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



The solution of this experiment was the weakest used. The 

 low value here set up, compared with that of the experiment before, 

 shows that the effect of impurities which contaminate the outer 

 water is much more marked as the dilution increases. 



(3) In the above experiments the outer water in which the 

 cells have been standing was exposed to the air. In the experi- 

 ment to follow, the outer vessel was closed by a cork, through 

 which the glass of the cell passed. The cork also had a manometer 

 passing through it containing mercury. The whole was rendered 

 air-tight by pouring molten paraffin wax into a mould round the 

 cork, and so covering it with a good layer of wax. That this was 

 air-tight was shown by the manometer steadily standing at different 

 levels under barometric changes. Since the osmotic pressure is 

 the excess of the internal pressure set up over the atmospheric 

 pressure, in this experiment corrections had to be made for the 

 changes in the manometer levels. 



Cell e. '0034 N solution of potassium sulphate. Theoretical 

 osmotic pressure at 0° C. = 57 '7 mm. 





Gauge 



levels 



Correc- 





Osmotic 



Time, etc. 







tions to 

 difference 



Tempera- 

 ture 



pressure at 

 given tem- 









narrow- 

 limb 



wide-limb 



of levels 



(degrees C.) 



perature 



Thursday, July 21 ... 



open 65*2 



58-2 









Pressure added 



111 



53-6 



— 



18-4 



— 



Saturday 



126-2 



51-4 



— 



18-5 



— 



Monday 



132 



51 



— 2*5 mm. 



18-4 



78-5 mm. 



Tuesday 



131 



51 



— 



18-1 



— 



Wednesday 



126 



51-4 



+ 4 



17-6 



78-6 



Thursday 



119 



52-4 



+ 12 



18 



78-6 



Thursday (week later) 



86-5 



56-2 



+ 23 



19-3 



53-3" - 



Followed by steady fall. 



From the table it will be seen that the maximum pressure was 

 one of 78"6 mm. at an average temperature of 18° C, and remained 

 steady through four days. 



The theoretical pressure at 18° C. = 61 "46 mm. 



78 ' 6 ; = 1-278. 



= 139, 



Hence 



61 46 



•278 



or 13'9 °/o °f the salt is dissociated. 



According to Jones'* cyroscopic determinations, a. is about - 90 



* Phil. Mag., Vol. xxxvi. p. 472. 



