﻿Opalescence of Gases in the Critical State. 1G9 



[H denoting the gas constant for 1 gr. molecule, N the 

 number of molecules in 1 gr. molecule]. 



Now the intensity of the opalescent light s, which is 

 scattered into unit conical angle in a perpendicular direction 

 to the incident light (its intensity being put equal to unity), 

 is connected with this magnitude by the relation 



•=!&*• (C) 



and this is the quantity which Kamerlingh-Onnes and Keesom 

 have measured experimentally for ethylene, at temperatures 

 ranging from o, 06 to 2°'41 above its critical point. A 

 detailed comparison of their results and those got for ether 

 by F. B, Young is not possible, as the latter ones are pub- 

 lished only in a rough graphical way, but the general outline 

 of the curve defining the opalescence as function of the 

 temperature is quite analogous (except the extrapolated part 

 for the immediate neighbourhood of 6 C ). 



The authors compare it with the above theory by intro- 

 ducing the value of r~ given by Yerschaffelt * for the 

 neighbourhood of the critical point 



dv v 



and find an approximate agreement, both considering the 

 relative values of s as depending on (6 — @ c ), and also its 

 absolute value. 



As a repetition and extension of such measurements is 

 highly desirable (with special precautions respecting purity 

 of the substance, which has been demonstrated by F. B. 

 Young to be a very important factor), it may be worth while 

 to consider more in detail the results we should expect for a 

 substance following, for instance, Yan der Waals' reduced 

 equation 



In the neighbourhood of the critical point 7r = cp — S = l it 

 assumes the approximate form 



A7r = 4A^[l-|A<£ + 2 A0 2 ]-O A</> 3 , 

 where A7r=7r — 1~ — -— &c, and the development (2) then 



* Verschafielt, Coram, from Phys, Lab. Leiden, Supplm. Xo. 6, p. 3 



(1003). 



