34 SCIENCE PROGRESS 



They require, however, to be somewhat modified, as will be 

 pointed out later. 



Contemporary with Faraday, Pluecker : examined some 

 minerals and many other substances by means of the electro- 

 magnet. He expressed the relative permeability of these 

 substances by a series of numbers, taking iron as a standard of 

 comparison, in somewhat the same manner as Delesse (see 

 below). 



While Faraday, Pluecker, and others were directing their 

 attention to the purely physical aspect of magnetic properties, 

 two French geologists, Fournet and Delesse, attended to the 

 matter more particularly with reference to its bearing on 

 minerals and rocks. Fournet's work 2 was directed towards an 

 explanation of the anomalies of terrestrial magnetism. For 

 this purpose he studied the magnetic properties of all the 

 important rock-forming minerals, not overlooking the fact that 

 even the felspars, due to ferromagnetic impurity, often affect 

 the magnetic needle. He distinguished magnetite, haematite, 

 and ilmenite as being magnetipolar. Other magnetic minerals 

 he described as showing simple magnetism — i.e. they were 

 attracted by a magnet, but gave no evidence of polarity. He 

 discussed the varying magnetic characters exhibited by rocks 

 of different types as well as minerals, and upon this basis pro- 

 ceeded to deal with the general effects of different rock masses 

 (granites, basalts, limestones, etc.) in modifying the earth's 

 magnetic field. It is noteworthy that Fournet did not use an 

 electromagnet in his observations. He relied on the use of 

 a simple permanent magnet, following the methods of H. B. de 

 Saussure and Haiiy. Fournet's paper is a compendium of 

 careful observations, and strikes one as being a document of 

 somewhat considerable value. Unfortunately, it appears not 

 to have received the attention it deserves. As an amplification 

 of the earlier work by Saussure, it merits careful consideration 

 by those who are interested in the bearing of petrography 

 on the subject of terrestrial magnetism. 



Of all the contributions dealing with the application of the 

 electromagnet to the study of minerals and rocks, those made 



1 Poggendorfs Annalen der Physik., Band lxxiv. p. 321, 1848. 



2 " AperQus sur le magnetisme des mineraux et des roches, et sur les causes 

 de quelques anomalies du magnetisme terrestre," Annates des Sciences, SotiHe" 

 d 1 Agriculture, etc., de Lyon, tome xi. 1848. 





