268 SCIEXCE PROGRESS. 



present are given b\' the ratio of the parts into which d^ is 

 divided by the stationary vertical line x = 5/6. The 

 quantity of liquid increases until in c the whole mixture is 

 liquid. Similar phenomena would be displayed by other 

 mixtures. When, however, the composition x is less than 

 X, (the line .r, touches the curve in C) no condensation 

 takes place. And for mixtures between the limits x = x^ 

 and X = x^ (the line x^ passes through P) the condensation 

 will be found to be retrograde, the quantity of liquid 

 reaching a maximum and diminishing until it is all eva- 

 porated. Mixture r^ has its critical point in C, and 10° C. 

 would therefore be the critical temperature for this mixture. 

 Co-existence of two phases which approach each other and 

 finally become identical must give rise to what was called 

 the critical phenomenon and we see that at 10" C. this 

 would take place for mixture jTj. In order to return to the 

 phenomena for a given mixture at different temperatures 

 we have to draw the same diagram for those tempera- 

 tures. It will then easily be seen that the phenomena 

 obtained are exactly those w^hich were described in the begin- 

 ning. It is found that the critical phenomenon belongs 

 to the one temperature i^ only if we do not take gravita- 

 tion into account. In lowering the temperature the curve 

 moves towards the left and at the critical temperature of 

 air would touch the line x = 0. Below that temperature all 

 mixtures would be condensable and no critical phenomena 

 would exist. 



Another instructive way of representing the experi- 

 mental results is obtained by drawing the border-curves for 

 the different mixtures in the /-/ diagram (fig. 4), first 

 studied by Duhem, The border curves have the shape of 

 loops, in the simplest case lying in between the vapour- 

 pressure curves for the two components, A curve may be 

 drawn enveloping the loops. The points of contact P 

 correspond to P in fig. i and fig. 3, The points C 

 where the loop has a vertical tangent also correspond to 

 the points C in figs, i and 3, The highest points of the 

 loops (M) correspond to M in fig, i. The difference 

 between C P and M is clearly brought out by this diagram,. 



