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, 1 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 2 

 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 lxix. 1899. 



