the Weights of Atoms. 287 



large in comparison with the wave-length as to exclude the 

 application of Rayleigh's theory. Try for example T= 1 0~ 3 A 3 

 (that is to say, the volume of the molecule l/1000th of the 

 cubic wave-length, or roughly diameter of molecule l/10th of 

 the wave-length) which seems small enough for fairly approxi- 

 mate application of Rayleigh's theory ; and suppose, merely 

 to make an example, D' to be the optical density of water, D 

 being that of ether'; that is to say, D'/D= (1-3337) 2 = 1*78. 

 Thus we have (D'-B) T/I) = -0001\> : and with\=6 . 10- 5 , 

 and with k- 1 =3-72.10°, (1) gives n = l'48.10 G , or about 

 one and a half million particles per cubic centimetre. 

 Though this is larger than the largest number for natural air 

 counted by Aitken, it is interesting as showing that Bouguer's 

 degree of opacity can be accounted for by suspended 

 particles, few enough to give no appreciable contribution to 

 refractivity, and yet not too large for Rayleigh's theory. 

 But when we look through very clear air by day, and 

 see how far from azure or deep blue is the colour of a 

 few hundred metres, or a few kilometres of air with the mouth 

 of a cave or the darkest shade of mountain or forest for back- 

 ground ; and when in fine sunny weather we study the 

 appearance of the grayish haze always, even on the clearest 

 days, notably visible over the scenery among mountains or 

 hills ; and when by night at sea we see a lighthouse light at 

 a distance of 45 or 50 kilometres, and perceive how little of 

 redness it shows ; and when we see the setting sun shorn of 

 his brilliance sufficiently to allow us to look direct at his face, 

 and yet only ruddy, rarely what could be called ruby red ; 

 it seems to me that we have strong evidence for believing that 

 the want of perfect clearness of the lower regions of our. 

 atmosphere is in the main due to suspended particles, too 

 large to allow approximate fulfilment of Rayleigh's law of 

 fourth power of wave-length. 



§ 62. But even if they were small enough for Rayleigh's 

 theory the question would remain, Are they small enough and 

 numerous enough to account for ihe refractivity of the atmo- 

 sphere ? To this we shall presently see we must answer 

 undoubtedly " No""; and much less than Bouguer's degree of 

 opacity, probably not as much as a quarter or a fifth of it, is 

 due to the ultimate molecules of air. In a paper by Mr. 

 <^uirino Majorana in the Transactions of the R. Accademia 

 dei Lincei (of which a translation is published in the Philo- 

 sophical Magazine for May 1901), observations by himself in 

 Sicily, at Catania and on Mount Etna, and by Mr. Gaudenzio 

 Sella, on Monte Rosa in Switzerland, determining the ratio 

 of the brightness of the sun's surface to the brio-fitness of the 



