366 PROCEEDINGS OP THE AMERICAN ACADEMY. 



" Losungsmittel ist derjenige Stoff des Gemenges, welclier bei dem 

 betrachteten Vorgange ausgeschieden wird." This view is heroically 

 logical, for it meaus that, when a salt crystallizes from a saturated 

 solution, the mother liquor consists of water dissolved in the salt. 



Nernst's position on the subject is doubtful. He puts solutions un- 

 der the head of physical mixtures and remarks : * " Die verdiinnten 

 Losingen sind Gemische welche eine Komponente in grossen Ueber- 

 schuss zu den iibrigen enthalten ; erstere bezeichnen wir in diesem 

 Falle als das Losungsmittel, letztere als geldste StofEe." On the other 

 hand, he draws a distinction between freezing out the solvent and 

 crystallizing out the solute, f He does not accept the view that the 

 salt is the solvent in a saturated solution ; but he does not suggest in 

 any way that there may be different laws for the solute and the solvent. 

 Planck is very clear and precise ; he defines dilute solutions in almost 

 the same words as Nernst, and goes on : $ " Bei einer beliebigen Losung 

 kann jeder Bestandtheil derselben als Losungsmittel oder als geloster 

 Stoff aufgefasst werden." This means that in a mixture of two liquids 

 either may be considered as the dissolved substance, and will therefore 

 decrease the partial vapor pressure of the other, and this decrease of 

 the vapor pressure will be greater the greater the concentration of the 

 dissolved substance. This is not in agreement with the facts. A 

 saturated solution of ether in water has the same partial vapor pres- 

 sures as a solution of water in ether sajturated at the same tempera- 

 ture. § For the moment we will consider ether as the dissolved 

 substance. In the first solution, the volume concentration is roughly 

 10% ; in the second, about 99% at 20° ; and yet this enormous change 

 of concentration has no effect on the partial vapor pressures. The 

 figures are still more remarkable if we consider solutions of chloroform 

 , in water and water in chloroform, when one of the components is 

 present in infinitesimal quantities. We must assume one of two things : 

 either that our present formula for the change of the vapor pressure 

 with the concentration is all wrong, since it does not admit of the 

 vapor pressure of one of the components passing through a mini- 

 mum ; or that there is a difference between solvent and solute, and 

 that each has its own law expressing the change of its vapor pressure 

 with the concentration. This time I prefer the second assumption, 

 with all that it implies. The equations of van 't Hoff and Raoult are 



* Tlieoretische Chemie, p. 115. t Ibid., p. 393. 



I GrunJriss der Thermoclicmie, p. 131. 



§ Wied. Ann., XIV. 219, 1881; Ostwald, Lelirbucli, I. 644. 



