12 PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
action due to structure, and to pull the crystal into the axial position between the 
points. In a field of uniform force, or between flat poles, the length sets equatorial, and 
it is the partial attainment of such a field, at a distance from the points, that causes 
the crystal to turn from axial to equatorial when it is raised or lowered. Beryl is a 
paramagnetic crystal, and when the influence of form is annulled, it always sets a line 
perpendicular to the axis of the crystal from pole to pole ; a cube of this crystal, at 
present in my possession, shows this deportment whether the poles are pointed or flat ; 
but in the specimen examined the dimension of the crystal along its axis was greatest, 
and hence the deportment described. It is needless to dwell upon each particular para- 
magnetic body: the same principle was observed in the preparation and choice of all of 
them ; namely, that the line which, in virtue of the internal structure of the substance, 
would set axial, was transverse to the length of the body. The directive action due to 
structure was thus brought into opposition with the tendency of magnetic bodies to 
set their longest dimension from pole to pole : between the points the latter tendency 
was triumphant ; at a distance, on the contrary, the influence of structure prevailed. 
The substance which possesses this directive action in the highest degree is carbo- 
nate of iron : when a lozenge, cloven from the crystalline mass, is suspended from the 
angle at which the crystallographic axis issues, there is great difficulty in causing the 
plate to set axial. If the points are near, on exciting the magnetism the whole mass 
springs to one or the other of the points ; and when the points are distant, the plate, 
although its length may be twenty times its thickness, will set strongly equatorial. 
An excitation by one cell was sufficient to produce this result. In the experiment cited 
the residual magnetism was found to answer best, as it permitted the ends of the 
plate to be brought so near to the points that the mass was pulled into the axial 
position. When the magnet was more strongly excited, and the plate raised so far 
above the points as to prevent its springing to either of them, it was most interest- 
ing to watch the struggle of the two opposing tendencies. Neither the axial nor the 
equatorial position could be retained ; the plate would wrench itself spasmodically 
from one position into the other, and, like the human spirit operated on by conflicting 
passions, find rest nowhere. 
The conditions which determine the curious effects described in the present chapter 
may be briefly expressed as follows : — 
An elongated diamagnetic body being suspended in the magnetic field, if the 
shortest horizontal dimension tend, in virtue of the internal structure of the sub- 
stance, to set equatorial, it is opposed by the tendency of the longest dimension to 
take up the same position. Between the pointed poles the influence of length usually 
predominates ; above the points and below them the directive action due to structure 
prevails. 
Hence, the rotation of such a diamagnetic body, on being raised or lowered, is 
always from the equatorial to the axial position. 
If the elongated mass be magnetic, and the shortest dimension of the mass tend. 
