the Weights of Atoms, 285 



have 6 - 830000 *=-8 ; [which gives 1/^=3720000 centimetres = 

 37*2 kilometres. 



§ 58. Suppose for a moment the want of perfect trans- 

 parency thus defined to be loholly due to the fact that the 

 ultimate molecules of air are not infinitely small and infinitely 

 numerous, so that the " suspended particles " hitherto spoken 

 of would be merely the molecules N 2 , 2 ; and suppose further 

 (D f —D)T to be the same for nitrogen and oxygen. The 

 known refractivity of air (/x— 1 = '0003), nearly enough the 

 same for all visible light, gives by equation (7) above, with n 

 instead of 2, 



^'-^ = ■0006. 



Using this in (1) we find 



,._ 29-76 



^-^xuo 6 (8) ' 



for what the rate of loss on direct sunlight would be, per centi- 

 metre of air traversed, if the light were all of one wave-length, 

 X. But we have no such simplicity in Bouguer's datum 

 regarding transparency for the actual mixture which consti- 

 tutes sunlight : because the formula makes h~ l proportional 

 to the fourth power of the wave-length ; and every cloudless 

 sunset and moonset and sunrise and moonrise over the sea, 

 and every cloudless view of sun or moon below the horizon of 

 the eye on a high mountain, proves the transparency to be in 

 reality much greater for red light than for the average un- 

 dimmed light of either luminary, though probably not so much 

 greater as to be proportional to the fourth power of the wave- 

 length. We may, however, feel fairly sure that Bouguer's 

 estimate of the loss of light in passing vertically through the 

 whole atmosphere is approximately true for the most luminous 

 part of the spectrum corresponding to about the D line, wave- 

 length 5*89 . 10~ 5 cm.j or (a convenient round number) 6 . 10~ 5 

 as Rayleigh has taken it. With this value for A, and 3*72 . 10 6 

 centimetres for A -1 , (8) gives ?? = 8 , 54.10 18 for atmospheric 

 air at 10° and at standard pressure. Now it is quite certain 

 that a very large part of the loss of light estimated by Bouguer 

 is due to suspended particles ; and therefore it is certain that 

 the number of molecules in a cubic centimetre of gas at 

 standard temperature and pressure is considerably greater 

 than 8-54 . 10 l \ 



