288 Prof. Tyndall on the Nature of the Force by which 



Cubes of crystallized bismuth. 



Repulsion when the force was directed 



Strength of magnet. 



3-6 



5-7 



8-4 

 100 

 11-9 



along the cleavage, across the cleavage. 



11-7 8 



34-8 23 



78 53 



111 76-5 



153 110 



It is manifest from this table that bismuth behaves as a body of 

 considerably superior diamaguetic power when the force acts 

 along the planes of cleavage. 



Let two indefinitely thin needles be taken from sucli a mass, 

 the one with its length parallel, and the other with its length 

 perpendicular to the planes of cleavage ; it is evident that if two 

 such needles be formed into a cross and subjected to experiment 

 in the manner above described, the former will act the part of 

 the more powerfully diamagnetic needle, and produce similar 

 effects in the magnetic field. 



We now pass on to the demonstration of the proposition, that 

 it is not necessary that the crystallized masses should be elon- 

 gated to produce the effects exhibited by the prisms in the ex- 

 periments already re- -p- 

 corded. Let us suppose ^' 

 the ends of our rect- 

 angular box to be com- 

 posed of cubes, instead 

 of elongated masses, 

 of crystallized bismuth, 

 and let the planes of 

 principal cleavage be 

 supposed to be jiarallel 

 to the face ab, fig. 5. 

 Let the continuous line 

 de represent an indefi- 

 nitely thin slice of the 

 cube passing through 

 its centre, and the dot 



5. 



ted line r/f a similar slice in a perpendicular direction. These 

 two slices manifestly represent the case of the cross in fig. 4 ; 

 and were they alone active, the rectangular box, in a uniform 

 field of magnetic force, must turn in the direction of the arrow. 

 Comparing similar slices in pairs on each side of those two cen- 

 tral slices, it is manifest that every pair parallel to the line de 

 represents a stronger mechanical couple than every corresponding 



