CONDENSATION AND CRITICAL PHENOMENA. 217 



does not exist. Perhaps the problem to find one which 

 would embrace the whole field of possible values for p, v, 

 T is insoluble, but the dogma of the continuity as founded 

 by Andrews and enlarged by Van der Waals seems in- 

 dependent of this equation. If we take Andrews' diagram 

 and complete it inside the border-curve in the manner of 

 Thomson and Van der Waals, we have the whole dogma 

 before us. The conditions outside the border-curve are to 

 be realised by experiment, those inside only under special 

 conditions or in the surface layer between the two phases. 

 The reality of the isothermal inside the border-curve is 

 hypothetical, but so acceptable that at present in similar 

 cases of co-existence of phases the existence of a theoretical 

 continuous transition between the phases is invariably 

 admitted. We might almost say that thermodynamics is 

 essentially dependent on hypotheses of that kind. 



In the light of the dogma it is easily seen that a 

 difference between liquid and vapour or gas does not exist. 

 In fact this point was made entirely clear by Andrews him- 

 self. It is, however, not unnecessary to lay some stress on 

 this point, as even now there seems to be a tendency in the 

 minds of physicists to define gas-vapour and liquid in a way 

 that shows that the bearing of the continuity is not suffi- 

 ciently understood. Very generally one finds a substance 

 called a gas (or a true gas) above the critical temperature 

 and vapour (or liquid) below the critical temperature. 

 This will do as a conventional distinction, but there is no 

 real foundation for it. One might just as well call a sub- 

 stance a gas above the critical pressure and a vapour (or 

 liquid) beneath it. The critical constants have a special 

 meaning /;/ ^/le critical point only, and the critical isothermal 

 is no line of demarcation between different conditions or 

 properties of the substance. 



In a second article the condensation and critical pheno- 

 mena of mixtures will be considered, to which we have been 

 led up naturally in discussing the experiments on so-called 

 pure substances. 



J. P. KUENEN. 



