Dandeno : Osmotic theories 287 



been, in fact, the basis for all modern conceptions of osmotic pres- 

 sure. His figures have entered into text after text, in both physics 

 and chemistry. The work is now regarded as a classic, and van*t 

 Hoff' s law gives it an emphasis which will carry It on through 

 time, for many years yet. Now, van't Hoff's law does not explain 

 the cause of osmotic pressure, it merely states it, and leaves the 

 explanation to be inferred. 



Let us point out some of the most serious objections to van't 

 Hoff's law as it relates to osmotic pressure. It bases its argu- 

 ment, from the experimental side, on Pfeffer*s researches and con- 

 clusions. These show several weaknesses which seriously affect van't 

 Hoff's theory as well as Pfeffer's own figures. Pfeffer states that 

 the osmotic pressure of cane sugar, when a gram molecular weight 

 is dissolved in a liter volume, is 22.4 atmospheres and this is what 

 it should be theoretically according to van't Hoff's theory. Sugar 

 is not an electrolyte, hence no account is to be taken of dissocia- 

 tion. In looking into Pfeffer's work * to examine minutely the 

 methods, this statement is found : *'The same pressure (that is, 

 22.4 atmos.) must be exerted by a solution of 342 grams of cane 

 sugar in i liter of water." Now, he surely could not have meant 

 this as stated, because it is not in accord with some of his own 

 statements, and is certainly not in accord with van't Hoff's law, 

 or any ordinary interpretation of it. Moreover, further on is given, 

 on pp. 146, 147 of Pfeffer's Plant Physiology, one statement re- 

 ferring to solution concentration; " i gram in 100 c.cm. of watcr^ 

 and this refers to a column heading on the preceding page, 

 '^Osmotic pressure of i gram in 100 c.c. of sohitiony From this 

 it appears as though he regarded the two statements, ^' dissolved^ 

 in a liter of water," and "dissolved to a liter of solution " as the 

 same thing. In the phrase '* 342 grams of cane sugar in one 

 liter of water," we have a concentration quite different from that 

 made by dissolving 342 grams to one liter of solution. The latter 

 is that to which van't Hoff's theory directly applies. Theoret- 

 ically, 342 grams to I liter volume gives 22.4 atmospheres pres- 

 sure. Now, it maybe said that Pfeffer made a mere typographical 

 error, when he said " in i liter of water." Of course, neither 

 Pfeffer, van't Hoff, nor Ostwald had ever actually developed a 

 pressure of 22.4 atmospheres. Pfeffer used weaker solutions, and 



*0.smoti£che Untersuchungen, 1S77, and Physiology of plants, 19C0. 



M 



