OF THALLIUM, INDIUM, AND TIN 23 



The difference between the observed and the ideal values is usually 

 great; in the case of the concentrated cell, Ci-C2, it amounts to 13 per 

 cent. Further study of the figures shows that as the dilution is increased, 

 this difference between the observed and calculated potentials diminishes, 

 becoming only about 0.6 per cent in the case of the very dilute cell 03-04. 

 Deviations from the theoretical are always positive ; the cell always gives 

 a potential higher than the value computed simply from its concentrations. 

 Cells of thallium amalgams thus appear to behave in a fashion similar to 

 those of cadmium with increasing dilution, although in the case of the 

 thallium cells the deviations are larger. Zinc varies in the opposite 

 direction. 



The results of these measurements and calculations are plotted graphi- 

 cally below according to the method employed by Richards and Forbes, 

 which affords a convenient method of noting the departure of the cells 

 from the gas law. In fig. 6 there are plotted as abscissae the logarithms of 

 the volumes occupied by a given weight of amalgamated thallium, taking 

 the volume of the most concentrated amalgam in cup Ci as unity. The 

 progress of the curve in the direction of ordinates between the points 

 corresponding to any two volumes indicates the extent of the deviation 

 from the theory of the electromotive force of the cell made from the two 

 indicated amalgams. The curve is built up by plotting first the results 

 with cell C, then those with cells A and B, and finally those with cell D. 

 In each case as the drawing progressed the " parent amalgam " was 

 started at its proper concentration on the curve already drawn ; and this 

 proceeding of necessity fixed the other points obtained from that particular 

 cell. If into each cell a two-phase amalgam, having a constant potential, 

 had been introduced, according to the excellent suggestion of Hulett and 

 De Lury, the construction of this curve would have been somewhat facili- 

 tated ; but the final result would have been identical. In this case greater 

 care about perfect constancy of temperature would have been necessary. 

 The regularity of the curve affords strong evidence of the accuracy of the 

 measurements. 



The curve for the thallium amalgams, like those for both zinc and 

 cadmium, shows that as dilution is increased the potential of any cell 

 approaches nearer and nearer to the requirement of the simple concentra- 

 tion law ; that is to say, the slant of the curve becomes less and less. Com- 

 plete horizontality would indicate complete fulfilment of the gas law. The 

 regular form of the curve indicates the absence of oxidation in the more 

 dilute amalgams, one of the most insidious sources of error in this sort of 

 work. Thallium amalgams are extremely sensitive to oxidation and its 

 elimination in these measurements is a source of gratification. 



The results depicted by this curve will be discussed later in connection 

 with the results for the other metals. 



