﻿302 M. H. Wild on the Absorption of Light by the Air. 



and since the densities are as the pressures, we shall have 



_P 



n ~ P 



if P represent the original pressure, and p the pressure of the 



rarefied air ; thus we have also 



E 

 a p =a P ?, 



where a p and a P represent the coefficients of transparency of the 

 air for the respective pressures p and P. 

 If we put the ratio determined by our previous experiments, of 



-2- =#, we have also __L- . 



flp a P = x P ~P 



Introducing x, P, and p, the above values, we get, referred to a 

 unit of path of 1 metre, 



fl 720 = 0-98935 August 29. 



« 715 =0-99521 „ 31. 



The latter, which from the reasons stated is the more reliable 

 result, almost agrees within the limits of error with that previously 

 ascertained. Without prematurely attributing too great weight 

 to this agreement, we may draw the conclusion that by means of 

 my photometer it is possible to determine the absorption of air in 

 tubes of no great length, those, for instance, which can be set up 

 in an ordinary room. 



From the previous observations it follows with certainty that 

 air in the lower layers is far less transparent than is ordinarily 

 assumed. In the case of air only half saturated with aqueous 

 vapour and heated to about 24° C, the absorption near the 

 ground is so considerable, that after traversing 300 metres the 

 luminous intensity has been diminished to about one-third. 

 However paradoxical and large this absorption of air may seem, 

 it is infinitely small compared with that of water. From the 

 above numbers we find that, after traversing the same distance in 

 water as pure as possible, the luminous intensity would only 

 amount to one quintillionth (that is, absolute darkness would 

 set in), and that, to diminish the intensity to only one-third, its 

 path in water need not be more than 5 metres. Prom my ob- 

 servations I can also deduce with certainty a greater degree of 

 transparency for red than for blue light, though I cannot yet 

 consider the quantitative difference definitely settled. This sup- 

 ports the ordinary explanation of morning and evening red ; yet 

 only further detailed observations can show whether Clausius's 

 theory of morning and evening red, as well as of the blue colour 

 of the sky, which has been accepted by most physicists, is cor- 

 rect or not. 



