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 séries 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? 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—ze. 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 Poggendorf’s Annalen der Phystk., Band \xxiv. p. 321, 1848. 
? “ Apercus sur le magnétisme des mineraux et des roches, et sur les causes 
de quelques anomalies du magnétisme terrestre,” Annales des Sciences, Socitté 
@ Agriculture, etc., de Lyon, tome xi. 1848. 
