CONDENSATION AND CRITICAL PHENO- 

 MENA. 



I. SINGLE SUBSTANCES. 



THE thirty years that have elapsed since the publication 

 of Andrews' investigation of carbonic acid have made 

 the dogma of " the continuity of the gaseous and liquid 

 states of matter" common property. It seems worth while 

 to review the history of that dogma up to the present 

 time, both from an experimental and a theoretical point of 

 view. The phenomena established by Andrews for car- 

 bonic acid consist in the occurrence of a so-called maximum 

 vapour-pressure increasing with temperature, and of cor- 

 responding maximum vapour-density and minimum liquid- 

 density, which approach each other the higher the tempera- 

 ture, and finally coincide in a point — the critical point. These 

 facts are all exhibited in the diagram added to his paper, 

 wherein/* and v are the two co-ordinates. The diagram ac- 

 companying this paper gives an approximate idea of Andrews' 

 diaoram for carbonic acid; the meaning^ of the dotted curves 

 added to it will appear later on. The curves drawn for con- 

 stant temperature, the isothermals, show a horizontal part, re- 

 presenting the process of condensation. A curve is drawn 

 through the points representing the volumes of the 

 saturated vapour and liquid curve. This is called the 

 border-curve, or the saturation-curve, and culminates at 

 the critical point C. The isothermals of higher tem- 

 perature have a continuous course, without the sudden 

 breaks which the lower ones show where the process 

 of condensation begins and ends, but still showing a 

 deviation from the shape pv = constant, consisting in 

 a flattening in the region of the critical point. 



Since then a number of other substances have been 

 investigated in the same or a similar manner. They all dis- 

 play qualitatively similar phenomena ; the existence of a 

 critical point has been established for almost all substances 



