34 Dr. Tyndall on the Progress of the Physical Sciences : 



Discs were taken from Iceland spar, carbonate of lime and iron, 

 and carbonate of iron, so that the axis in each instance lay in 

 the plane of the disc. The axis in every case turned slowly 

 into the equatorial position and finally remained there. 



A disc of arragonite exhibited the same deportment. The axis 

 stood equatorial. In this experiment particular precautions are 

 necessary, which shall be immediately pointed out. 



Beryl cut into the form of a shallow cylinder set its axis from 

 pole to pole. The plane of most eminent cleavage stood con- 

 sequently equatorial. 



A disc of tourmaline set itself so that the line perpendicular to 

 the axis stood equatorial. 



In all non-conducting substances it is known that the induc- 

 tion which takes place on the approach of an electrified body 

 continues for some time after the said body is removed. This 

 fixation of the fluid may be readily demonstrated with a disc of 

 glass. 



In rock-crystal and topaz this polarization is exhibited so 

 strongly, that if a disc be held for an instant between the two 

 poles, it will continue to assert this position against an inten- 

 tional torsion of the suspending fibre. 



Although similar phsenomena of polarity were exhibited more 

 or less by all the crystals examined, still, if we except arragonite, 

 they never attained to such a degree of intensity as to prevent 

 the assumption of the positions above described. 



If the crystal be a conductor, a continuance of the electric 

 state is not observed after the exciting cause has been removed. 

 A cylinder of bismuth, the axis of which was perpendicular to 

 the plane of most eminent cleavage, always set itself so that the 

 said plane formed an angle of 90° with the line joining the 

 poles. 



In the investigation on the magneto-optic properties of cry- 

 stals* carried out by Dr. Tyndall and myself, the position of 

 crystals between the poles of a magnet is referred to the peculiar 

 .aggregation of the material particles. The question occurs, Do 

 not the phsenomena above described permit of being referred to 

 a similar origin ? 



A fine powder of sulphate of barytes was mixed to a paste 

 with gum- water and pressed together in one direction. From 

 the mass when dried a circular disc was taken, so that the line 

 of compression was parallel to the plane of the disc. The direc- 

 tion of greatest compression takes up between the poles a posi- 

 tion similar to that assumed by the short diagonal of heavy spar 

 — it sets equatorial. 



* Philosophical Magazine, July 1850. 



