THE ELECTROMAGNET 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 at a certain distance, the ilmenite is withdrawn and the garnet 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, 



