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.).  Or the crystallization may be irregular; or the substance may not be quite pure; and if we consider how minute a quantity of matter will alter greatly the conducting power of water, it will seem not unlikely that a little extraneous matter diffused through the whole or part of a cube, may produce effects sufficient to account for all the irregularities of action that have been observed.

1699.  An important inquiry regarding the electrical polarity of the particles of an insulating dielectric, is, whether it be the molecules of the particular substance acted on, or the component or ultimate particles, which thus act the part of insulated conducting polarizing portions (1669.).