Magneto- optic Properties of Crystals. 15 



development of magnetic power, an electric stream is aroused 

 in the copper coin, which circulates round the coin in a di- 

 rection opposite to that of the current which passes from the 

 battery round the coils of the magnet. The effect of this in- 

 duced stream is to create a polar axis in the copper; and 

 when the direction of the stream is considered,* it is easy to 

 see that the north end of this axis must face the north pole 

 of the magnet, and will consequently be repelled. On looking 

 therefore at the penny, apparently attracted as above de- 

 scribed, we must, if we would conceive rightly of the matter, 

 withdraw our attention from the coin itself, and fix it on a 

 line passing through its centre, and at right angles to its flat 

 surface; this is the polar axis of the penny, the repulsion of 

 which causes the apparent attraction. 



We do not mean to say that any such action as that here 

 described takes place with a bismuth crystal in the magnetic 

 field. The case is cited merely to show that the " approach " 

 of the bismuth crystal, noticed by Mr. Faraday, may, be really 

 due to repulsion ; and the "recession " of the sulphate of iron 

 really due to attraction. 



Our meaning will perhaps unfold itself more clearly as we 

 proceed. If we take a slice of apple, about the same size as 

 the penny, but somewhat thicker, and pierce it through with 

 short bits of iron wire, in a direction perpendicular to its flat 

 surface, such a disc, suspended in the magnetic field, will, on 

 the evolution of the magnetic force, recede from the poles and 

 set itself strongly equatorial ; not by repulsion, but by the 

 attraction of the iron wires passing through it. If, instead of 

 iron, we use bismuth wire, the disc, on exciting the magnet, 

 will turn into the axial position ; not by attraction, but by 

 the repulsion of the bismuth wires passing through it. 



If we suppose the slice of apple to be replaced by a little 

 cake made of a mixture of flour and iron filings, the bits of 

 wire running through this will assert their predominance as 

 before ; for though the whole is strongly magnetic, the supe- 

 rior energy of action along the wire will determine the position 

 of the mass. If the bismuth wire, instead of piercing the 

 apple, pierce a little cake made of flour and bismuth filings, 

 the cake will stand between the poles as the apple stood ; for 

 though the whole is diamagnetic, the stronger action along 

 the wire will be the ruling agency as regards position. 



Is it not possible to conceive an arrangement among the 

 particles of a magnetic or diamagnetic body, capable of pro- 

 ducing a visible result similar to that here described? If the 

 magnetic and diamagnetic forces be associated with the par- 

 ticles of matter, is it not a reasonable inference, that the closer 



