164 Prof. Tyndall on the Natwe of the Force by which 



and continues to do so when raised or lowered. A sphere of 

 compressed bismuth, on the other hand, sets its Une of compres- 

 sion ahvays 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 

 hoi'izoutal, the repuhion 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, hovvcver, 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 compels 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 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 



