( 185 ) 



curve meets the solubility curve, it is possible to pi'ovc iu a very 

 simple way tlie necessity of the appearance of retrograde soliclilicatiou 

 in /; and ^. 



Here however we must at once point out (hat, as will be discussed 

 presently, i-etrograde solidification also occurs bol ween /> r/. The fact 

 that theory requires this, can only be pro\'ed mathematically ^). 



Returning to tig. 2, we must still state that the curve q b uniting 

 the plaitpoints of the different ^>-.i'-loops, is ^■ery steep and, as far as 

 it has been observed, parallel to the first part of the plaifpoinlcurve 

 a p. This course however will probably change towards a higher 

 temperature, for if the plaitpointcurve possesses a maximum, which 

 is probably the case, then the projection of the plaitpointcurve on 

 the ^>.u-plane must also show a maximum. 



The />-.i'-sections below the 

 temi)erature 203° are not 

 drawn in tig. 2, as the scale 

 is too small to render the i)ar- 

 ticulars conspicuous. There- 

 fore this part of tig. 2 is 

 separately reproduced on a 

 larger scale in tig. 4. 

 p In accordance with the pre- 

 ceding we see that, though 

 at 200° no retrograde con- 

 densation occurs, instead of 

 it there appears retrograde 

 solidification. Soon however 

 the situation changes here, 

 for already at 196° retro- 

 Y grade condensation could be 



Fig. iu. ()l)served. 



What was observed when going from |)oint q to a higher tempe- 

 rature, is naturally also found in point />, but here towards a lower 

 temperature. This is illustrated by figures 4^/ and 4A : fig. 4^/ applies 

 to temperatures above point ([ and fig. 4/^ ai)plies to leni|)eratures 

 below i)oint />. In both figures three /^-/'-sections are repi-esented 

 schematically ; the sections 1 and 2 differ but slightly in tcni|)crature, 

 and 3 a[)plies to a temperature considerably (bfferenl from f hat witli 

 which 2 corresponds. 



In tig. 4a section 1 aj)plies to the lowest and 3 lo the highest of 



1) VAN DER Waals, 1. c. 



