14? Messrs. J. Tyndall atul H. Knoblauch on the 



ing it freely in the neighbourhood of the crystal. The ten- 

 dency of the needle was always to place itself parallel to the 

 magnecrystaliic axis. 



Crystals of bismuth lost their power of pointing at the mo- 

 ment the metal began to fuse into drops over a spirit-lamp or 

 in an oil-bath. " Crystals of antimony lost their magnecry- 

 stallic power below a dull red heat, and just as they were 

 softening so as to take the impression of the copper loop in 

 which they were hung." Iceland spar and tourmaline, on 

 the contrary, on being raised to the highest temperature which 

 a spirit-lamp could give, underwent no diminution of force; 

 they pointed equally well as before. 



Mr. Faraday finally divides the forces belonging to crystals 

 into two — inherent and induced. An example of the former 

 is the force by which a crystal modifies a ray of light which 

 passes through its mass; the second is developed exclusively 

 by magnetic power. To this latter, as distinct from the other, 

 Mr. Faraday has given the name magnet ocrystallic. To ac- 

 count for crystalline action in the magnetic field, we have, 

 therefore, the existence of three new forces assumed :— the 

 optic axis force, the magnecrystaliic force, and the magneto- 

 cry st all i c force. 



With regard to the experimental portion of Mr. Faraday's 

 labours on this subject, we have only to express our admira- 

 tion of the perfect exactitude with which the results are given. 

 It appears to us, however, a matter of exceeding difficulty to 

 obtain a clear notion of any such force as he has described ; 

 that is to say, a force proceeding from the pole of a magnet, 

 and capable of producing such motions in the magnetic field, 

 and yet neither attractive nor repulsive. 



That a crystal of bismuth should approach the magnetic 

 pole, and that a crystal of sulphate of iron, should recede 

 therefrom, appears, at first sight, anomalous, but certainly 

 not more so than other phsenomena connected with one of 

 Mr. Farada/s most celebrated discoveries, and explained in 

 a beautiful and satisfactory manner by himself. 



If we hang a penny from its edge in the magnetic field, and 

 so arrange the suspending thread, that the coin, before the 

 magnetic power is developed, shali make an angle of 45°, or 

 thereabouts, with the line joining the poles ; then, on closing 

 the circuit, and sending a current round the magnet, the coin 

 will suddenly turn, as if it made an effort to set itself from 

 pole to pole ; and if its position beforehand be nearly axial, 

 this effort will be sufficient to set it exactly so; the penny 

 thus behaving, to all appearance, as if it were attracted by 

 the poles. 



The real cause of this however is repulsion. During the 



