442 Method of Measuring High Osmotic Pressures. [Apr. 21, 



connected with the negative pole of the battery, was suspended. This 

 electrode was surrounded by a porous pot so as to prevent the solution, 

 when it had been rendered alkaline by the passage of the current, from 

 reaching the membrane on the tube. As mentioned by Morse, we 

 found that the alkaline solution acted injuriously on the membrane. 

 The porcelain tubes were filled with the copper solution, and were 

 furnished with copper electrodes. The voltage used was 100; and 

 when the resistance of the tubes had risen to a constant A^alue, they 

 were taken down and soaked in distilled water for several days. They 

 were then again set up, and the current passed until the resistance was 

 again constant, generally at a higher value than before, upon which 

 they were taken down and washed. This process was repeated till no 

 further change in resistance took place. 



The highest resistance obtained was 170,000 ohms ; but out of some 

 fifty tubes only eight or nine reached this figure. These latter were 

 those selected for the experiments, and it was found advisable to 

 re-make the membranes eleetrolytically after they had been subjected 

 to considerable pressures. 



[May 21. — We have lately found that the membranes are greatly 

 improved if they be re-made eleetrolytically under high pressure. The 

 pressure should be applied to a very strong solution of sugar in which 

 the potassium ferrocyanide has been dissolved and which surrounds the 

 outside of the tube ; the inside of the tube being filled with the usual 

 copper sulphate solution and remaining under atmospheric pressure.] 



Results. — The following results with cane sugar were obtained ; the 

 experiments were made more for the purpose of testing the method 

 than to get accurate observations of the osmotic pressures ; but we 

 think they are within 10 per cent, of the true values : — 



Concentration in 

 grammes per 

 litre. 



Pressure at which 

 the " turning point " 

 occurred. 



Pressure when 

 " turning point " 

 corrected for guard- 

 ring and solution 

 leaks. 



Pressure deduced 

 from Boyle's law. 



120 7 



10 "5 atmos. 



9 *5 atmos. 



8 *4 atmos. 



180 -0 



15 7 „ 



14 -4 „ 



12 -5 „ 



240 -0 



23 -6 „ 



21 -3 



16 -7 „ 



360 -0 





37-0 „ 



25-1 „ 



420 -0 



45-9 „ 



43 -0 „ 



2y -2 „ 



Advantages of the Method. — It may be of use briefly to point out what 

 seem to be the advantages of this method : — 



* In this experiment the actual " turning point " was not readied. 



