10 
PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
These experiments might be extended indefinitely, but sufficient are here to enable 
us to deduce the law of action. In the first place we notice, that all those substances 
which set equatorial between the points, and axial above and below them, are diamag- 
netic ; while all those which set axial between the points, and equatorial above and 
below them, are paramagnetic. When any one of the substances here named is 
reduced to the spherical form, this rotation is not observed. I possess, for example, 
four spheres of calcareous spar, and when any one of them is suspended between the 
points, it takes up a position which is not changed when the sphere is raised or lowered; 
the crystallographic axis sets equatorial in all positions. A sphere of compressed car- 
bonate of iron, suspended between the points, also sets that diameter along which the 
pressure is exerted from pole to pole, and continues to do so when raised or lowered. 
A sphere of compressed bismuth, on the other hand, sets its line of compression 
always equatorial. The position taken up by the spheres depends upon the molecular 
structure of the substances which compose them ; but, when the mass is elongated, 
another action comes into play. Such a mass being suspended with its length hori- 
zontal, the repulsion of its ends constitutes a mechanical couple which increases in 
power with the length of the mass ; and when the body is long enough, and the local 
repulsion of the ends strong enough, the couple, when it acts in opposition to the 
directive tendency due to structure, is able to overcome the latter and to determine 
the position of the mass. In all the cases cited, it was so arranged that the length of 
the body and its structure should act in opposition to each other. Tartaric acid and 
citric acid cleave with facility in one direction, and, in the specimens used, the planes 
of cleavage were perpendicular to the length of the body. In virtue of the structure 
these planes tended to set equatorial, but the repulsion of the elongated mass by the 
points prevented this, and caused the planes to set axial. When however the body was 
raised or lowered out of the sphere of this local repulsion, and into a position where 
the distribution of the force was more uniform, the advantage due to length became 
so far diminished that it was overcome, in turn, by the influence of structure, and 
the planes of cleavage turned into the equatorial position. In the specimen of salt- 
petre the shortest horizontal dimension was parallel to the axis of the crystal, which 
axis, when the influence of form is destroyed, always sets equatorial. A full crystal 
of calcareous spar will, when the magnetic distribution is tolerably uniform, always 
set its axis at right angles to the line joining the poles ; but the axis is the shortest 
dimension of the crystal, and, between the points, this mechanical disadvantage com- 
pels the influence of structure to succumb to the influence of shape. A cube of 
calcareous spar, in my possession, may be caused to set the optic axis from pole to 
pole between the points, but this is evidently due to the elongation of the mass along 
the diagonals ; for, when the corner of the cube succeeds in passing the point of the 
pole, the mass turns its axis with surprising energy into the equatorial position, 
round which it oscillates with great vivacity. Counting the oscillations, I found 
that eighty-two were performed by the cube, when its axis was equatorial, in the time 
