NO. 8 WATER-VAPOR TRANSPARENCY FOWLE 7 



atmospheric absorber than aqueous vapor may have removed the 

 energy of this wave-length from the solar beam. 



TEMPERATURE AND PRESSURE EFFECTS 



The effect of pressure on the absorption of radiation by gases has 

 been investigated principally by Miss Eva von Bahr.^ In general 

 the absorption of the same quantity of gas is increased by increasing 

 its pressure. The absorptive power exerted by a gas present in an- 

 other gas does not depend merely upon its partial pressure but upon 

 the total pressure of the mixed gases as well. For instance, let the 

 absorption of a given quantity of a gas in a tube be measured ; if the 

 gas be then allowed to expand, in such a manner that the weight of 

 gas in the path remains the same, the absorption would in general 

 decrease. If a non-absorbing gas be then admitted until the total 

 pressure is restored, the original value of absorption will be regained. 

 This absorbing power of a gas in general increases with increasing- 

 pressure somewhat rapidly at first, then more slowly, and the maxi- 

 mum constant power of absorption for a gas in not reached until 

 under pressures ranging from 50 cm. for CSo to 400 cm. for CH^ 

 and COo. 



Unfortunately the variation of the transmission with the pressure 

 has been determined for only one of the absorption bands now under 

 consideration. Miss von Bahr gives for the water-vapor band at 

 2.^ fi the following values for the absorption of a constant amount 

 of vapor under the varying pressures : 



105 mm. 4.6 per cent. I 405 mm. 



235 " 7.2 " { 570 " 



370 " 8.6 " 755 " 



8.5 per cent. 

 10.6 ■' 

 12.0 



The increase in pressure was produced by introducing dry air 

 which exercised practically no absorption at this place in the spec- 

 trum. Miss von Bahr states that the " change of its absorption as 

 dependent upon the total pressure is in general (for the same gas) 

 the same in the diiTerent bands." ^ 



The present research is carried out on vapor at y6 cm. total pres- 

 sure whereas part of the vapor in the atmosphere is at a considerably 



^Annalen der Physik, 29, p. 780, IQ09 ; 33, p. 585, 1910; Verhandlungen der 

 Deutsche Physikalischegesellschaft, 15, p. 673, 1913. 



' Die Anderung der Absorption auf Grund des Gesamtdruckes ist in 

 allgeraeinen die Gleiche in vcrschiedenen Banden. 



