186 



OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



tions only a little less dilute than those whose osmotic pressure is to 

 be determined. 



The ratios of osmotic to gas pressure between 0° and 25°, though con- 

 stant for each concentration, are all greater than unitj^. The excess 

 varies from 6 per cent in the 0.2, 0.3, and 0.4 normal solutions, on the 

 one side, to 8.3 per cent in the 0.1 normal solution; and on the other, to 

 11.4 per cent in the normal solution. The increase in ratio from the 0.4 

 through the succeeding concentrations exhibits a certain amount of 

 regularity. The increment between the 0.4 and 0.5, and also between 

 the 0.5 and 0.6, is about 0.7 per cent. All the succeeding increments, 

 i. e., those between the 0.6 and 0.7, the 0.7 and 0.8, the 0.8 and 0.9, and 

 the 0.9 and 1 .0 concentrations, are approximately 1 .0 per cent. A notice- 

 able feature of the 0.2, 0.3, and 0.4 weight-normal solutions is the fact 

 that their osmotic pressures are all about equally (6 per cent) in excess 

 of the calculated gas pressure of the solute. 



Having found that the law of Gay-Lussac does hold for the osmotic 

 pressures of cane-sugar solutions between 0° and 25°, one is inclined to 

 believe that they should also conform to the law of Boyle, and to seek 

 for some rational explanation of the facts: 1st, that the ratios in ques- 

 tion are excessive, i. e., above unity; and 2d, that they are not propor- 

 tional to the supposed concentration of the solutions. The most 

 obvious general explanation (if one attempts to reconcile the pressures 

 between 0° and 25° to the view that the law of Boyle, as well as that of 

 Gay-Lussac, does hold) is hydration of the solute, which may be pre- 

 sumed to have the effect of concentrating the solutions. But if one 

 attempts to work out the precise degrees of hydration which would 

 account for the variations of ratio from concentration to concentration, 

 he is quickly entangled in certain hazardous assumptions respecting the 

 relations of solvent to solute and the effect of these upon the osmotic 

 pressure. In the writer's opinion, judgment as to the applicability of 

 Boyle's law to the osmotic pressure of cane-sugar solutions at tempera- 

 tures below 25° should be suspended until much more is known about 

 the osmotic pressures of the aqueous solutions of other substances. 



