Force and Osmotic Pressure. 383 



nearly in the case of zinc. But for j VdF, assuming that 

 Boyle's law does not hold, we might still make use of the con- 

 ception of the solution-pressure, and obtain quite a moderate 

 value for it. The actual form of the equation is not essential 

 to the argument ; but assuming the form already given, 



r_ V-6' 



we may form a rough estimate as follows : — 



Take the case of zinc in zinc-chloride solution : dissociation 

 may practically be neglected, for in the most saturated solutions 

 it is already very small. 1—x may be put equal to unity (in 

 Nernst's theory this is the case, the expression only entering 

 into the electromotive force of a concentration-cell on account 

 of the potential-difference between the two solutions). Then 



Ere= f T VdP = RTlog~ +&(n-P). 

 Ji>, r i 



For P x = 22 atmospheres (normal solution) E = 0'5 volt, 

 whence the left-hand side is 



0-5 x 2 x 96540 joules = 9'654 x 10 11 ergs ; 



RT is about 2'4 X 10 10 ergs at ordinary temperatures, and 

 for b we will simply assume the volume of a gram-molecule 

 of zinc chloride in the solid state =46 c.c. nearly. It follows 

 that II is about 20,000 atmospheres, a not impossible value. 

 For the most alkaline metals it might be three or four times 

 as great. 



The above calculation is of course very uncertain, both on 

 account of the form of equation assumed and the values of the 

 numerical constants : it is intended only to give the order of 

 magnitude of the solution-pressure, and for tbis it may serve. 

 Whether there is any real meaning attached to the solution- 

 pressure and to the process (common to Nernst's theory and 

 the above) of extrapolating the electromotive force of a con- 

 centration-cell into regions of unattainable concentration, is a 

 question to which I will not offer an answer. The reasoning- 

 may be taken merely as showing how to avoid the extra- 

 vagantly high pressures usually quoted. 



(v.) Osmotic Pressure of Concentrated Solutions. 

 Returning to the question of concentration-cells and the 

 actually measurable phenomena they exhibit, it would appear 

 that the chief use of electromotive measurements at the 

 present time may be to determine the osmotic pressure of 

 solutions of salts. The concentration attainable in salt solutions 



