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, 1 Langguth, 2 and Witter. 3 



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 mode 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. 



3 " Die Entwicklung der magrietischen Aufbereitung," Zeitschrift f. Electro- 

 chemie, Band vi. 1899. 



3 Zeitschrift/. Angewandte Chemie, 1899. 



