THE ELECTROMAGNET IN PETROGRAPHY 33 



Bellevue's hint, Cordier 1 proceeded to examine a large number 

 of sands obtained from the river beds, lake and sea shores, in the 

 vicinity of volcanoes. By means of a bar magnet he separated 

 the magnetite from these sands, and proved it by analysis to 

 be richly titaniferous. Throughout his later work on the 

 volcanic rocks themselves, Cordier used the magnet freely in 

 his researches, and was able to prove the world-wide distribution 

 of titaniferous magnetite as a constituent of eruptive rocks. 2 



The introduction by Sturgeon in 1825 of an electromagnet 

 bent in horseshoe fashion, disclosed the fact that by such means 

 a magnetic field of great intensity could be obtained. Following 

 this method of construction, M. Pouillet, about 183 1, had an 

 electromagnet made which was capable of supporting a weight 

 of about 800 kilograms. With it he was able to prove that 

 many substances were attractable by a magnet which were not 

 previously known to be magnetic. 



About 1845 Faraday 3 made his famous experiments with the 

 electromagnet, and examined a large number of substances, 

 including some minerals, distinguishing them as paramagnetic 

 or diamagnetic according as they were attracted or repelled by 

 the poles of a magnet. 



It is worthy of note that Faraday's paramagnetic substances 

 included manganese, chromium, cerium, and their compounds. 

 The magnet used by Faraday is interesting because of the 

 fact that he used adjustable pole pieces, which enabled him 

 to vary the distance between the poles, and thus to vary the 

 intensity of the magnetic field. (" The poles are 6 in. apart, the 

 ends are planed true, and against these move two short bars 

 of soft iron 7 in. long, 2\ in. wide, and 1 in. thick, which can be 

 adjusted by screws and held at any distance less than 6 in. from 

 each other. The ends of these bars form the opposite poles of 

 contrary name ; the magnetic field between them can be made of 

 greater or smaller extent, and the intensity of the lines of magnetic 

 force be proportionately varied " : loc. cit. vol. iii. p. 29.) Such 

 adjustable pole pieces are very useful as a means of varying the 

 intensity of the magnetic field, in making mineral separations. 



1 " Recherches sur differens produits volcaniques," Journal des Alines, 

 No. 124, vol. xxi. 1807. 



3 " De la maniere d'etre du fer titane dans les roches volcaniques, et du role 

 qu'yjoue ce mineral," Journal des Mines, No. 133, vol. xxiii. 1808. 



3 "Experimental Researches in Electricity," London, 1855. 



