224 TRANSACTIONS OF THE 
upon the relative concentrations of water vapor, hydro- 
chloric acid, oxygen and chlorine in the gases passing over 
the chloride. Too great a concentration of water vapor 
and too small a concentration of the chlorine gases will 
convert silver chloride into metallic silver and prevent 
its volatilization. The value of the equilibrium constant 
for this reaction has been obtained by the Bureau of 
Mines at the Pacific Station, Berkeley, California, and it 
is found that the concentration of water vapor sufficient 
to hydrolyze silver chloride and produce metallic silver, 
lies well within the range of possibilities in current metal- 
lurgical practice. 
The hydrolysis of lead chloride or of copper chlo- 
ride has so far received less attention because experience 
with ores shows that it is probably a less important factor 
with these metals. Experience in chloride volatilization 
with zinc-containing ores has demonstrated that the zinc 
is very little volatilized. This fact is not due to non-vola- 
tility of the zine chloride for this compound is very 
readily liquified and boiled. It is undoubtedly largely 
due to the readiness with which zinc chloride hydro- 
lyzes, though other causes may be contributory. 
The problem of the chemistry of the volatilization 
process may also be attacked from another direction, 
namely, by studying the behavior of sodium chloride at 
high temperatures. The vapor pressure curve of salt has 
never been determined. Data do exist which show that 
salt hydrolyzes to a considerable extent with water vapor, 
but more information is desired upon these problems 
and experiments are under way to secure it. 
To summarize, it seems that physical chemistry may 
contribute to the development of the chloride volatili- 
zation process by stating the conditions under which each 
metallic chloride will be formed from mixtures of common 
salt and the commonly occurring minerals of that metal; 
the chemical stability of the chloride in the presence of 
oxygen water vapor and other substances which may pos- 
sibly react with it, and the vapor pressure of the chloride 
at various temperatures. 
The question of recovery of the vaporized chlorides 
has not been discussed here. Originally the weakest link 
