of Crystalline Bodies between the poles of a Magnet. 181 



ever, showed that those whose optical axes were attracted 

 contained protoxide of iron in considerable quantity, while 

 those whose optical axes were repelled contained no trace of 

 this metal. 



Here, then, we have two crystals perfectly alike in optical 

 respects, but chemically different ; the change in the position 

 of the optical axis between the poles being doubtless due to 

 this difference. This seems to reduce that position to a mere 

 function, so to speak, of the chemical nature of the substance. 

 Could a salt of iron be introduced as an isomorphic substitute 

 for some other constituent in the whole class of diamagnetic 

 crystals, it is exceedingly probable that the position of the 

 optical axis between the poles would in most of these cases be 

 reversed, and this without in the slightest degree interfering 

 with the optical properties of the crystal. It is even likely 

 that Nature, as in the case before us, furnishes many examples 

 of this isomorphic substitution. If this be true, then the po- 

 sition of the optical axis between the poles is a mere accident, 

 and the introduction of it can only serve to render this already 

 difficult subject unnecessarily complex. 



On bringing a circular disc of gutta percha, which, in its 

 manufacture, appeared to have a fibrous structure imparted to 

 it, between the excited poles, the direction of these fibres set 

 itself strongly axial. This action was so decided, that a par- 

 allelogram, three-quarters of an inch long and half as wide, 

 with the fibres crossing it transversely, set itself stiffly equa- 

 torial. This can by no means be referred to the distance of 

 the parallelogram from the poles, or to any other of those 

 circumstances by which diamagnetic action is said to be ex- 

 hibited ; our voltaic power varied from one to twenty cells of 

 Bunsen's battery, but the result remained specifically constant ; 

 further, on being hung edgeways, the parallelogram stood 

 strongly magnetic; and when one pole was removed, the 

 whole mass was attracted by the other. 



Whence, then, this apparent diamagnetism of the gutta 

 percha ? The answer to this question will perhaps throw a 

 light upon the complicated phaenomena exhibited by crystals 

 generally. The equatorial position of the gutta percha is 

 manifestly due to the comparative facility with which the mag- 

 netic force can act in the direction of the fibre. Let us sup- 

 pose the parallelogram suspended, and the circuit closed; 

 every point of its substance is now affected, but not with equal 

 force in all directions ; in the direction of the fibre the action 

 is strongest, and may be represented by the longer diameter 

 of an egg, in the centre of which the point may be imagined. 

 All lines drawn from this centre to the shell will represent the 



