450 



crystal be suspended along any one of them, the two others set, one 

 axially, and the other equatorially. As there are two optic axes, 

 situated in the plane of the axes of greatest and least elasticity, so 

 there are two magnetic axes, characterized by the property already 

 mentioned. 



Among crystals, the author selected for special examination red 

 ferrocyanide of iron, sulphate of zinc, and formicate of copper. The 

 first is paramagnetic, the second diamagnetic, and in both cases the 

 principal axes of magnetic induction are determined by the planes of 

 crystalline symmetry. The setting of elongated prisms, as well as 

 of long cylinders and short cylinders or circular plates, cut in various 

 selected directions from the crystals, is described in detail. The use 

 of both cylinders and circular plates, cut with their axes in the 

 same direction, obviated any objection which might be raised attri- 

 buting the setting to the external form, since, so far as was due to 

 mere form, a cylinder and a circular plate would set with their axes 

 in rectangular directions. 



' Formicate of copper differs from the former crystals in having but 

 one plane of crystalline symmetry, and accordingly in having but one 

 principal axis of magnetic induction determined by the crystalline 

 form. The existence of three principal magnetic axes, having the 

 property already mentioned, was demonstrated experimentally, and 

 the directions of those two which were not determined by the cry- 

 stalline form, were ascertained by experiment. In this crystal the 

 axes of greatest and least induction, and consequently the magnetic 

 axes, lie in the plane of symmetry ; and the existence of two mag- 

 netic axes was demonstrated, and their positions were determined. 



In conclusion, the author gives a list of crystals, classified according 

 to their paramagnetic or diamagnetic characters, and the order of 

 magnitude of the magnetic inductions in the direction of their prin- 

 cipal axes. He also remarks that some crystals, of which instances 

 are given, though belonging according to their form to the biaxal 

 class, have two of their principal magnetic inductions so nearly 

 equal that they cannot be distinguished from magnetically uniaxal 

 crystals ; while others, though not belonging to the tesseral system, 

 have all their principal inductions so nearly equal that they cannot 

 be distinguished from amorphous substances. 



