THE ELECTROMAGNET IN PETROGRAPHY — 35 
by Delesse rank among the most important.’ 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 magnétique) 
of minerals as being to some extent a character of specific 
importance, and made a magnetically quantitative comparison 
of alarge 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 
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 bea 
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 magnétique des mineraux et des roches,” 
Annales des Mines, 1848, tome xiv. p. 429; also abstract of the same in Aznale 
de Chimie et de Physique, tome xxvi. 1849. 
