and other Minerals when eaposed to Heat. 31 
to; I have always, however, avoided it in practice, by repeating 
every series of experiments in an inverted order, by which we 
obtain two observations at equal distances from a mean state of 
electric tension, the mean of which will give strictly comparable 
results. 
The principal application which I made of this method of ob- 
serving, was to attempt to discover some relation between the 
form and dimensions of crystals of tourmaline, and their electric 
power. 
M. Becaueret, in a Second Memoir on the Tourmaline, pub- 
lished in 1828,* announced the rather extraordinary circum- 
stance, that long Tourmalines did not become at all electric by 
heat, and that their facility of being excited was generally in- 
versely proportional to their length. Dr Tomson, in his work 
on Heat,+ mentions the former assertion, and observes that it 1s 
one which he has never had an opportunity of trying. As my 
experiments have been generally made with black Tourmalines 
from Van Diemen’s Land, some of which are of great length, this 
point early occurred to me as one deserving of investigation. 
As these inquiries seem at no period to have excited much at- 
tention in this country, and as of late nothing whatever has been 
done upon them, these observations may prove the more inte- 
resting. 
The longest Tourmaline employed by M. BEecqueREL, was 
six centimetres, or 3.2 English inches m length, with a diameter 
of about .08 inches. My largest Tourmaline is 3.25 inches, or al- 
most precisely the same, with a diameter little different. Instead of 
finding this crystal “ tout a fait refractoire,” as M. BecquerEL 
describes his, it proved uniformly susceptible of powerful excite- 
ment, under the very same treatment which I was accustomed to 
D ieee Nee yt Ae ees 
* Ann. de Chimie. + P. 477. 
