﻿168 Prof. M. S. Smolnchowski on 



the same as Ravleigh's blue of the sky, and will transform 

 into it in a continuous manner with elevation of temperature. 



The first attempt at a kinetic explanation is that given by 

 Krister, with which I became acquainted through F. B. 

 Young's paper. But I think its inadequacy is apparent by 

 comparison with the above theory. For the immediate 

 cause of opalescence are differences of (optical) density, not 

 of temperature; differences of temperature could act only in 

 an indirect way, by influencing density. But deviations in 

 density must exist, quite independently of Maxwell's devia- 

 tions in temperature, since they are a necessary direct outcome 

 of the chance grouping of molecules. 



The kinetic theory given above is the only one yielding 

 any definitive quantitative relations, while nothing of this 

 kind is to be expected from the capillary theory, as it rests 

 on a wholly arbitrary and uncontrollable supposition. They 

 have been controlled ; with a favourable result in general, in 

 a very important experimental research by Prof. Kamerlingh- 

 Onnes and Dr. Keesom*. 



1 had pointed in my paper on Lord Rayleigb/s formula 

 for the index of absorption 1i of a medium containing in 

 unit volume n particles of volume T 



-?-? O' «> 



as defining the absorption by the coarse-grained substance, 

 when connected with the above formulas for the deviations 

 in density, and with the known relation between the index 

 of refraction //, and the density, and I had drawn some 

 quantitative conclusions therefrom, without however writing 

 down the explicit final formula, fir I considered it only as 

 indicating roughly the order of magnitude, since our case 

 presents some peculiarities not fitting at all with the suppo- 

 sitions underlying Rayleigh's calculation. 



Einstein, however, has arrived in a very remarkable way 

 by explicit calculation of the components of the dispersed 

 waves at the same formula which follows from the above 

 reasoning f, namely 



(-!) 



h -JT ST" -Tr—T- • • (0) 



* Comm. from Phys. Lab. Leiden, Xo. 104. p. 15 (1908). 



t Einstein, Ann. d. Phys. xxxiii. p. 1275 (1910), his formula (18), 

 p. 1293. The numerical result for air given by me, Ann. d. Phys. sxv. 

 p. 216 (1908), is vitiated by a slip of calculation, the value ought to be 

 doubled. [Addendum] Keesom has published now a short reprint of 

 the above paper (Ann. d. Phys. xxxv. p. 591, 1911), where the 

 deduction of this formula is given in full. 



