and Attached Water. 447 



energy, but always a lesser capacity for saturation than a still 

 less saturated one. (Witness anhydrous sulphate of magne- 

 sium compared with the crystalline salt.) Let us suppose that 



at a temperature t° a weight iv of water dissolves - of s, the 



weight of the salt. Then using the symbol ^ to denote either 

 physical adhesion or cohesion, we have the condition of equili- 

 brium or saturation expressed by the equation 



<i-S~(-+3-(i-D~.(i-3 



+ (» + lM" + ;)' 



Or, since the tensions of physical adhesion and cohesion are 

 independent of quantity, 



At a temperature t®, 



where <i is + or — according generally as t° is > or < 

 than t°. 



The expression 



implies that the water is not saturated ; while 



implies that it is supersaturated. The above equations are the 

 general ones for all salt solutions above 0° C. For tempera- 

 tures below 0° C. another element manifests itself with greater 

 force as the temperature is lowered, namely the physical affi- 

 nity between the solid salt and solid water. If in fig. 1 we 

 draw a straight line parallel to and below the zero axis so as to 

 cut any one tracing in two points (it never can be cut in more 

 than two points), we see that solidification may ensue at the 

 same temperature in two differently strong aqueous solutions 

 of the same salt. The intersection with the left-hand branch 

 is the condition of equilibrium resulting from the equality be- 

 tween: — on the one hand, ice -^ ice together with salt --^ water ; 

 on the other, salt solution ^ water, 



ice^ice + s^iv — [ iv+ - } ^w 



=( w+ 9 



The intersection on the right-hand branch shows the condi- 





