( 187 ) 



the whole isotherm of sohibilit}' points to a double retrog'rade solidi- 

 fication, has not been ascertained as yet. What has been found, is 

 that below 240" the upper part of the isotherm of solubility runs 

 toward the right, which points to an increase of solubility of anthra- 

 quinone in the fluid phase with increase of pressure ^), whereas 

 above 250° a reversed course was found. Between 240'" and 250° a 

 change of direction seems to have taken place, and in this range it 

 might be possible to ascertain the course foretold by van der Waals. 

 As however the small range of temperature 240^^250° corresponds 

 with a great difference in concentration, tiie point when the change 

 of direction takes place is not easily ascertained. 



The results obtained at temperatures between 203° and 247° are 

 represented in fig. 6. Here the isotherms of solubility for the fluid 

 phases at 210°, 220°, 230" and 240° are drawn. All these isotherms 

 show, as predicted by van der Waals '), the phenomenon of retro- 

 gi'ade solidification, and the nearer we get to point q, in other words 

 the nearer to 247°, the larger the region of this retrograde solidification. 



Hi 



:.h 



zs 



II: 



-_-V(- 



..J 



d 



Ether. 



Anthraquinone. 



X 



Fig. 7. 



1) Tliis iuniit'tic-e of pirssure lias been examined up to niore than '0^ ,iliiii)S|»]i('i-es. 

 -) loc. cit. 



13 



Proceedings Royal Acad. Amsterdam. Vol. VII. 



