452 Mr. PL S. Willows on the Variation of the 



melts, and at a higher temperature we reach the melting- 

 point of another, this latter melting causing the fall in 

 resistance. The liquid occupies the space between the crystals 

 of solid matter and while fluid dissolves some of the zinc. 

 When the amalgam is cooled the liquid which has the higher 

 melting-point persists until a temperature corresponding to 

 that at R (fig. 3) is reached, when the whole mass becomes 

 solid again. The fall in resistance that ensues when the 

 amalgam is allowed to stand might be explained on the 

 supposition that when solidification takes place the zinc which 

 has been dissolved is not entirely separated, and this slow 

 separation causes the fall in resistance. Some support is lent 

 to this view of partial fusion by the fact that when amalgam 

 is heated in a beaker, at a temperature somewhere below 100°, 

 a few small globules of liquid are squeezed from the interior 

 and appear on the surface, although the mass as a whole 

 shows no signs of liquefaction. The temperature at which 

 this occurs was not definitely fixed. Against this theory 

 it may be pointed out that it is improbable that the liquid 

 should persist when the amalgam is cooled for 40° below its 

 melting-point, as the straight part of the resistance-curve 

 from 100° to 25° (fig. 3) would lead us to infer. Also from 

 curve C (fig. 6) we see that whatever is the change that is 

 completed at 36°, if this is only partially brought about by 

 the amalgam being heated to 34°, then this changed state 

 exists while the temperature falls to 15°, and takes several 

 weeks to be completely destroyed. This would be unlikely 

 if it were a simple case of fusion. 



A theory which covers most of the results obtained is the 

 following : — 



From what has been said at the beginning of this section 

 it is seen that when zinc combines with excess of mercury 

 there is, on the whole, an absorption of heat. If, on the 

 other hand, a compound of zinc and mercury exists in solution 

 in mercury, and it is by some means dissociated, there will 

 be an absorption of heat if the dissociated zinc still remains 

 in solution ; but if the zinc, in addition to being dissociated, 

 goes out of solution, there will be an evolution of heat. 

 There would be an absorption of heat if a solid compound 

 was dissociated. 



The amalgam is looked on as consisting of a mixture of 

 zinc, mercury, and compounds of zinc and mercury. As the 

 temperature is raised to 36°, one of the compounds becomes 

 unstable, and one of two things may occur ; either the com- 

 pound is dissociated wholly or in part, or a new and more 

 stable compound is formed by further combination with the 



