98 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



ing quotations from van't Hoff's memorable paper.* At the conclu- 

 sion of Section I, entitled "Der Osmotische Druck, Art der Analogie, 

 welche durch dessen Einfuhrung entsteht," he says (page 483) : 



"Von diesem praktischen Vorteil werden wir in Nachfolgenden Nutzen 

 ziehen, speziell zur Erforschung der fur ideale Losungen giltigen Gesetze,fiir 

 Losungen also, die derartig verdunnt sind, dass sie den idealen Gasen an die 

 Seite zu stellen sind, und in denen somit die gegenseitige Wirkung der gelosten 

 Molekiile zu vernachldssigen ist, wie audi der von diesen Molekulen eingenom- 

 mene Raum bei Vergleich mit dem Volum der Losung selbst." 



The succeeding three sections, namely, II, III, and IV, are entitled: 



II. "Boyle's Gesetz/tir verdilnnte Losungen." 



III. " Gay-Lussac's Gesetz/w verdilnnte Losungen." 



IV. "Avogadro's Gesetz/wr verdiinnte Losungen." 



Again, he says (page 498) : 



"Noch trefflicher ist dass der so allgemein auch fur Losungen angenom- 

 mene G uldberg und Waagesche Satz thatsachlich als einf ache Schlussf olgerung 

 aus den oben fur verdiinnte Losungen aufgestellten Gesetzen entwickelt werden 

 kann." 



It is equally certain that some of those who were foremost in adopting 

 the new views concerning osmotic pressure, and who became the most 

 effective agents in promoting their publicity and general acceptance, 

 failed to make clear the full significance of van't Hoff's reservations; 

 for we find stated, without due qualification, and thereafter constantly 

 repeated as models of concise yet comprehensive definition, proposi- 

 tions like the following: 



"Dissolved substances exert the same pressure, in the form of osmotic pressure, 

 as they would exert were they gasified at the same temperature without change of 

 volume." [Again] * * * "d. h. der osmotische Druck gelosten Rohrzucker ist 

 gerade so gross wie der Gasdruck den man beobachten wiirde, wenn man das 

 Losungsmittel entfernte, und die geloste Substanz den gleichen Raume bei gleicher 

 Temper atur in Gasform erfullend zuruckliesse." 



Much confusion and futile discussion would have been saved if it 

 had been clearly explained in connection with all such statements: 



1. That van't Hoff intended to apply the equation PV = KT only 

 to " ideal solutions," i. e., to solutions so dilute that neither the volume 

 nor the mutual attractions of the solute molecules are of importance. 



2. That when more concentrated solutions are to be dealt with, it 

 will obviously be necessary to modify the simple equation for "ideal" 

 solutions in a manner analogous to the modification by van der 

 Waals of the equation for "ideal gases." 



3. That, because of the solvent, the case of solutions is more complex 

 than that of gases, and that, for this reason, the general equation for 

 them may be more complex than the equation of van der Waals for 

 gases. 



*Zeitschrif t fur physikalische Chemie, I, 479. 



