286 Lord Kelvin on 



§ 59. This conclusion drawn by Rayleigh from his dynami- 

 cal theory of the absorption of light from direct rays through 

 air, giving very decidedly an inferior limit to the number of 

 molecules in a cubic centimetre of gas, is perhaps the most 

 thoroughly well founded of all definite estimates hitherto 

 made regarding sizes or numbers of atoms. We shall see 

 (§§ 73 ... 79 below) that a much larger inferior limit is found 

 on the same principles by careful consideration of the loss of 

 light due to the ultimate molecules of pure air and to suspended 

 matter undoubtedly existing in all parts of our atmosphere, 

 even where absolutely cloudless, that is to say, warmer than 

 the dew-point, and therefore having none of the liquid 

 spherules of water which constitute cloud or mist. 



§ 60. Go now to the opposite extreme from the tentative 

 hypothesis of § 58 and, while assuming, as we know to be 

 true, that the observed refractivity is wholly or almost wholly 

 due to the ultimate molecules of air, suppose the opacity 

 estimated by Bouguer to be wholly due to suspended particles 

 which^ for brevity, we shall call dust (whether dry or moist). 

 These particles may be supposed to be generally of very 

 unequal magnitudes : but, for simplicity, let us take a case 

 in which they are all equal, and their number only l/10000th 

 of the 8*54 . 10 18 , which in § 59 we found to give the true 

 refractivity of air, with Bouguer's degree of opacity for \= 

 6 . 10 -5 . With the same opacity we now find the contribution 

 to refractivity of the particles causing it, to be only l/100th 

 of the known refractivity of air. The number of particles of 

 dust which we now have is 8*54.10 u per cubic centimetre, or 

 1107 per cubic wave-length, which we may suppose to be 

 almost large enough or quite large enough to allow the 

 dynamics of § 56 for refractivity to be approximately true. 

 But it seems to me almost certain that 8'54.10 14 is vastly 

 greater than the greatest number of dust particles per cubic 

 centimetre to which the well-known haziness of the clearest 

 of cloudless air in the low T er regions of our atmosphere is due ; 

 and that the true numbers, at different times and places, may 

 probably be such as those counted by Aitken * at from 42500 

 (Hyeres, 4 p.m. April 5, 1892) to 43 (Kingairloch, Argyll- 

 shire, 1 p.m. to 1.30 p.m. July 26, 1891). 



§ 61. Let us, however, find how small the number of par- 

 ticles per cubic centimetre must be to produce Bouguer's 

 degree of opacity, without the particles themselves being so 



* Trans. K. S. E. 1894, vol. xxxyii. part iii. pp. 675, 672 



