CONDENSATION AND CRITICAL PHENOMENA. 203 



that could be tested, and more or less accurate determina- 

 tions have been made of their critical constants, especially 

 of the critical temperature. But though the general features 

 of the phenomena described by Andrews have been con- 

 stantly confirmed by subsequent investigations, there is a 

 considerable amount of uncertainty as to the details of the 

 phenomena during condensation and near the critical point. 

 It is maintained by a number of experimenters that neither 

 agree fully with the simple conclusions to be drawn from 

 Andrews' diagram. In the first place the existence of a 

 constant vapour-pressure independent of the quantities of 

 vapour and liquid present is denied. The fact is that even 

 Andrews himself did not find a constant condensation- 

 pressure, but one which increased slowly during compression 

 and somewhat more rapidly near the end of the condensa- 

 tion. At i3'i° C. the pressure at the beginning of the pro- 

 cess of condensation was 48 'o atmospheres, and at the end 

 49*5 ; at 21 '5° C. the corresponding pressures were 58*9 and 

 6 1 '3. But he ascribed this increase of pressure to the 

 influence of a small admixture of permanent gas (1/500). 

 Exactly similar phenomena, essential for mixtures, as we shall 

 see in a second article, have been observed by a number of 

 experimenters, and some of them are bold enough to assert 

 that impurities were out of the question in their case at 

 least, or try to prove that the way in which the pressure in- 

 creased was inconsistent with the idea that this increase 

 should be due to impurities. Even if we knew exactly 

 how a mixture say of ether and an unknown second 

 substance would behave when condensed, it would still 

 be questionable if in the experiments the equilibrium 

 between the two phases, liquid and vapour, was always 

 properly established, for even small admixtures occasion 

 very marked retardation in the establishment of equilibrium. 

 One has only to think of the slowness of diff"usion under 

 any circumstances to understand what it may come to in 

 a long and narrow high pressure tube. Moreover, we 

 cannot help noticing that in a few cases experiments have 

 been made with substances that did not show any increase 

 of vapour-pressure at all. Young and Thomas prepared 



