THE ELECEROMAGNET IN PETROGRAPHY 47 
magnetic and less magnetic material is, as a rule, mechanically 
enclosed in the most magnetic portion of the first separation. 
Hence the operation should be repeated till no residue remains 
behind, or till the separation is satisfactorily made. In the 
presence of fine powder it is practically impossible to get a 
good separation in the dry way, due to the fact that non-magnetic 
grains are mechanically enclosed in a mesh of magnetic grains— 
a defect which seems to be largely eliminated by having the 
grains immersed in water during the separation. 
The actual details of the separation will depend upon the 
character of the specimen, and must be left to the judgment of 
the operator. A little experience suffices to indicate the best 
course to adopt in any given case. If a number of minerals of 
suitably varying permeability be present, the most permeable 
are first of all separated in a comparatively weak field, after 
which the field is intensified to secure the separation of a less 
magnetic ingredient. 
The manner in which the grains are presented to the poles 
depends upon whether they are strongly or weakly attracted. 
In the former case the grains should not be permitted to touch 
the poles, while in the latter case the poles and the grains 
should be brought into contact. Thus, if the poles be inserted 
in a mixture of ilmenite and garnet grains, with a field of 
medium intensity, both minerals are extracted. But if the 
grains be not allowed to touch the poles, it will be found that, 
at a certain distance, the ilmenite is withdrawn and the garnet 
left behind. Further, by this means less non-magnetic material 
is enclosed with the ilmenite than is the case when the poles are 
inserted among the grains. On the other hand, in the case of a 
mixture of monazite, zircon, and quartz, the poles may be in- 
serted among the grains, in order to extract the monazite, which 
is only slightly magnetic. 
It is, of course, not possible by the magnetic method to 
separate minerals which agree or differ only slightly in per- 
meability. For instance, hornblende cannot be separated from 
augite of similar composition ; garnet and rhombic pyroxene are 
commonly inseparable ; and some chromite cannot be separated 
from garnet. 
As an example of separation, we may cite the following. 
The specimen was a concentrate obtained by washing a river 
sand. The minerals present were ilmenite, magnetite, garnet, 
