and other Minerals when exposed to Heat. 33 
tensities,—1, 2, 5, 4, 3, the areas following the natural order of the 
numbers. Other series being taken with sets of crystals, 1.2 and 
1.8 inches long, gave similar indications of irregularity,* but the 
area of section has so far a general influence, that where the dif- 
ferences are considerable, the thickest crystal has almost univer- 
sally the greatest power. The relative forces are so connected, 
that we can hardly impute the irregularities to any general law ; 
the differences, as I shall immediately illustrate by reference to 
another class of experiments, must in all probability be attributed 
to a variable structure in specimens of the same mineral, as well 
as in those of different species. 
I took a crystal 14 inches long, and carefully determined the 
intensity of its electricity, which, by a mean of three experiments, 
gave 45° of deviation. I immediately broke it at one-fourth of 
its length from one end, the two portions being then heated and 
their intensities determined each three times, the mean of the 
larger portion gave a deviation of 47°, of the smaller 43°, the 
mean of which gives precisely the original force. As far as in- 
tensity goes, the diminution of length would not therefore ap- 
pear to be favourable to the development of electricity. With 
a view of procuring through a larger range of dimension, the in- 
fluence of length alone, I selected a series of tourmalines whose 
sections were as nearly equal as possible, the diameter being 
about ;!,th inch, and one of which was the very long crystal be- 
fore mentioned. ‘This experiment was made with great care ; a 
* Nos. Intensity. Intensity. 
1.2 long. 18 long: 
1 (thickest) 82 54° 
2 77.5 40° 
3 50° 34° 
4 57.5 35.5 
5 65° 
6 (thinnest) 34° 
VOL. XIII. PART I. E 
