certain binary alloys in high vacua. 33 
quantity of lead in the form of a thin mirror was condensed. Such 
an experiment might indicate that magnesium is the more 
volatile at that temperature, but it would be premature to con- 
clude that this is the case in the absence of knowledge of the 
relative viscosities of the two vapours. 
While the experiments recorded in the present communication 
have not been successful in effecting the isolation of intermetallic 
compounds of the metals under consideration, a comparison of the 
result obtained in the case of the magnesium-zinc series with that 
obtained with the magnesium-lead series is not without interest. 
The maximum on the freezing point curve of the magnesium-zine 
series corresponding to the compound is remarkably sharp, while 
the maximum on the freezing point curve of the magnesium-lead 
system is rounded. It is clear that the less sharp the summit of 
the curve, the more the compound will tend to dissociate into 
its constituents on heating above its melting point. It is, there- 
fore, not surprising that all the author's attempts to distil the 
compound Mg,Pb resulted in showing that, in the state of vapour, 
this compound is largely dissociated. 
VOL. XVII. PT. I. 3 
