812 Mr. F. B. Young on the 



& 



coil, to cause the meniscus to disappear when the temperature 

 of the vapour jacket was as much as 8° below C , so that the 

 low T er portion of the ether was still presumably in the liquid 

 state. Owing to the great difference in density the transition 

 zone was then extremely well marked. Its depth was so slight 

 that it had almost the appearance of a convex meniscus. 

 Closer examination, however, led to the conclusion that the 

 appearance of convexity was due to the refractive effect upon 

 horizontal lines placed behind (fig. 5), the effect being pro- 

 duced by the presence in a cylindrical tube of a medium 

 gradually increasing in density downwards. The apparent 

 slight convexity of the meniscus mentioned in Section C (4) 

 may have been due to this cause. The matter is here men- 

 tioned because a convexity of the meniscus in the neighbour- 

 hood of the critical temperature has previously been 

 recorded (10) ; it is now suggested that in such cases the 

 temperature C had already been slightly exceeded and that 

 the so-called meniscus was in reality a very narrow transition 

 zone. 



If the bell was filled with vapour at a temperature con- 

 siderably below 6 C and the heating current was then switched 

 on, the liquid was observed to rise and fill the bell (the upper 

 meniscus was for this purpose embraced by the coil). If, 

 however, the temperature was not far below 6 C the meniscus, 

 after rising a short distance, became nebulous and faded 

 away as it rose. A similar effect is mentioned by Amagat (11), 

 who found it occur when carbon dioxide slightly below its 

 critical temperature was subjected to slow compression. It 

 was probably due to the evolution of heat by the vapour as it 

 condensed under compression. By diminishing the heating- 

 current and consequently the rapidity with which the 

 meniscus rose, it was possible to make the meniscus rise 

 higher before disappearing, the latent heat of vaporization 

 having more time in which to dissipate. 



F. Opalescent Phenomena. 



(1) Occurrence. — The opalescent effects described in 

 Section E afford some information concerning the conditions 

 which favour the production of opalescence. On reducing 

 the temperature to 6 C after the production of the artificial 

 transition zone, a dense opalescence appeared in that zone 

 only, no visible phenomena occurring above or below as the 

 temperature fell. Although the temperature of the ether 

 varied through the tube, each layer of ether passed through 

 C in turn ; since marked opalescence was exhibited only by 

 that part of the ether in which the meniscus appeared, and 



