THE ELECTROMAGNET IN: PETROGRAPHY 43 
through a glass tube, which was placed vertically between the 
poles of the magnet. The tips of the poles were only 5 mm. 
apart, and were chisel-edged, so as to concentrate the lines of 
force. Mann had little or no confidence in the use of the electro- 
magnet quantitatively. He made numerous attempts to esti- 
mate the amount of augite in his phonolites by this method, 
doubtless encouraged by Doelter’s example, but his results 
showed so much variation that he abandoned them as untrust- 
worthy. As already pointed out, however, such rocks can 
scarcely be expected to yield even approximately accurate 
quantitative results. 
It is beyond the scope of this article to deal with the 
many and elaborate inventions made during the last twenty 
years or so, by Wetherill and others, in applying the electro- 
magnet on a large scale to ore-dressing operations. ‘Those 
requiring information on this matter may be referred to the 
book by M. Korda already quoted (published by L’Eclairage 
Electrique, 40, Rue des Ecoles, Paris), and also to the descrip- 
tions given by Richards,! Langguth,? and Witter.’ 
From the foregoing review of work done on the subject of 
the magnetic method of mineral separation, it is clear that the 
possibilities of the method have been somewhat fully considered 
by the various workers who have used it. In spite of this fact, 
however, any one wishing to use the electromagnet in petro- 
graphical work would find some difficulty in getting helpful 
guidance as to the best mede of procedure. To such, the 
following practical hints, tested in the course of some experience 
with the electromagnet, may be found useful. 
3. THE APPARATUS REQUIRED 
If expense can be disregarded, a magnetic separator of the 
Wetherill pattern is well worth having, especially when experi- 
ments are to be done on a large scale. A good example of the 
use of such a separator in this way is provided by a recent 
investigation as to the commercial value of certain heavy placer 
1 Ore Dressing, vol. ii. 1903. Published by The Engineering and Mining 
Journal. 
? “Die Entwicklung der magnetischen’ Aufbereitung,” Zeztschrift f- Electro- 
chemie, Band vi. 1899. 
3 Zeitschrift f. Angewandte Chemie, 1899. 
