. 



Before we proceed further we need to examine another 

 question. Let the carbonic acid in the air be, for instance, 

 the same as now (K = l for vertical rays), and the quantity 

 of water-vapour be 10 grammes per cubic metre (W = l for 



* Cf. above, pages 246-248, and Lang-ley's curve for the solar spec- 

 trum, Ann. d. Ch. et d. Phys. ser. 6, t. xvii. pp. 323 and 326 (1889) ; 

 ' Prof. Papers/ No. 15, plate 12. 



