of Electro-Magnetism by Heat. 57 
charged through a spiral wire, without magnetising a needle in- 
closed within it, provided the discharge be made gradually, and 
without a shock. The tourmaline, which has been mentioned as 
analogous to the metals, in receiving polarity by heat, I found, 
even when strongly excited, to have no magnetic action, either 
when two silver wires were coiled round its extremities and con- 
nected with the galvanoscope, or when the same wire was con- 
tinued throughout; but it has not, I believe, been noticed, though 
it might have been expected, that in the last case the wire prevents 
the tourmaline from acquiring the opposite polarities. There is 
a singularity in the electric properties of the tourmaline, which 
was pointed out to me by Professor Henslow in the Abbé Hauy’s 
Traité de Mineralogie: if it be exposed to a low temperature, 
its extremities assume the opposite electricities; on increasing the 
temperature, the electric polarities diminish, at length altogether 
cease, and are afterwards resumed, but in the opposite states; that 
end which was positive becoming negative, and vice versa. Very 
unexpectedly, I discovered a similar phenomenon in the metallic 
electro-magnetism. As bismuth and antimony are oppositely af- 
fected by heat, and nearly to the same extent, it seemed probable 
that in certain proportions they might neutralize each other, and 
a compound bar would be produced, in which the electro-magnetic 
effects would cease. This is certainly the case, but these bars 
possess, what may be called a moveable zero. I have four bars, 
with different preportions of antimony and bismuth: the first 
slowly exhibits with the large compass 4° deviation, as antimony, 
then, on continuing the heat, returns to zero, passes through it, 
and deviates 5° as bismuth; the second deviates 3° as antimony, 
returns to zero, deviates 4° as bismuth, and again returns to zero 
as the bar begins to melt; the third deviates through 7°, and 
just returns to zero at its melting point; the fourth deviates 7° in 
the same manner, but returns only to 4°. These results were ob- 
Vol. Il. Part I. H 
