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



We have tacitly assumed that the actual semi-permeable membrane is impermeable 

 to the air dissolved in both solution and solvent ; so that the s of the equation is the 

 actual change in volume under the conditions of the experiment. This element will 

 be given by the density measurements in conjunction with the measurements of 

 the compressibility of the solutions and of water, which in all cases contain 

 dissolved air. 



The u term is similarly obtained ; although this term, as will be seen later on, is of 

 no importance in this work, it may be so when dealing with liquids that are more 

 volatile than water ; then the compressibility of air-containing solvents will have 

 to be obtained with considerable accuracy. It should be noted that the P of the 

 limit represents the actual osmotic equilibrium pressure due both to the dissolved salt 

 and the dissolved air in the solution measured against the back osmotic pressure of 

 the air dissolved in the otherwise pure solvent. 



On the other hand, the v term is, in one respect, different from the actual conditions ; 

 it is vapour pressure in air when both solution and solvent are under the sum of the 

 pressures of the atmosphere and their own vapour pressures, while the actual 

 experiments are made when the two liquids are under the pressure of the atmosphere 

 alone. It is thought that this slight difference may be neglected. 



Evaluation of the Integrals. 



f\+i> 

 sete. It will be seen on p. 198 (Table IV.) that the coefficient of 

 A+ ", w 



compression of any one solution varies but little at different pressures ; it was, 

 therefore, thought that its mean value would be accurate enough for the purpose in 

 view, and the following table gives the resulting volumes of the several solutions 

 when under compression. Columns (1) and (2) are a repetition of those on p. 181 

 (Table lA.) ; (3) gives the number of grammes of water to one gram-molecular weight 

 of anhydrous salt ; (4) gives the observed equilibrium pressure, taken from a graph 

 when necessary ; (5) gives the mean coefficient of compression per atmosphere, also 

 taken from a graph when necessary; (6) to (12) give the volumes of the respective 

 solutions which contain one gram-molecule of salt when under the pressure given 

 at the head of the respective columns. 



The numbers in the horizontal line between the brackets have not been used in the 

 reduction it was considered that the changes in volume were too small to give 

 reliable results. 



On plotting the volumes in any one of the columns (6) to (12) against the figures in 

 column (3), it will be seen that the resulting curve is practically a straight line, hence 

 we may take the change in volume (at any given pressure) caused by a change in 

 concentration due to the loss of one gramme of water as constant, and this volume 

 ehange will be given by the ratio of the difference in volume divided by the 



