﻿172 On Opalescence of Gases in the Critical State. 



introduce the empirical coefficients given by Verschaffelfc 

 (loc. cit.) instead of Van der Waals' equation, only the 

 numerical values have to be changed. The equation of the 

 border curve is then given by 



^^lzpB-STv/AT+lO-OAr. 



For points in the one-phase space we get, instead of (10), 



!£ = -<&[iOA3 + 18-4At"]; . . . (U) 



OV Vc |_ J 



for the two-phase state, instead of (11), 



!^ = -18-4£ c [~Aa + Arl ; .... (13) 

 for the point of separation into two phases 



|^ = _^ri8-4AT + 28 9AT 32 l . . (U) 



(3 V Vc L J 



where the upper and lower sign relate to a specific volume 

 smaller or larger respectively than the critical volume. 



The quantitative measurements of opalescence, alluded to 

 above, have been made hitherto only for the one-phase state, 

 and they were compared with theory under the assumption 

 that the specific volume had exactly the critical value 

 (A0 = At = O). As this seems a somewhat doubtful point, 

 which according to the above formulas maybe of considerable 

 importance, measurements on several tubes, with differ em 

 values for the specific volume, are very desirable. 



To make analogous observations in the two-phase state 

 will be much more difficult, as the establishment of thermic 

 equilibrium is a much slower process there, owing to the 

 thermal effects of local evaporation or condensation. F. B. 

 Young's paper demonstrates in a very instructive way the 

 persistency of minute thermal inequalities and the enormous 

 importance of this factor. Ko doubt also the fact observed 

 by Travers and Usher and F. B. Young, that for slowly 

 rising temperature the opalescence of the "receding" phase is 

 greater, is to be explained in this way. Mr. Young rightly 

 observes (p. 813) that the density of the receding phase must 

 approach the critical density when the meniscus is dis- 

 appearing, which can be explained by those thermal effects ; 

 this means that its temperature must be higher than that 

 of the increasing phase, and therefore more favourable for 

 opalescence. 



Approximate equality of opalescence in both phases ought 



