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149 



we say that the potential of A is higher 

 than that of B, and the cause of the 

 current is assigned to the difference of 

 potential between the two bodies. Dif- 

 ference of potential may thus be 

 thought of as a difference of electrical 

 le\el. The unit in which difTerence of 

 potential is measured is called the volt. 



It was found that freshly cast sticks 

 of cadmium were negative to the solu- 

 tion of cadmium iodide in which they 

 were immersed, but the magnitude of 

 this difference was not constant under 

 apparently identical conditions. Upon 

 prolonged immersion in the solution of 

 cadmium iodide, however, a constant 

 dift'erence of potential was established, 

 the average difference between the ini- 

 tial and final values being abottt 0.0093 

 volt. 



\^arious hypotheses were advanced 

 to account for these facts, but the only 

 one which has thus far survived the 

 searching test of experiment, is that 

 which assumes the existence of two or 

 more forms of the metal cadmium. 

 Such different forms of the same ele- 

 ment are known as allotropic tnodifica- 

 tions, 



Allotropic modifications of an element 

 undergo transformation from one form 

 into the other at a definite temperature 



known as the transition temperature. 

 Obviously at this particular tempera- 

 ture there should be no difference be- 

 tween the electrical behavior of freshly 

 cast cadmium and that which has been 

 immersed for some hours in a solution 

 of cadmium iodide. This was actually 

 found to be the case at about 70° C. 

 Therefore, this temperature may be 

 considered as a close approximation to 

 the transition temperature of the two 

 forms of cadmium. 



Furthermore, if we were to construct 

 a cell having for its two poles sticks 

 of freshly cast cadmium, and cadmium 

 which had been transformed by pro- 

 longed immersion in a solution of cad- 

 mium iodide, and having as an electro- 

 lyte a solution of a cadmium salt, the 

 direction of the current furnished by 

 such a cell should be conditioned by 

 its temperature. Below 7o°C the fresh- 

 ly cast electrode should be positive, 

 while above 70° it should be negative. 



A careful series of experiments 

 served to completely verify the correct- 

 ness of this prediction. 



Among other facts it was observed 

 that whereas freshly cast cadmium has 

 a bright silvery lustre, that which has 

 been immersed in a solution of a cad- 

 mium salt for twenty-four hours ac- 

 quires a dull gray color. 



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