MEASUREMENT OF OSMOTIC PRESSURE 569 



all measurements except those made with two tubes of pre- 

 eminent excellence. 



The Numerical Results. — The figures obtained in the measure- 

 ments of concentrated solutions were as follows : 



Cane sugar 





Dextrose 





Galactose 



Mannitol 



Concentration. 

 G/litre. 



l8o'I 

 300 - 2 

 420-3 

 540-4 

 660-5 



750-6 

 99-8 

 199-5 

 3192 

 448-6 

 548-6 

 250 

 380 

 500 

 500 

 100 

 no 

 125 



51-9 (calc.) 



Pressure in 

 atmospheres. 



1395 12-45 (calc.) 

 26-77 



43"97 

 67-51 



100-78 



13374 

 1321 

 29-17 



5319 



87-87 



121*18 



35"5 

 62-8 

 958 

 97"3 

 131 

 146 

 167 



The results are shown graphically in fig. 8, in which the 

 diagonal lines show the values calculated from van't Hoff's 

 equation. 



Experiments on Calcium Ferrocyanide. — The experiments on 

 calcium ferrocyanide, published in 1908 and 1909, are note- 

 worthy as extending the measurements of osmotic pressure to 

 aqueous solutions of salts. In the case of the more concentrated 

 solutions the osmotic pressure was correlated with the vapour 

 pressure by means of a thermodynamic formula. In a formula 

 put forward by Prof. A. W. Porter the compressibility of the 

 solution and solvent were taken into account and these quan- 

 tities were therefore measured but deviations amounting to 2\ 

 per cent, were found between the calculated and observed 

 values. A modified equation was therefore developed in which 

 the thermodynamic cycle was calculated for operations carried 

 out under atmospheric pressure instead of in a vacuum. The 

 deviations were then reduced to less than 0-5 per cent. ; but it 

 is noteworthy that the correct assumptions to be made in 

 working out the thermodynamic cycle were only determined 



