EXPERIMENTAL EXAMINATION OF THE INDICATIONS OF THE PROOF PLANE. 451 
angle a, the particles are differently circumstanced, and have one or two sides free. 
Resting, therefore, on the previous fact, that the capacity of a conducting body, con- 
sidered as a whole, and upon which we accumulate electricity, is affected by this 
circumstance, we might be led to conclude that the proof plate of small thickness 
should actually receive more electricity at the angle a than at the centre c, it being 
there less exposed to the opposing forces, whatever they be, which tend to contravene 
the induction. Now if we extend the thickness of the proof plane P, we place the 
distant points more without the influence of these forces ; hence its inductive capa- 
city becomes more perfect, so that when the thickness or other extension is suffi- 
ciently great to render the inductive capacity the same at the centre c as at the 
angle a, then the quantity of electricity abstracted is also the same, as is found by 
experiment (36.) (41.) Tables VIII. and IX. 
49. It is somewhat doubtful therefore, whether we can really take the proof plane 
as an element of a charged body, since it forms no integral part of the surface, 
as is the case with the point touched. It may, however, be still open to the same 
influence as that which affects the capacity of the whole area to which it is applied ; 
so that the disposition of electrified bodies to yield up their electricity at points 
or edges may as easily arise from the superior attractive force generated by a more 
perfect induction in external bodies, in the way just stated, as from an original 
concentration of the charge upon such points or edges. In short, we really know 
nothing of the actual distribution of electricity upon a charged surface, except 
through the medium of other bodies in some way applied to it. I have already 
endeavoured to show * that an electrified substance only gives off electricity by the 
influence of an attractive force, set up between it and some other substance : 
hence an electrified sphere or other body perfectly insulated in the best vacuum 
which can be obtained, under ordinary circumstances ; will, if placed without 
the influence of any source of attraction, retain its electrical state for an indefinite 
period -f*. It is therefore not until we present a neutral conducting body to an insu- 
lated charged body that we begin to disturb the electrical distribution, which may 
have been previously uniform. 
50. In the course of this and the preceding communication I have ventured occa- 
sionally to scrutinize the prevailing theories of electricity, and advert to the opinions 
entertained by many profound inquirers in this department of science. I would not, 
however, be thought insensible to their claims on our confidence. The researches 
of many distinguished philosophers on the Continent, together with those which have 
reflected so much honour on the science of our own country, must necessarily receive 
from every impartial mind the warmest admiration. It must not, therefore, be for- 
gotten, that whilst detailing a series of facts carefully deduced by induction and ex- 
* Philosophical Transactions for 1834, p. 242. 
3 m 2 
t Ibid. p. 244. 
