710 PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 
affinities on different bounding planes, between a growing crystal and the fluid from 
which it is being formed, and the cleavage properties (different specific capacities 
for resisting stress in different directions) afford the primary illustrations of this 
statement. It is probable that the proposition asserted is a universal proposition in 
the sense, that there is no kind of physical agency falling under the category referred 
to, which does not meet with different capacities for receiving it in different direc- 
tions in some crystals. There certainly may be, and probably are, crystals which 
transmit certain physical agencies equally in all directions. Crystals of the cubical 
system, for instance (unless possessing the conceivable dipolar rotatory property* * * § , 
from which some, if not all, are certainly exempt), conduct heat and electrieity 
equally in all directions, and have equal magnetic inductive capacities and equal 
thermo-electric powers. But thermal and electric conductivity, magnetic inductive 
capacity, and “thermo-electric power-f"” are undoubtedly different in different direc- 
tions in many, if not in all, crystals not of the cubical system. Many crystals have 
not shown any marked difference in their absorption of light according to the direc- 
tion of its propagation through them ; but some undoubtedly do show a difference of 
this kind, to such a degree as to give sensibly different colour to light passing 
short distances through them in different directions^:. Faraday had good reason, 
after making the discovery of the induction of electro-polarization in non-conducting 
substances^, to try the specific directional qualities of crystals used as dielectrics ; 
and although he found no sensible differences in the inductive capacities of the 
crystals (rock crystals and Iceland spar) which he tried for this kind of action, in 
different directions, it appears highly probable that induced electro-polarization will 
sooner or later be ascertained to be no exception to the general rule. 
105, Another very general principle is, that any directional agency applied to a 
substance may give it different capacities in different directions for all others. 
Whether or not this is true as a universal proposition, events have proved that the 
probability of its being true in any particular case is quite sufficient to warrant an 
experimental inquiry. Brewster discovered that mechanical stress induces in glass 
directional properties with reference to polarized light, which are lost as soon as the 
stress originating them is removed. These properties were shown by Fresnel to be 
of the same kind as the property of double refraction possessed by a natural crystal. 
Experiments made by Sir David Brewster and Mr. Clerk Maxwell prove that 
isinglass and other gelatinous substances dried under stress, thin sheet gutta-percha 
* See “Dynamical Theory of Heat,” § 168; also §§ 163, 166, 167, 169 to 171, Transactions of the Royal 
Society of Edinburgh, May 1854. See also Professor Stokes “ On the Conduction of Heat in Crystals,” Cam- 
bridge and Dublin Mathematical Journal, Nov. 1851. 
■f Or thermo-electric difference from a standard metal. See “ Dynamical Theory of Heat,” § 140. 
t Most crystals not of the cubic system, even when nearly colourless, exhibit difference. See Haidingee’s 
‘ Researches.’ 
§ Experimental Researches in Electricity, Series XIV. §§ 1688, 1689, 1692 to 1698. June 1838. 
