208 TRANSACTIONS OF THE 
are not particularly high grade, some of them being of 
such a lean character that it is a question whether means 
for their profitable treatment are at present in existence. 
There is one characteristic that these complex sulphide 
ores possess in common, that is, the minerals of lead, zinc, 
and iron which carry the values are very intimately 
associated. It is scarcely possible to conceive the extent 
to which this intimate association of the minerals exists 
without recourse to microscopic examination. The 
microscope shows that it is necessary to resort to ex- 
tremely fine grinding to liberate the individual minerals, 
which is necessary in order that they may be recovered 
and separated into marketable products of lead and zinc. 
When polished surfaces of specimens of complex sulphide 
ores are examined under the microscope the nature and 
extent of this intimate mineral association is readily 
seen and in order to apply this knowledge in a practical 
manner it is but a logical step to apply the principles of 
micrometry. 
The average size of the mineral grains as they exist 
in these ores can be measured by micrometric methods 
applied during the examination of polished surfaces pre- 
pared from representative specimens of the ores in ques- 
tion and with this information as a guide it is not a 
difficult matter to determine with approximate accuracy, 
the degree of grinding that will place the ore in a condi- 
tion in which the mineral grains composing it are most 
amenable to recovery by the treatment selected. Thus 
by a small amount of preliminary investigation the basic 
requirement for successful concentration has been met, 
i. e.; the mineral particles are freed or unlocked and in 
a condition in which their separation and recovery is a 
matter of the application of proper methods of treat- 
ment. Nothing could be more simple than this method 
of gaining the desired information and since it is one of 
the vital essentials to success in actual practice this pro- 
cedure should be made use of in mills and ore testing 
laboratories as a prerequisite to experimental or pre- 
liminary testing of ores by the processes of gravity con- 
centration or flotation. 
The same method of procedure is indicated in the 
case of ores in which the value-carrying minerals are 
