100 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



present time must fail to convince, and is bound to absorb in futile 

 discussion much energy which might be more profitably employed in 

 finding out what the facts of osmotic pressure really are. 



It has been intimated by some of our friendly colleagues that the 

 adoption at the beginning of our investigation (1) of the weight-normal 

 system for the solutions ; (2) of the practice of referring the gas pressure 

 of the solute to the volume of the solvent; and (3) of the custom of 

 always stating the ratio of observed osmotic pressure to the calculated 

 gas pressure of the solute are all inconsistent with the general attitude 

 toward the subject which is professed above — that, in fact, all three 

 of the itemized practices are indicative of preformed judgments in a 

 case which we were professedly attempting to investigate without 

 prejudice. This plausible indictment calls for some defense on each 

 of its specifications. 



Long before taking up the investigation of osmotic pressure, the 

 author had been accustomed to point out certain defects of the usual 

 "volume-normal" system of making up solutions, and to maintain that, 

 while it was advantageous and correct for merely analytical purposes, 

 it was both disadvantageous and illogical whenever any phenomenon 

 was to be studied in which the influence of the solvent upon the solute 

 was involved. It was maintained that, in cases of the latter kind, 

 the true concentration of a solution is determined by the numerical 

 ratio of the molecules of the solute to those of the solvent rather than 

 by the number of solute molecules in a given space. An illustration 

 frequently used for the purpose was the case of cane sugar and glucose. 

 In a volume-normal solution of cane sugar at 0°, the numerical ratio 

 of solute to solvent molecules is about 1 to 44.1, while in a volume- 

 normal solution of glucose at the same temperature the ratio is about 

 1 to 49.2. In other words, with respect to the solvent, the cane-sugar 

 solution is 11.5 per cent more concentrated than that of glucose. When 

 stated in terms of osmotic pressure, the difference is about 3.7 atmos- 

 pheres, notwithstanding the fact that equal volumes of the two solutions 

 contain the same number of solute molecules. 



Another illustration of the difficulties which are encountered when 

 volume-normal solutions are employed was the following example of the 

 effect of what may be called decimal dilution. Suppose a 0.1 volume- 

 normal solution of cane sugar to be made up by diluting 100 cubic centi- 

 meters of a normal solution to 1,000 cubic centimeters. With respect 

 to the relative numbers of solute molecules contained in equal volumes, 

 the new solution is one-tenth as concentrated as that from which it was 

 made, but with respect to ratios of solute to solvent molecules, namely, 

 1:44.1 and 1: 544.1, the concentration of the diluted solution is not 

 0.1, but 0.081 normal. 



When it came to a choice of systems, it was concluded by the author 

 and his colleague, Frazer, that the only justification for the use of the 



