THE ELECTROMAGNET IN PETROGRAPHY 35 



by Delesse rank among the most important. 1 It is all the 

 more necessary to emphasise this work by Delesse since it 

 appears to have been hitherto almost completely ignored. 

 He regarded the magnetic power {pouvoir magnetique) 

 of minerals as being to some extent a character of specific 

 importance, and made a magnetically quantitative comparison 

 of a large number of minerals. To do this he crushed them, 

 and experimented with grains of uniform size in a field of 

 constant intensity. The maximum weight of such grains 

 adhering to the poles of the magnet, under uniform conditions, 

 was taken as an index of what he called the magnetic power of a 

 mineral. Thus, two minerals had the same magnetic power if, 

 under similar conditions, equal weights of them adhered to the 

 poles of the magnet. If the adhering weight of the grains of 

 one mineral was twice that of another, the magnetic power 

 of the former was twice that of the latter. In short, the 

 magnetic power was proportional to the weight of the 

 adhering grains, and consequently 



t 

 x — p 



where x = magnetic power of a mineral, 



p = maximum weight of adhering grains of this mineral, 

 P = maximum weight of adhering grains of some 



standard substance, the magnetic power of which 



may be taken as unity. 



This method of comparison has been stigmatised as primitive 

 and objectionable; but although it cannot lay claim to great 

 exactitude, it is at least simple and feasible, and proves to be a 

 very efficient and useful means of comparison. Especially when 

 we consider the fact that the magnetic field used is not uniform, 

 and that the permeability of a mineral is generally far from 

 constant, it seems clearly undesirable to sacrifice practical 

 utility, in the effort to attain a more than requisite degree of 

 accuracy. 



Using this mode of comparison, Delesse examined numerous 

 minerals, taking steel as his standard. The following list is 

 compiled from his results ; the number opposite a mineral 



1 See particularly " Sur le pouvoir magnetique des mineraux et des roches," 

 Annales des Mines, 1848, tome xiv. p. 429; also abstract of the same in Annate 

 de Chimie et de Physique, tome xxvi. 1849. 



