CHAPTER V. 

 THE WEIGHT-NORMAL SYSTEM FOR SOLUTIONS. 



The solutions employed in this investigation have been made, from 

 the beginning, by dissolving a gram-molecular weight of the substance, 

 or a decimal part of the same, in 1,000 grams of water. Moreover, in 

 calculating the theoretical gas pressure of the solute at any temperature 

 the volume of the solvent in the pure state, and not that of the solution, 

 has been adopted as the standard. The solutions so made have been 

 called "weight-normal" to distinguish them from "volume-normal" 

 solutions, or those made by dissolving the same quantities of substance 

 and diluting the solutions to a volume of 1,000 cubic centimeters. 

 Perhaps a better name for such solutions would have been "solvent- 

 normal." 



There have existed a widespread and persistent misapprehension of 

 the reasons which led to the adoption of the weight-normal system for 

 osmotic -pressure measurements and a quite general misunderstanding 

 of the nature of the advantages which were expected from its employ- 

 ment. Unfortunately, but perhaps not altogether unnaturally, it has 

 been inferred by many that the preference shown for this system has 

 somehow committed the author and his associates to the view that 

 under it the osmotic pressure of aqueous solutions will be found to 

 conform necessarily to the gas laws. This appears to be the interpreta- 

 tion of Findlay, who, in his recent excellent work* on osmotic pressure, 

 has reduced the weight-normal system, as employed by the writer 

 and his co-workers, to the form of a general equation which he calls the 

 " equation of Morse," and has then proceeded to show — though by 

 evidence which is not entirely convincing to the writer — that it must 

 fail in the case of highly concentrated solutions. It is hoped that the 

 following somewhat discursive presentation of the subject will serve to 

 clear up some of the misunderstandings which have arisen. 



The fact that van't Hoff expressly limited his deductions concerning 

 osmotic pressure to extremely dilute solutions, in which neither the 

 aggregate volume of the solute molecules nor their mutual attractions 

 are of moment, has been — certainly until recently— too often ignored 

 or too feebly emphasized. That he had a clear vision of some of the 

 complications which must arise when it was attempted to deal with 

 the osmotic pressure of concentrated solutions, and that he, on that 

 account, deliberately excluded these as something for which the simple 

 equation PV = KT is inadequate, is convincingly shown by the follow- 



*" Osmotic Pressure," by Alexander F.'nilay. Longmans, Green & Co., 19 13. 



97 



