398 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



the freezing point sufficiently large to give the desired accuracy. Few 

 metals depress the freezing point at all, and those which do show a 

 eutectic point 1° or 2° below the normal. In a preliminary testing out 

 of this idea, the resistance of a 5 per cent Cd amalgam was measured 

 at 21°. 5. Beginning at between 1000 and 2000 kgm. the pressure 

 coefficient was found abnormally large over the pressure range up to 



LJ 



|007oHG. IOO%CD. 



PROPORTION H6 AND CD. 



Figure 17. Showing the raising of the melting curve of mixtures of mer- 

 cury and cadmium with increasing pressure. 



and including 12,000 kgm., which is the freezing pressure of pure mer- 

 cury at 21°. 5. The time required in reaching eciuilibrium was also 

 abnormally long. Both these facts indicate the separating out of some 

 component from the amalgam. Between 1 2,000 and 1 3,000, this process 

 of separation seemed to have been completed, and from 13,000 to 

 13,500 the behaviour was entirely normal. The explanation is simple. 

 The effect of pressure is to raise the entire melting diagram Hg— Cd, 

 the Hg end more than the Cd end. (See Figure 17.) This raising is 

 presumably accompanied by a shifting of the proportions of the two 

 metals present in the eutectic mixture. The initial temperature and 

 proportion of the two metals present in the amalgam may be represented 

 by the point A (in this case ^ = 21°.r) and x = 5 per cent), but when the 

 pressure has risen sufficiently to bring the melting curve- up to A, with 

 further rise of pressure A slides along the rising curve, always at con- 



