220 TRANSACTIONS OF THE 
densing the vapors seems to offer possibilities. The pro- 
cess of thus separating the mixed metallic content of an 
ore from the other constituents by converting the metals 
into chlorides and vaporizing these chlorides has been 
under experiment for about thirty years. It shoud be 
borne in mind that to present smelting processes are 
economically satisfactory for the richer grades of ore, 
but that to meet the world’s future demands for metals 
it is becoming increasingly necessary to do two things; 
first, reduce losses in the smelting of ores, and second, 
develop methods of economically treating poorer grades 
of ore. Present experimentation on the chloride vola- 
tilization process is directed rather more to the second 
of these objects, but it is to be noted that success is 
hardly to be achieved in any treatment of low grade ores 
unless recoveries are fairly complete. The fact that 
metal chlorides may be vaporized from ores of several 
types is well established, but this is not sufficient. It 
must also be established that the vaporization, and sub- 
sequent recovery of the vapors can be quite complete. 
Pure science may perhaps be of assistance here in des- 
cribing the conditions under which the maximum com- 
pleteness may be expected. Let us first consider briefly 
the physical principles which underlie the process of 
vaporization. 
Every solid or liquid substance may be considered 
to have some tendency to send out molecules from its 
own body into any space surrounding it A- metal, such 
as gold, appears to have no tendency to waste its sub- 
stance by evaporation, for an actual decrease in weight 
of a piece of gold has never been authentically recorded. 
Our belief that it is, nevertheless, continually wasting 
away by evaporation, though at a rate far too small to 
detect, rests upon an abstraction. 
The first general law of evaporation is to the effect 
that evaporation into any enclosed space continues until 
a certain fixed pressure of vapor is attained, after which 
a condition of equilibrium persists unless the temperature 
is changed or part of the vapor is removed. 
The change in the characteristic vapor pressure with 
change in temperature is of great importance. The law 
