Evaporation and Dissociation. 457 



at one point, and is also a tangent to the apex. There are, 

 therefore, two volumes corresponding to each of these pres- 

 sures. Since no gas can be submitted to a negative pressure, 

 those portions of an isothermal line representing the truly 

 gaseous condition of matter never extend below the horizontal 

 line of zero pressure ; only those portions of the isothermal 

 which proceed towards the inferior apex fall below this line. 

 An isothermal line below zero pressure is therefore cut only 

 twice by a line of equal pressure, and there are therefore two 

 volumes corresponding to each pressure. At each inferior 

 apex, however, the horizontal line is a tangent to the curve, 

 and there is therefore only one volume corresponding to a 

 given pressure. 



On referring back to Plate IX., it will be seen that the 

 pressures corresponding to the superior apices of each iso- 

 thermal line, when mapped, produce the curve AC ; and 

 those corresponding to the inferior apices, the curve BC. 

 The surface bounded by these curves and the line of zero 

 pressure corresponds to portions of the isothermal lines, 

 including pressures between the two apices, and each point in 

 the surface is the locus of intersection of three isochoric lines. 

 Below the line of zero pressure the isochoric lines cor- 

 responding to the gaseous state are absent ; and hence each 

 point is the locus of intersection of only two isochors. The 

 isothermal lines above and below the limits of pressure given 

 by the apices are cut only once by any line of equal pressure; 

 hence the isochors outside the area ACD, and above the line 

 of zero pressure, do not intersect. The apex C of the curvi- 

 lateral triangle ACD is the point of highest temperature and 

 pressure at which intersection can take place, and therefore 

 represents the critical point ; it is also the common point of 

 intersection of the three pressure-temperature curves. 



Referring now to Plate VIII., in which the isochoric lines 

 in the neighbourhood of the critical point are shown on a 

 larger scale, it will be seen that the isochoric lines above 

 a volume not far removed from 4 cub. centim. per gram cut 

 the ordinary vapour-pressure curve CE on one side, while 

 those below the volume 3*75 evidently cut the vapour-pressure 

 line on its other side. There must therefore be an isochoric 

 line which does not cut the curve at all, but forms a tangent 

 to its end-point. That isochor gives the critical volume. It 



may be determined by calculating the value of -£ at the 

 critical temperature. This value of — is identical with 



