v 
and other Minerals when exposed to Heat. 3: 
The unequal temperature of the portion of any section, pre- 
vents, as I have already observed, all the parts from giving a 
maximum intensity at once. This will diminish the total effect, 
but as all the parts afford the same kind of electricity, the re- 
sultant can never be null on this account. Therefore, even if 
the irregularities of amount did not compel us to admit innate 
varieties of structure or electric disposition in different specimens, 
stubborn facts must force us to some such conclusion. In the 
course of my researches, I have met with a crystal of tourmaline, * 
possessing no external irregularities of structure, (the termina- 
tions, however, of the crystal are not preserved), which has the 
singular property of presenting in cooling a vitreous pole at both 
ends. Having ascertained this point, I proceeded to examine 
the electricity of its parts, by means of CouLome’s Proof-plane, 
by which the electricity of any portion is insulated and examined. 
As I expected, I found the central portion of the crystal resi- 
nously electrified. This remarkable fact is not unexampled. 
Havy has recorded the case of a crystal of topaz which had a si- 
milar property, which, from its analogy to known facts in the phe- 
nomena of magnetism and of double refraction, Dr Brewster 
conceived to be owing to the union of two distinct crystals, 
with the vitreous poles in contact, as in that case resinous elec- 
tricity was developed at both ends. Be this as it may, the ex- 
ample of tourmaline which I have cited, proves that the junction 
of the separate crystals, if such exist, may be imperceptible, and 
as the probability that such irregularity should exist, however 
caused, is in proportion to the length of the specimen, this may 
perhaps explain the want of excitability observed by BecquerEL 
in very long crystals. 
The phenomena of tourmaline, though entirely electric, bear 
so strong an affinity to those of magnetism, that the study of 
* It is No. 3. of the Series, at the foot of p. 32. 
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