Magneto-optic Properties of Crystals. 31 



the model coinciding with that which represented the optical 

 axis. This model imitated, in every respect, the deportment 

 observed by Mr. Faraday. Between the near points the opti- 

 cal axis stood from point to point, between the distant points 

 equatorial. It cannot however be imagined that the optic 

 axis force survived the pounding, dissolving and precipitating. 

 Further, this optic axis force is a sword which cuts two ways; 

 if it be assumed repulsive, then the deportment of carbonate 

 of lime and iron is tmexplainable: if attractive, it fails in the 

 case of Iceland spar. 



It is a remarkable fact, that all those crystals which exhibit 

 this phenomenon of turning round, cleave, either perpendi- 

 cular to their axes, or oblique to them, furnishing a resultant 

 which acts in the direction of the perpendicular. Beryl is an 

 example of the former ; the crystal just examined, Iceland 

 spar, is an example of the latter. This is exactly what must 

 have been expected. In the case of a magnetic crystal, clea- 

 vable parallel to its length alone, there is no reason present 

 why the axial line should ever be forsaken. But if the clea- 

 vages be transverse, or oblique, so as to furnish a line of elec- 

 tive polarity in the transverse direction, two diverse causes 

 come into operation. By virtue of its magnetism, the crystal 

 seeks to set its length axial, as a bit of iron or nickel would 

 do ; but in virtue of its mechanical structure, it seeks to place 

 a line at right angles to its length axial. For the reasons 

 before adduced, if the near points be used, the former is tri- 

 umphant; if the points be distant, the latter predominates. 



We noticed in a former paper a description of gutta-percha 

 of a fibrous texture, which, on being suspended between the 

 poles, was found to transmit the magnetic force with peculiar 

 facility along the fibre. A piece was cut from this substance, 

 exactly the same size as the tourmaline crystal, described at 

 the commencement of this section. The fibre was transverse 

 to the length of the piece. Suspended in the magnetic field, 

 the gutta-percha exhibited all the phenomena of the crystal. 



One of the sand-paper models before described is still more 

 characteristic as regards this turning round on the removal 

 of the poles to a distance. We allude to that whose magnetic 

 layers of emery are perpendicular to its length. The deport- 

 ment of this model, if we except its greater energy, is not to 

 be distinguished from that of a prism of beryl. Between the 

 near points both model and crystal stand axial, between the 

 distant points equatorial, and between the flat poles the de- 

 portment, as before described, is exactly the same. The 

 magnetic laminae of beryl occupy the same position, with re- 

 gard to its axis, as the magnetic laminae of the model, with 



