Prof. Pliicker on the Magnetic Induction of Crystals. 479 
The results thus obtained from theory, and verified by experiment, 
with reference to an amorphous ellipsoid, were compared with the 
results obtained from the observation of crystals, and manifested a 
complete agreement. According to this theory, the magnetic induc- 
tion within a crystal is, like the elasticity of the luminiferous ether, 
determined by means of an auxiliary ellipsoid. As there are three 
rectangular axes of optical elasticity, so there are three principal 
axes of magnetic induction, characterized by the property that if a 
erystal 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 formiate 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. 
Formiate 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 ery- 
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. 
