178 THE EARL OF BERKELEY, MR. E. G. J. HARTLEY AND DR. C. V. BURTON: 



For the Solvent : 



Specific volume at hydrostatic pressures p , TT W u pa . 



Vapour pressure when the solvent is in contact with its own vapour alone TT M . 

 Specific volume of the vapour when under various pressures . . v, a , v, , &c. 



An examination of the equation will show that the following data are required : 



(1) The densities of solutions of different concentrations at the temperature at 



which the work was carried out, namely, C. 



(2) A series of measurements of the relative lowering of the vapour pressures of the 



solutions at C. 



(o) A corresponding series of measurements of the direct osmotic equilibrium 

 pressures for the purpose of comparison with those to be deduced from the 

 measurements of (2) by means of the equation. 



(4) Measurements of the compressibilities of the solutions and of water. 



(5) A knowledge of the change in the specific volume of water vapour with 



pressure this knowledge we assume is given by REGNAULT'S work. 



Choice of Solute. 



Owing to the fact that, in the direct measurement of osmotic pressures, the copper 

 ferrocyanide membrane is semipermeable to but few substances, the choice is strictly 

 limited. In calcium ferrocyanide a substance was found which seemed likely to throw 

 considerable light on osmotic phenomena. 



Aqueous solutions of this substance have the following properties : The copper 

 ferrocyanide membrane is practically impermeable to the salt. The salt is very 

 soluble, so that strong solutions can be obtained and hence high equilibrium pressures ; 

 the solutions at all strengths are apparently stable enough at C. to allow the 

 necessary data to be obtained. The solutions show a considerable shrinkage on 

 dilution, and this fact makes it possible to determine experimentally the s of the 

 equation. 



In this part of the communication the work is limited to concentrations of from 

 30 to 50 gr. anhydrous salt to 100 gr. of water. These limits are imposed 

 upon us by the fact that weaker solutions have vapour pressures differing little 

 from that of pure water, and, consequently, measurements of the relative lowering of 

 their vapour pressures would entail very lengthy experiments,* while the stronger 

 solution has an osmotic equilibrium pressure of about 130 atmospheres, which is near 

 the utmost pressure we can obtain with our apparatus. 



* Even for a solution of 30 gr. anhydrous salt in 100 gr. water it is necessary to pass the air current 

 through the vessels for a period of seven days before the difference between the loss of weight of the 

 solution and that of the water is sufficient to give accurate results. 



