THE “ELECTROMAGNET- IN PETROGRAPHY ‘33 
Bellevue’s hint, Cordier! proceeded to examine a large number 
of sands obtained from the river beds, lake and sea shores, in the 
vicinity of voleanoes. 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.” 
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 1831, 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 * 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, 24 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”: doc. cit. vol. ili. 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 différens produits volcaniques,” /ournal des Mines, 
No. 124, vol. xxi. 1807. 
? “De la maniére d’étre du fer titané dans les roches volcaniques, et du réle 
qu’y joue ce minéral,” Journal des Mines, No. 133, vol. xxiii. 1808. 
3 “ Experimental Researches in Electricity,” London, 1855. 
