64 ELECTROCHEMICAL INVESTIGATION OF LIQUID AMALGAMS 



takes account only of the space occupied by the solvent, where 



R T AT 



TT= . - In ~. The latter method will evidently, as has been found by 

 vr AI 



Richards and Forbes, give a much higher value. For the cadmium cell 

 this was found to be 8.56 millivolts at o C., instead of about 8.3 given by 

 the equations above, the actually observed value being 8.67 at o. 55 



In any case, it is clear that the new method of calculating the results 

 from the equation of Raoult throws no light upon the major deviations 

 of the cells from the equation of Cady, for these deviations are far too 

 great to be explained by such insignificant alterations in the numbers 

 predicted by theory, and some of the changes are in the wrong direction. 

 On this account, it was thought unnecessary to recalculate the new theo- 

 retical values for each case. 



As an outcome of these considerations, one may say that while the 

 equation of Cady in one or other of its forms affords a fairly satisfactory 

 means of calculating the temperature coefficient of an amalgam cell (and 

 probably also of other cells in which there is but little change of heat 

 capacity), and the best available means of finding the potential without 

 electrical measurement, it does not afford a good method of determining 

 the heat of dilution. This latter quantity is to be much more accurately 

 found with the help of the equation of Helmholtz, to which the reader's 

 attention is now directed. 



EQUATION OF HELMHOLTZ. 



In the first part of this monograph the temperature coefficients of the 

 cells consisting of amalgams of thallium, indium, and tin were used for 

 computing the heat of dilution, according to the equation of Helmholtz. 

 The same calculation may now be applied to zinc, cadmium, and lead. 

 Turning first to the case of zinc, we may take the cell Mi-M3 where 

 71-0 = 24.237 millivolts and ATT between o and 29.96 C. = 2.799 millivolts." 

 Because the temperature coefficient has been shown to be very nearly if not 

 quite independent of the temperature, the value given may be used at o. 

 Then 



ir vp ...................................... 4679.2 joules 



ATT 



................................... 4926.0 j oules 



U .................................. 246.8 joules 



This value for the heat of dilution, 246.8 joules, or 59.0 calories, 

 is considerably greater than the value 52 joules found by actual ther- 

 mochemical experiments. The difference is due in part to the fact that 



55 Richards and Forbes, Carnegie Institution of Washington, Publication 56, p. 

 62 (1906). The values there given are for 23. 

 68 This paper, p. 45. 



