Resistance of certain Amalgams with Temperature. 451 



§ 8. Probable Cause of the Changes in the case of 

 Zinc Amalgams. 



It has been observed by Regnauld * that in the formation 

 of amalgams of certain metals, viz., zinc, cadmium, and 

 others, there is an absorption of heat. Phipson f found that 

 when lead and bismuth, mixed in certain proportions, were 

 alloyed with mercury there was a fall in temperature 

 amounting in one case to 27°. This shows that in some 

 cases the formation of a compound of mercury and another 

 metal may be masked owing to the absorption of heat fol- 

 lowing from the solution of the compound in excess of mercury 

 being greater than the evolution of heat arising from the 

 actual formation of the compound. 



Lord Rayleigh \ has pointed out that the difference in the 

 behaviour of alloys and pure metals, as far as concerns their 

 electrical resistance, may arise from the heterogeneity of the 

 former. From this point of view, when a current is passed 

 through an alloy, it sets up between the particles of the dif- 

 ferent metals a series of Peltier effects proportional to the 

 current, and these create an opposing E.M.F. also propor- 

 tional to the current. It follows that this opposing E.M.F. 

 would be indistinguishable experimentally from a resistance. 

 If the alloy is a true chemical compound, this back E.M.F. 

 would not exist unless we assume the Peltier effects to take 

 place between the separate atoms forming a molecule of the 

 compound, which does not seem probable. 



If we look on the amalgam as containing particles of zinc 

 immersed in mercury or a compound of mercury and zinc, 

 then a rise in temperature might probably cause different 

 compounds to be formed, and the Peltier effects due to these 

 would be different from those existing originally. Part of 

 the decrease in resistance in the neighbourhood of 70° might 

 be explained in this way, but it would not account for the 

 whole, for Lord Rayleigh finds for copper and iron mixed in 

 equal volumes that the increase in resistance would be about 

 2 - 6 per cent, only; while, as Table 1. shows, the fall at Q 

 (fig. 3) is, in some cases, as much as 16 per cent., and this 

 although the thermo-electric properties of zinc and mercury 

 are less pronounced than for copper and iron. 



A simple method of explaining some of the results is as 

 follows : — The amalgam consists of a mixture of several dif- 

 ferent compounds of mercury and zinc together with zinc and 

 mercury in the free state. At 36° one of these compounds 



* Regnauld, Compt. Rend. vol. li. p. 778, vol. Hi. p. 533. 

 f Phipson, Bull. iSoc. Chim. vol. v. 

 \ Rayleigh, Nature, June 1896, p. 154. 

 9 12 



