of an inch in the side. The average of many
experiments gave 100 for the specific inductive capacity coinciding with
the direction of the optical axis, and 98.6 and 99.92 for the two other
directions.

1694. Lord Ashley, whom I have found ever ready to advance the cause of
science, obtained for me the loan of three globes of rock crystal belonging
to Her Grace the Duchess of Sutherland for the purposes of this
investigation. Two had such fissures as to render them unfit for the
experiments (1193. 1698.). The third, which was very superior, gave me no
indications of any difference in the inductive force for different
directions.

1695. I then used cubes of Iceland spar. One 0.5 of an inch in diameter
gave 100 for the axial direction, and 98.66 and 95.74 for the two cross
directions. The other, 0.8 of an inch in the side, gave 100 for the axial
direction, whilst 101.73 and 101.86 were the numbers for the cross
direction.

1696. Besides these differences there were others, which I do not think it
needful to state, since the main point is not confirmed. For though the
experiments with the first cube raised great expectation, they have not
been generalized by those which followed. I have no doubt of the results as
to that cube, but they cannot as yet be referred to crystallization. There
are in the cube some faintly coloured layers parallel to the optical axis,
and the matter which colours them may have an influence; but then the
layers are also nearly parallel to a cross direction, and if at all
influential should show some effect in that direction also, which they did
not.

1697. In some of the experiments one half or one part of a cube showed a
superiority to another part, and this I could not trace to any charge the
different parts had received. It was found that the varnishing of the cubes
prevented any communication of charge to them, except (in a few
experiments) a small degree of the negative state, or that which was
contrary to the state of the inductric ball (1564. 1566.).

1698. I think it right to say that, as far as I could perceive, the
insulating character of the cubes used was perfect, or at least so nearly
perfect, as to bear a comparison with shell-lac, glass, &c. (1255). As to
the cause of the differences, other than regular crystalline structure,
there may be several. Thus minute fissures in the crystal insensible to the
eye may be so disposed as to produce a sensible electrical difference
(1193.).