166 Prof. Tyndall on the Nature of the Force hj which 



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 

 powder between two clean plates of copper, and squeezing them 

 together in a strong vice. The line of compression in diamag- 

 netic bodies always sets equatorial, when the field of force 

 is uniform, or approximately so j but between points the repul- 

 sion of the ends furnishes a couple strong enough to overcome 

 this directive action, causing the longest dimension of the 

 mass to set equatorial, 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 equatoi'ial, the latter axial. Raised or lowered, 

 the former set axial, the latter equatorial. The simple substi- 

 tution 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 dimen- 

 sion at right angles to the axis, the attraction of the ends of 

 such a mass is sufficient to overcome the directive 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 prepara- 

 tion 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 trium- 

 phant ; at a distance, on the contrary, the influence of structure 

 prevailed. The substance which possesses this directive action 

 in the highest degree is carbonate of iron : when a lozenge, 



