34 Prof. Forses’s Experiments on the Electricity of Tourmaline 
direct and reversed series were taken, and several of the deter- 
minations independently repeated. The mean deviations of the 
needle of the electroscope will be given in the following Table : 
No. Length. Intensity. 
1 3.25 inch. 19.°5 
2 2.10 82° 
3 1.60 60° 
4 1.55 60° 
5 1.35 89° 
6 1.19 68° 
We thus see that the long crystal holds a high place among 
those of equal section with it, and we have at the same time an 
additional proof of the native irregularities of different crystals. 
It is well known that the artificial arrangement which repre- 
sents best the phenomena of the tourmaline, is that of a series of 
insulated plates of glass arranged parallel to one another, suit- 
ably coated, and with the contiguous coatings connected by tin- 
foil. If one end of this battery be charged from an electrical 
source, while the other communicates with the ground, the plates 
at one extremity will partake of an excess of the electricity com- 
municated, whilst those at the other will have the opposite spe- 
cies in excess, and a large proportion of the range in the centre 
will exhibit no traces of free electricity: hence, by shortening the 
pile (supposing the plates very numerous), no change will take 
place in the intensity of the free electricity, but the intensity will 
bear a direct relation to the swzface of the plates, or the section 
of the pile. So far analogy supports the increase of intensity 
with the diameter of the tourmaline ; but when we come to con- 
sider the mode of charging, it fails, and leaves us in great doubt 
as to whether the length of a crystal, if its structure be perfectly 
uniform, should have any influence or not. I have found short 
crystals of a considerable area, and so formed as to have a large 
surface, perhaps the most energetic. 
