22 Messrs. J. Tyndall and H. Knoblauch on the 



to that just described. Both crystals set their planes of clea- 

 vage equatorial. Both are diamagnetic. 



Let us now examine a magnetic crystal of similar form. 

 Sulphate of nickel is, perhaps, as good an example as we can 

 choose. Suspended in the magnetic field with its axis hori- 

 zontal, on exciting the magnet the axis will set itself from pole 

 to pole ; and this position will be persisted in, even when the 

 axial dimension is shortest. Suspended from its end, the cry- 

 stalline prism will take up an oblique position with considerable 

 energy. When the crystal thus suspended has come to rest, 

 mark the line along its end which stands axial. Let a knife- 

 edge be laid on this line, and pressed in a direction parallel to 

 the axis of the prism. The crystal will yield before the edge, 

 and discover a perfectly clean plane of cleavage. 



These facts are suggestive, The crystals here experimented 

 with are of the same outward form ; each has but one cleavage ; 

 and the position of this cleavage, with regard to the form of 

 the crystal, is the same in all. The magnetic force, however, 

 at once discovers a difference of action. The cleavages of the 

 diamagnetic specimens stand equatorial ; of the magnetic^ axial, 



A cube cut from a prism of scapolite, the axis of the prism 

 being perpendicular to two of the parallel faces of the cube, 

 suspended in the magnetic field, sets itself with the axis of the 

 prism from pole to pole. 



A cube of beryl, of the same dimensions, with the axis of 

 the prism from which it was taken also perpendicular to two 

 of the faces, suspended as in the former case, sets itself with 

 the axis equatorial. Both these crystals are magnetic. 



The former experiments showed a dissimilarity of action 

 between magnetic and diamagnetic crystals. In the present 

 case both are magnetic, but still there is a difference ; the axis 

 of the one prism stands axial, the axis of the other equatorial. 

 With regard to the explanation of this, the following fact is 

 significant. Scapolite cleaves parallel to its axis, while beryl 

 cleaves perpendicidar to its axis ; the cleavages in both cases, 

 therefore, stand axial, thus agreeing with sulphate of nickel. 

 The cleavages hence appear to take up a determinate position, 

 regardless of outward form, and seem to exercise a ruling 

 power over the deportment of the crystal. 



A cube of saltpetre, suspended with the crystallographic axis 

 horizontal, sets itself between the poles with this axis equa- 

 torial. 



A cube o /topaz, suspended with the crystallographic axis 

 horizontal, sets itself with this axis from pole to pole. 



W T e have here a kind of complementary case to the former. 

 Both these crystals are diamagnetic. Saltpetre cleaves parallel 



