Magneto-optic properties of Crystals. 1 7 



not that which has been here effected by artificial means occur 

 naturally ? Must it not actually occur in most instances? for, 

 where perfect homogeneity of mass does not exist, there will 

 always be a preference shown by the forces for some particular 

 direction. This election of a certain line is therefore the rule 

 and not the exception. It will assist both the reader and us 

 if we give this line a name ; we therefore propose to call it 

 the line of elective polarity. In magnetic bodies this line will 

 stand axial, in diamagnetic equatorial. 



"The relation of the magnecrystallic force," says Mr. Fa- 

 raday, " to the magnetic field is axial and not equatorial." 

 This he considers to be proved by the following considera- 

 tions : — suppose a crystal of bismuth so suspended that it sets 

 with its maximum degree of force, then if the point of suspen- 

 sion be moved 90° in the axial plane, so that the line which in 

 the last case stood horizontal and axial, may now hang verti- 

 cal, then the action is a minimum : now, contends Mr. Fara- 

 day, if the force were equatorial this change in the axial plane 

 ought not to have affected it; that is to say, if the force act 

 at right angles to the axial plane, it is all the same which 

 point of the plane is chosen as the point of suspension. 



This seems a fair conclusion ; but the other is just as fair 

 — that, if the force be axial, a change of the point of suspen- 

 sion in the equatorial plane cannot disturb it. Mr. Faraday 

 finds the line of maximum force in sulphate of nickel to be 

 parallel to the axis of the prism. Whatever, therefore, be 

 the point of suspension in the plane perpendicular to the axis, 

 the action ought to be the same. On examining this crystal 

 it will probably be found that two opposite corners of the 

 parallelopiped are a little flattened. Let the prism be hung 

 with its axis horizontal and this fattening vertical, and after 

 the evolution of the magnetic force let the oscillations of the 

 prism be counted. Move the point of suspension 90° in the 

 equatorial plane, so that the flattening shall be horizontal, and 

 again count the oscillations. The numbers expressing the 

 oscillations in both cases will be very different. The former 

 will be a maximum, the latter a minimum. But if the force 

 be axial this is impossible, therefore the force is not axial. 



Whatever be the degree of conclusiveness which attaches 

 itself to the reasoning of Mr. Faraday drawn from bismuth ; 

 precisely the same degree attaches to the above drawn from 

 sulphate of nickel. The conclusions are equal and opposite, 

 and hence destroy each other. It will probably be found that 

 the reasoning in both cases is entirely correct ; that the force 

 is neither axial nor equatorial, in the sense in which these 

 terms are used. 



Phil, Mag, S. 3. Vol. 37. No. 247. July 1850. C 



