THE ELECTROMAGNET IN PETROGRAPHY 37 
lends much support to this view, and it explains many 
peculiarities somewhat consistently. But it has long been 
known that the influence of any chemical constituent in deter- 
mining the magnetic power of a substance is not strictly 
additive, and is seriously modified by its mode of combination. 
Thus compounds of copper with the halogens are said to be 
paramagnetic,! although pure copper and the halogens are 
separately diamagnetic. Alloys exhibit some striking effects : 
for example, iron and nickel, both of which are powerfully 
magnetic, are stated to be capable of yielding an alloy which 
is practically non-magnetic. Such examples, however, appear 
to be somewhat exceptional. The investigations of St. Meyer? 
show that, as a rule, a compound consisting of two diamagnetic 
elements is itself diamagnetic, and that a compound consisting 
of two paramagnetic elements is itself paramagnetic. But the 
rule does not apply additively in a quantitative sense: for 
instance, the magnetic power of a compound consisting of two 
paramagnetic elements is less than the sum of the separate 
magnetic powers of the two elements. Again, when water 
enters into the composition of a mineral, it does not exert its 
full diamagnetic effect. The physical condition of a mineral is 
also an important factor in determining its magnetic permeability. 
Some of Delesse’s observations on the magnetic character- 
istics of the various groups of minerals are worthy of being 
recounted. The members of the amphibole group showed 
variation in accordance with the iron percentage. Actinolite 
exhibited a feebler power than common hornblende; antho- 
phyllite was feebler still, while tremolite was practically non- 
magnetic. The pyroxene group showed a variation analogous 
to that of the amphiboles. 
He thought the relatively feeble power displayed by biotite 
might be explained by the presence of fluorine, which he stated 
to be probably diamagnetic. Pluecker had already shown that 
the other halogens were diamagnetic. He was surprised at 
the feeble magnetic power displayed by the chlorites, seeing 
that they were comparatively rich in iron oxides and poor in 
silica. As, however, he included water in his list of dia- 
magnetic substances, we might have expected him to attribute 
1 St. Meyer states that this behaviour is due to impurity. 
2 “Magnetisirungszahlen anorganischen Verbindungen,” Stefan Meyer, Wied. 
Ann. der Physik, Band |xix. 1899. 
