BOOKS AND CURRENT LITERATURE 



235 



the exprossion is, strictly speaking, incorrect. A solution does not, in 

 itself, have any osmotic pressure; and the term is used, in a somewhat 

 loose manner certainly, to denote the hydrostatic or mechanical pres- 

 sure which wouhl he produced if the solution were separated from the 

 pure solvent by a membrane or septum which is permeable only to 

 the solvent It is osmosis that produces the osmotic pres- 

 sure, not osmotic pressure which produces osmosis. Nernst, it is true, 

 identifies the osmotic pressure with the expansive force which brings 

 about diffusion; but it does not seem to the writer to be wise to use 

 these two terms synonymously, although the 'expansive force' can, of 

 course, be measured by the osmotic pressure" (p. 4, footnote). The 

 reviewer prefers to employ the term diffusion tension (for either solvent 

 or solute) to denote this expansive force within the solution, which 

 may or not become mechanically evident as osmotic pressure. Without 

 a membrane between the two solutions, or between the solution and 

 the pure solvent, there is thus no osmotic pressure; osmotic pressure 

 develops when a suitable membrane is introduced and the membrane 

 plays as important a part in this as does the fulcrum in the action of 

 a lever. If the membrane is not perfect, the osmotic pressure never 

 equals the diffusion tension. 



Findlay emphasizes strongly the fact that the van't HofT theory of 

 solutions "never claimed to be valid except for very dilute solutions, 

 and for these the theory is perfectly valid" (p. 46). He points out 

 that solutions of higher concentration -should not be treated from the 

 standpoint of this theory, nor from that of van der Waal's equation, 

 but should be approached from the standpoint of an ideally perfect 

 solution, the general equation for which should be valid for any con- 

 centration. 



A satisfactory chapter on the cause of osmosis and the action of 

 the semipermeable membrane closes the book. Osmotic pressure "is 

 a function not of the concentration, expressed in grams or gram-mole- 

 cules per litre, but a function of the molar fraction" (p. 66). This 

 fraction is the number of molecules of solute divided by the number 

 of molecules of solvent in the solution. It is pointed out that the 

 bombardment theory of the origin of osmotic pressure has been prac- 

 tically abandoned by physical chemists, excepting for elementary expo- 

 sition. "The probability of its revival does not, however, appear to 

 be excluded" (p. 70). 



The reviewer has neyer been able to perceive any very great differ- 

 ence between the theory of attraction of the solvent and that of the 

 bombardment of the membrane by the solute particles, as far as osmotic 



THE PLANT WORLD, VOL. 16, NO. 8, AugUSt. 1913 



