PROFESSOR TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 11 
required to perform fifty-nine, when the axis stood from pole to pole. Heavy spar 
and coelestine are beautiful examples of directive action. These crystals, as is well 
known, can be cloven into prisms with rhombic bases : the principal cleavage is 
parallel to the base of the prism, while the two subordinate cleavages constitute the 
sides. If a short prism be suspended in a tolerably uniform field of force, so that its 
rhombic ends shall be horizontal, on exciting the magnet the short diagonal will set 
equatorial, as shown in fig. 2. If the prism be 
suspended with its axis and the short diagonal 
horizontal, the long diagonal being therefore 
vertical, the short diagonal will retain the equa- 
torial position, while the axis of the prism sets 
axial as in fig. 3. If the prism be suspended with 
its long diagonal and axis horizontal, the short 
diagonal being vertical, and its directive power 
therefore annulled, the axis will take up the 
equatorial position, as in fig. 4. Now as the 
line which sets equatorial in diamagnetic bodies 
is that in which the magnetic force acts most 
strongly*, the crystal before us furnishes a per- 
fect example of a substance possessing three 
rectangular magnetic axes, no two of which are 
equal. In the experiment cited in Table II., the 
mass was so cut that the short diagonal of the 
rhombic base was perpendicular to the length 
of the specimen. Carbonate of tin, and the other 
powders, were compressed by placing the pow- 
der between two clean plates of copper, and 
squeezing them together in a strong vice. The line of compression in diamagnetic 
bodies always sets equatorial, when the field of force is uniform, or approximately so ; 
but between points the repulsion of the ends furnishes a couple strong enough to over- 
come this directive action, causing the longest dimension of the mass to set equa- 
torial, and consequently its line of compression axial. 
The antithesis between the deportment of diamagnetic bodies and of paramagnetic 
ones is perfect. Between the points the former class set equatorial, the latter axial. 
Raised or lowered, the former set axial, the latter equatorial. The simple substitution 
of an attractive for a repulsive force produces this effect. A sphere of ferrocyanide 
of potassium, for example, always sets the line perpendicular to the crystallographic 
axis from pole to pole ; but when we take a full crystal, whose dimension along its 
axis, as in one of the cases before us, is six times the dimension at right angles to the 
axis, the attraction of the ends of such a mass is sufficient to overcome the directive 
* Phil. Mag. Ser. 4. vol. ii. p. 177. 
c 2 
