UTAH ACADEMY OF SCIENCES 223 
may be converted into chlorides by merely heating with 
sodium chloride. Beyond the fact that the heavy metal 
chlorides are formed, however, comparatively little is 
known as to the actual chemistry involved. Rarely is 
all the gold, silver or copper extractible by leaching. 
A part is almost always retained because of physical 
or chemical reasons not at all obvious and the partic- 
ular ore used, the mechanical preparation, such as de- 
grees of fineness, and the details of roasting procedure 
all affect the recoveries. 
The various explanations offered as to the chemistry 
of chloridizing roasting,—and these have been rather 
numerous and conflicting, must therefore be regarded as 
hypotheses only. It has, for example, been claimed that 
the presence of a certain small percentage of sulphur in 
the ore is essential and that this sulphur by first form- 
ing sulphates of the metals makes possible the formation 
of chlorides by direct metathesis. Others claim that the 
chlorine must be liberated as free chlorine gas to be ef- 
ficient in combining with the metals. The hypothesis has 
also been advanced that the formation of sulphur chlo- 
rides is an essential intermediate step in the process. 
In order to make progress in a problem so complex 
it is necessary to analyze it and subdivide it into as 
many and as simple problems as may be necessary for the 
securing of convincing answers. The obvious primary 
subdivision in this case is a study of each metal in its re- 
actions with sodium chloride. In the studies of the Bu- 
reau of Mines the greatest progress has thus far been 
made with silver. It has been shown that all the commonly 
occurring silver minerals, with the exception of the chlo- 
ride, dissociate at elevated temperatures in the presence 
of oxygen giving metallic silver. The problem then be- 
comes how may metallic silver be converted into its 
chloride. If chlorine gas or hydrochloric acid gas is pas- 
sed over heated silver, chloride of silver is formed. If 
water vapor is passed over heated silver chloride metal- 
lic silver is produced and hydrochloric acid, oxygen and 
chlorine gas pass off. The reaction between silver chlo- 
ride and water vapor is therefore a reversible reaction 
and the complete volatilization of silver chloride depends 
