PKOFRSSOll TYNDALL ON THE DIAMAGNETIC FORCE, ETC. 
41 ) 
needle of the powerfully diamagnetic substance bears to that of the feeble one. An 
inspection of the table at page 44 will show that this must be the case. 
It is also shown in the following table, that in masses of crystallized bismuth 
the diamagnetic repulsion acts with very different energies in different directions. 
Cubes were taken from a mass of bismuth with the planes of principal cleavage 
parallel throughout to two opposite faces of each cube. The cubes were placed upon 
the ends of a torsion balance, and the diamagnetic repulsion was accurately measured 
when the force acted parallel to the planes of cleavage. The cubes were then turned 
90° round, and the repulsion was measured when the force acted perpendicular to the 
planes referred to. 
Cubes of crystallized bismuth. 
Repulsion when the force was directed 
Strength of magnet. 
3-6 
along the cleavage. 
117 
across the cleavage. 
8 
57 
34-8 
23 
8-4 
78 
53 
10-0 
111 
76-5 
11-9 
153 
110 
It is manifest from this table that bismuth behaves as a body of considerably superior 
diamagnetic power when the force acts along the planes of cleavage. 
Let two indefinitely thin needles be taken from such 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 elongated to produce the effects exhibited by 
the prisms in the experiments already recorded. Fig. 5. 
Let us suppose the ends of our rectangular box to 
be composed of cubes, instead of elongated masses, 
of crystallized bismuth, and let the planes of prin- 
cipal cleavage be supposed to be parallel to the 
face a h, fig. 5. Let the continuous line d e repre- 
sent an indefinitely thin slice of the cube passing 
through its centre, and the dotted lineg/ a simi- 
lar 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 m pairs on each side of those two central slices, it is 
MDCCCLV. H 
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