60 ENERGY CHANGES INVOLVED IN DILUTION OF AMALGAMS. 



hydrargyrate containing two or more atoms of the solute to the molecule. 

 This equally plausible contingency simply postulates that a balanced reaction 

 such as 2Zn % Zn 2 or 2ZnHg n % Zn 2 Hg n -m + Hg m exists in the solution. 

 Such a reaction would of course be pushed from right to left by diluting the 

 amalgam. 



It has been suggested also that changes of heat capacity should be con- 

 sidered in probing to the uttermost the equation of Cady. In the present 

 case, however, this possibly disturbing effect is eliminated, for we have 

 shown that the change of heat capacity which takes place on diluting either 

 zinc or cadmium amalgam is so slight as to cause no suspicion that this phe- 

 nomenon can have anything to do with the observed irregularities. 



The most probable disturbing agencies having thus been discussed in 

 general, the particular cases under consideration may be considered. As has 

 been made clear, the amalgams of the two metals vary in opposite ways from 

 the theory, cells of zinc amalgams giving potentials too low and cells of 

 cadmium amalgam giving potentials too high. The two must, then, be con- 

 sidered separately ; and of the two, cadmium may most conveniently be con- 

 sidered first, because it presents the least irregularity. 



One of the striking facts in relation to cadmium amalgam is the fact that 

 its heat of dilution is so small as to be negligible. Therefore, the equation 

 of Helmholtz reduces practically to the form 



ttvF = vVT% 

 a 1 



making the thermodynamics of the problem as simple as possible. 



The only irregularity which this cell of cadmium amalgams manifests is 

 the following, namely, with all except the most dilute amalgams 



ttvF > RThi^- 



Thus from the volume-energy point of view the cadmium acts like hydro- 

 gen a gas " more than perfect." 



But in this equation the left-hand member represents a statement of fact, 

 and R and T are definite quantities whose product is only very slightly 

 uncertain. Therefore, it seems probable that the only remaining term, the 

 ratio of v 2 to v x , is not correctly chosen. 



It will be remembered that this term was always evaluated on the assump- 

 tion (i) that the metal dissolved in mercury is strictly monatomic, (2) that 

 it forms no compound with the solvent, and (3) that it obeys the laws of 

 perfect gases with exactness. If the first condition is not satisfied, the poten- 

 tial will be lowered ; if the second or the third is violated, the reverse effect 

 will probably prevail. 



