PHENOMENA OF A SPHERE CHARGED ON ITS INNER SURFACE. 
44 1 
wire d by an insulating handle n d, and subsequently the charged sphere a a'. Let 
the mercury contained within the sphere be now poured off ; we have then a spherical 
body, upon the interior surface of which there is a powerful accumulation of free 
electricity. Insulate this charged sphere, and touch its interior surface with an in- 
sulated proof plate. The plate will not, on being again withdrawn, exhibit the least 
electrical indication, although electricity in a free state is known to exist upon its 
inner surface. If, however, we connect the proof plane with an insulated conducting 
rod projecting beyond the sphere, electricity will be freely taken up, as in the case of 
a body similarly circumstanced, and placed within a charged sphere of metal. 
32. It is of no consequence to this experiment whether the glass sphere have sub- 
sequently to charging an external coating or not. Free electricity is everywhere dif- 
fused on its interior surface, and is easily communicable to any body capable of re- 
ceiving it. The following experiment is sufficiently illustrative of this. 
Experiment M. — Let a circular plate of glass d d', fig. 23, be placed on a conducting 
plate c, whose diameter is about half that of the glass ; place a similar plate c' upon 
its upper surface, immediately opposite the conducting plate c below, so as to give 
the glass two moveable coatings ; charge this system by communicating electricity 
to the upper plate ; remove the coatings, and place the glass on an insulating support; 
if the charged side be now touched with the proof plane, electricity will be freely 
taken up. 
33. We have here a direct experimental fact, showing that the neutral state of the 
proof plate is by no means evidence of the non-existence of electricity upon the in- 
terior surface of a charged sphere, and therefore no influence of this kind can be 
logically deduced from the experiment in question (21.). Moreover, the theory itself 
assumes, upon certain principles in the 12th section of Newton, that the action of a 
spherical stratum of electricity upon any point placed within it is equal to zero : should 
electricity therefore really exist upon the interior surface of the sphere, it could not, 
if the theory be true, be imparted to a body placed wholly within it. The experi- 
ment therefore is in this point of view irrelevant ; but if we take the experiment as a 
mere fact abstracted from hypotheses, it must necessarily be considered in connexion 
with other facts, such as those above stated (31.) (32.) ; in either case, however, it is 
clearly no evidence of the non-existence of electricity upon the interior surface of a 
charged shell. 
34. The experiments last mentioned (L.), (M.) are instructive ; they show that it is 
not only from the absence of electricity upon the interior surface of a charged shell 
that we fail to electrify a small insulated body placed within it, but that this result 
may also arise from incapacity of the given body itself to take up electricity under 
certain circumstances. Pursuing therefore these facts, unfettered by any hypothetical 
view of electricity, we are immediately led to examine under what conditions one 
body can receive electricity by communication from another, as in the case of touching 
a charged conductor with a small insulated disc of inconsiderable thickness ; since 
3 L 
MDCCCXXXVI. 
