148 Prof. H. L. Callendar and Mr. H. T. Barnes. 



In order to exhibit the nature of these results on a convenient 

 scale, it is necessary to plot, not the whole percentage, as is usually 

 the case, but the small differences from the linear formula given in 

 the fourth column. It is then apparent that there is a singular 

 point in the curve of solubility at a temperature of 39° C. At this 

 point, as the temperature rises, the rapidly increasing solubility of 

 the heptahydrate begins to exceed that of the hexabydrate, which is 

 also increasing, but less rapidly. At temperatures above 39°, a 

 solution containing only crystals of the heptahydrate becomes super- 

 saturated with respect to the hexahydrate, so that if any crystals of 

 the latter are formed or introduced, there will be rapid crystallisa- 

 tion, and the strength of the solution will diminish. At 39° C. the 

 solubilities of the two hydrates are equal and amount to 41*1 per 

 cent, of the solution ; but whereas the rate of increase of the solu- 

 bility of the heptahydrate is 033 per cent, per 1° C, that of the 

 hexahydrate is only 022 per cent, per 1°. 



The crosses marked R. and S. in fig. 4, at the points 0° and 20°, 

 correspond to the values given by Roscoe and Schorlemmer plotted 

 on tne same scale. The remaining observations of Roscoe and 

 Schorlemmer, and also those of Etarcl, differ so much from ours that 



Fig. 4. 



1 



r 



—2 



Curve 



of D/f) 



T erence 



a 



l 







from 







* 





L/nea 



- formo 



/&. 













.4 





— y 



L 







/#* 











A' 













0° 2 



0° 6 



0° 4 



0° 5 



0° 6 





M 











they could not be included in the limits of the page. The observa- 

 tions of Poggiale do not show the diminution of solubility above 

 80° C. whicb Etard gives. There is a general agreement in the 

 results quoted, especially between those of Roscoe and Schorlemmer 

 and Poggiale, but if the differences may be taken as indication of the 

 order of accuracy attained, it is evident that the results of these 

 observers could not be expected to show the critical point at 39° C. 



By interpolation on the curve given in fig. 4, we find the following 

 values for the sol ability as given by our observations. 



