62 ELECTROCHEMICAL INVESTIGATION OF LIQUID AMALGAMS 



This approximate constancy of the value of TT obs. -n- calc. at different 

 temperatures had already been observed by Cady, 62 and was recognized by 

 him as proving the heat of dilution of the cell was constant over the range 

 of temperature used. His confirmation was far less exact than ours, 

 however. The relation is not only of theoretical interest, but is also 

 useful practically, as it offers a means of checking the potentiometer 

 measurements. 



The deviations from the exact fulfilment of Cady's equation must be 

 ascribed, as they were in the previous paper, to the inexactness of the 



expression In ~, as a means of estimating the free energy of the osmotic 



C<i 



effect. As was said before, these irregularities can hardly be traced with 

 exactness until precise measurements of the osmotic pressures of the amal- 

 gams have been made; and such are not yet available. The paper of 

 Richards and Forbes, already so often quoted, amplifies the obvious fact 

 that the formation of hydrargyrates in solution will tend to increase the 

 observed potential, while the polymerization of the dissolved metal will 

 tend to diminish it. This paper disclosed also the fact that if in the case 

 of cadmium allowance is made for the space occupied by the dissolved 

 cadmium, a large part of the difference between the theoretical and 

 observed values is eliminated. According to a previously made similar 

 observation of Morse and Frazer, the excessive osmotic pressure of 

 sugar solutions is to be corrected in a similar way. 



Very recently Lewis M has pointed out in an interesting paper that a 

 more generally accurate method of expressing osmotic effect is found in 

 the generalization expressed essentially as follows : ' The activity of a 

 substance is proportional to its mol-fraction." Thus instead of expressing 

 the electrically manifested osmotic effect of a concentration cell by the 



T~) r T~* T^ *T~* I 



equation TT= --,{- hi C} one may express it as ir= =- In , / , 

 vF c , vF n + Nj n + N, 



where n signifies the numbers of gram-molecules of dissolved substance 

 and N l and N 2 those of the solvent in the two amalgams respectively. 

 This is essentially an application of the equation of Raoult to electro- 

 motive force. On the same pattern the equation of Cady would become 



U RT , n + N., 



ft ^. H -pr In - ^. 



vF V F n + N, 



Neither of these equations is given in just this form by Lewis in his 

 paper, but each is an immediate outcome of his reasoning. 



E2 Journ. Phys. Chem., 2, page 562. 



53 Am. Chem. J., 34, i (1905) ; 37, 324, 425- SS8; 38, 175 (1907). 



M G. N. Lewis; J. Am. Chem. Soc., 30, 668 (1908). See also Lewis, Zeit. phys. 

 Chem., 61, 163 (1007). In connection with this latter article, read journ. phys. 

 Chem., 4, 389 (1900). 



