DISCUSSION OF EVIDENCE. 147 



The strong mineral acids show some hydrating power, but the com- 

 plexity of the hydrate formed by these substances seems to pass 

 through a maximum. The acids thus differ from the salts. 



Some 13 non-electrolytes were studied as to their hydration, and 

 none of them showed any appreciable hydration. The same applies 

 to the organic acids that were studied in this connection. 



The following general relations were brought out by the work of 

 Jones and Getman. The total amount of water held in combination by 

 the dissolved substance increases as the concentration of the solution 

 increases. From what is known of mass action, this would be expected. 



The number of molecules of water combined with 1 molecule of the 

 dissolved substance generally increases from the most concentrated to 

 the most dilute solution studied. In some cases, however, the number 

 of molecules of combined water seems to pass through a maximum. 

 These results, we believe, give us the approximate amounts of com- 

 bined water, and certainty the relative hydrating powers of the different 

 compounds with which we worked. 



One other relation bearing on the question of hydration in aqueous 

 solution was brought out by the work of Jones and Getman, 



RELATION BETWEEN THE MINIMA IN THE FREEZING-POINT CURVES AND 

 THE MINIMA IN THE BOILING-POINT CURVES. 



It has already been pointed out that if we plot the molecular lower- 

 ings of the freezing-point as ordinates against the concentrations of 

 the solutions as abscissa?, the curves have a well-defined minimum; 

 from this minimum they rise both with dilution and with concentration. 

 What is the meaning of this minimum? The curves rise with dilution 

 because of increasing dissociation ; they rise with increasing concentration 

 because the total amount of combined water increases with the concen- 

 tration of the solution ; and consequently the lowering of the freezing- 

 point of the ever-decreasing amount of sol vent water becomes greater and 

 greater. The minima in freezing-point curves are, then, the points where 

 the two opposite effects increase in dissociation with dilution and in- 

 crease in combined water with concentration, just offset one another. 



The rise in the boiling-points of solvents produced by dissolved sub- 

 stances was also studied, and at different concentrations. Boiling-point 

 curves were plotted analogous to the freezing-point curves; i. e., molec- 

 ular rise in the boiling-point as ordinates and concentrations as abscissae. 

 These curves also had minima, and we interpreted the minima here in 

 a manner analogous to our interpretation of the minima in the freezing- 

 point curves they are the points of equilibrium between increasing dis- 

 sociation with dilution, and increasing hydration with concentration. 



A comparison of the freezing-point with the boiling-point curves 

 brought out a relation of interest, and we believe of some importance 

 in the present connection. The minima in the boiling-point curves 



