62 



ENERGY CHANGES INVOLVED IN DILUTION OF AMALGAMS. 



Calculated thus, b in cell 1-5 is found to be 15.8 milliliters, in cell 1-2, 14.4 

 milliliters, in cell 2-5, 12.7 milliliters, and in cell 3-8, 21 milliliters, for an 

 amount of amalgam containing a gram-atom of cadmium. Owing to the 

 extreme dilution, the value for cell 3-8 has a very great probable error and 

 may be rejected. The average of the others is 14.3 milliliters, a quantity 

 very near the gram-atomic volume of cadmium, 13.0. Thus if from the 

 actual volume of the amalgam the space originally occupied by the cadmium 

 is subtracted, the remaining volumes v 5 and v 9 very nearly fulfill the equation 



RTln **. 



irv. 



F. 



v. 



The following table illustrates this relation, and is comprehensible without 

 further comment : 



Evidently, while the correction greatly improves the agreement between 

 the theoretical and the observed values, there is still a slight discrepancy, 

 especially with the stronger amalgams. The " useless volume " is in most 

 cases slightly larger than the volume of the cadmium. Does this signify 

 that the cadmium expands on amalgamation, while causing the mercury 

 which surrounds it to contract somewhat more than it expands? This is 

 conceivable, although the total effect on amalgamating a gram-atom of cad- 

 mium is a contraction of about 1.3 milliliters, as calculated from the densi- 

 ties. Or, on the other hand, does some of the mercury, combining with the 

 cadmium, remove itself from the possibility of functioning as a solvent and 

 thus add to the useless volume ? Yet another possibility also exists. It may 

 be, as has been already suggested, that this value of the useless volume is 

 really nothing but an accidental balance between the opposing tendencies 

 that the true value is really considerably larger, including at least a gram- 

 atom of mercury in addition to one of cadmium, but that the true value does 

 not appear because of the counterbalancing presence of polymerization. It 

 will be seen that in the case of zinc this latter effect seems to be the pre- 

 dominant one. 



Further light could be obtained by determining the osmotic pressure of the 

 cadmium in solution, because this would be dependent upon the sum of the 

 osmotic pressures of the various molecular species, instead of having their 

 effect counterbalance. If the deviation of this value of the osmotic pressure 



