698 Proceedings of Royal Society of Edinburgh. [sess. 
force the motion to the outside where pressure was least. If we 
suppose for a moment the charge could he retained there — as it is 
in hollow non-conducting bodies like the Leyden jar — the pressure 
would be greatly increased, owing to the opposite sides raining, as 
it were, electricity and encroaching on one another. The motion, 
in short, is self-forced to the outside, where it has greater freedom. 
Suppose the charge to he negative — then the greater pressure on 
the outside than in the interior would force so much of the motion 
as remained into the interior surface so as to bring it up to the 
neutral state. To put it in the language of the text-hooks — “the 
mutual induction ” of the opposite sides would retain the normal 
amount of motion in the interior atoms, leaving the exterior 
negative. 
If, though, an electrified body he lowered into the interior of a 
hollow conductor without touching it, as in the ice-pail experiment, 
the inside will he found to he electrified with electricity of the 
opposite kind to that of the body. This is explained by the fact 
that, supposing the body to he positive, the pressure in the 
interior is increased by the radiation from the body, and which 
forces the greater part of the motion in the atoms of the conductor 
to the outside, leaving the inside negative and the outside positive. 
On the other hand, suppose the body to be negative — then the 
greater pressure on the outside than in the inside, in presence of 
the negative body, forces the motion to the inside, making it 
positive and the outside negative. What follows if the electrified 
body, whether positive or negative, be allowed to touch the 
conductor is already explained in the text-books. 
25. The Eledrophorus. 
The explanation of the electrophorus is very simple. When 
the metallic disc is placed on the negatively electrified resinous 
cake, the small radiation of electricity by the cake permits of the 
expansion of the atoms of the disc next it, i.e. motion is drawn 
to them at the expense of the atoms on the far side of the disc. 
In other words, the greater pressure on the side of the disc farthest 
from the cake presses the motion to the side of least pressure, 
which becomes positive, and the far side equally negative. When 
