222 _TRANSACTIONS OF THE 
the chlorides of the metals are in general much higher 
than the pressures of the metals themselves at the same 
temperatures and if the metals can readily be converted 
into chlorides volatilization of these compounds may pos- 
sibly prove a desirable process. 
The vapor pressures of but few metal chlorides have 
been experimentally determined. The Pacific Station® 
of the Bureau of Mines has found the vapor pressure of 
lead chloride to be 1 mm. at 567° and 50 mm. at 747°. 
The vapor pressure of Ag is 1 mm. at about 850° and 50 
mm. at about 1200°. Since silver is usually present in 
minor quantities it is not so essential that it should have a 
high vapor pressure. The vapor pressures of cuprous 
‘chloride, of zinc chloride, of iron chloride and of gold 
chloride are not known but it is known that these com- 
pounds volatilize even more readily than lead chloride. 
Temperatures of 1000° Centigrade or even higher are 
readily obtainable and the mere volatilization of the 
metal chlorides therefore, offers no theoretical difficul- 
ties. 
Formation of the metal chlorides and keeping them 
undecomposed once they are formed present more dif- 
ficult problems. 
The process of forming metal chlorides by subject- 
ing them to the action of chlorine gas has long been 
known and it is entirely feasible in the laboratory but 
the engineering difficulties associated with the handling 
of this very corrosive gas as well as its relatively high 
cost and small production have thus far prevented its 
wide application. Instead, attention is centered on the 
direct metathesis of metal compounds with the compara- 
tively cheap sodium chloride. 
Formation of copper chlorides by the roasting of 
calcined cupriferous pyrite mixed with common salt has 
been in use for over seventy years as the Longmaid- 
Henderson process for the lixiviation of copper. The 
chloridizing roasting of gold and silver ores prepara- 
tory to lixiviation has been practiced nearly as long. 
It is thus evident that gold, silver and copper minerals 
5Located at University of California, Berkeley, California. 
