66 ELECTROCHEMICAL INVESTIGATION OF LIQUID AMALGAMS 



heat of dilution of cadmium amalgams is positive, not negative. In our 

 experience, this assumption is contrary to fact. With the help of Dr. H. 

 L. Frevert one of us has found that solid cadmium amalgams produce a 

 large cooling effect on dissolving in more mercury, and there is every 

 reason to believe that the dilution of liquid cadmium amalgams is like- 

 wise an endothermic reaction, although its thermal effect is so small as to 

 make accurate measurement difficult. The dilution of a 3 per cent 

 cadmium amalgam with an equal bulk of mercury would evolve over 30 

 joules of heat if Carhart's theory were correct, and this would have raised 

 the temperature of the calorimetric system by 0.02. So large a thermal 

 effect could not have escaped detection. 



This example remains the most precise verification of the Helmholtz 

 equation which has ever been offered, coming within the experimental 

 error of about o.i per cent. The interesting measurements of C'arhart " 

 show an average deviation of nearly 2 per cent. 



Turning now to lead, we find results very like those of zinc and tin. 

 Let us take the cell Pi-P2, of which TT O = 0.008960 and ATT for 29.96 C. 

 = 0.001175. Then 



rr vp ....................................... 1730 joules 



ATT 

 vFT -- .................................... 2068 joules 



U .................................... - 338 joules 



The attempt was made to verify this value by actual thermochemical 

 experiment, using the same apparatus as in the other cases already men- 

 tioned in the previous papers. The apparatus was not suited for exact 

 quantitative work, but the test was enough to show a very decided cooling 

 effect (of 0.018 C.) in the apparatus, and to confirm in sign and in order 

 of magnitude the figure calculated from the electrical measurements. 



Calculated in the same way from the electrical measurements of the 

 other lead cells, the values for the heat of dilution are found to decrease 

 as the dilution increases. Figures for three lead cells are given in table 

 21 to serve as typical examples of this phenomenon, which of course 

 appears in the measurements with other metals also, in so far as their 

 degrees of accuracy permit. It is interesting to note that the maximum 

 cooling effect of dilution has not been wholly reached in a solution contain- 

 ing only o.i per cent of lead (or I gram-atom in 15 liters) ; for an amal- 

 gam of this considerable dilution is still found to absorb 20 calories more 

 upon dilution to fourteen times its volume. This last exceedingly attenu- 

 ated material would probably absorb very little more on further dilution ; 

 hence the limiting value is probably not far off. According to these results, 



58 Carhart, Phys. Rev., March, 1908. 



