238 Mr Barlow, On the Osmotic Pressure of Aqueous Solutions. 



air, the conductivity increases at first uniformly with the time, 

 almost as a straight line, falling off after the sixth day. The 

 flattening at the end indicates that the curve is tending to a 

 constant value. In Curve II, the more curved portion is at the 

 beginning, and the latter part is practically a straight line with 

 the same slope (very little steeper) for the sixth day as for the 

 fifteenth. These differences are caused by the additional disturb- 

 ing effect of the air in Curve II. 



In Curve I, the solution obtained from the soluble portion 

 of the pot, more and more nearly reaches its saturation strength, 

 owing to the very small quantity that will dissolve, and con- 

 sequently the rate of change in the conductivity of the water falls 

 off. In Curve II, the further portions of the curve are kept from 

 flattening by the impurity which is steadily absorbed from the air. 

 It will take a much longer time for the water to become saturated 

 with very soluble carbon dioxide (which will be the chief impurity) 

 than with the slightly soluble part of the clay ; hence the curve 

 becomes practically a straight line and apparently continues as 

 such for some distance beyond the limits of the diagram. 



There is one other experiment recently performed, which shows 

 in a direct way that the osmotic pressure is very considerably 

 reduced by the impurities in the outer water. 



Cell e had been left through the vacation (from March 27 to 

 April 28) with a solution of potassium sulphate that should give 

 a pressure of 219 mm. On March 27, the cell was left with the 

 gauge level afc 235"8 (osmotic pressure attained = 208 mm, at this 

 level), the cell being in a small outer vessel. A month later, the 

 level was at 92*6, the cell being well under water outside. The 

 comparatively small volume of the outer water must have become 

 very impure during this month. It was removed and replaced by 

 freshly boiled distilled water at the temperature of the laboratory. 

 The following were the gauge levels during the next few days. 







Tempera- 





Time, etc. 



Gauge 

 level 



ture of 

 water 



.- 







(degrees C.) 





Friday, April 28 



92-6 



11-8 





Saturday 



109 



11-5 





Monday, May 1 



127-6 



11-4 





Tuesday 



131-8 



11-6 





Outer water replaced by new 









water 









Wednesday 



136 



11-5 





Thursday 



138-6 



11-5 



Rise of 46 mm. 



Monday, May 8 



136-6 



11-6 



Falling 



