ing the point, illustrate the production of the current exceedingly
well, and justify the same conclusions; it being remembered that in such
cases the effect upon the discharge is of two kinds. For the current may be
interfered with by stopping the access of fresh uncharged air, or retarding
the removal of that which has been charged, as when a point is electrified
in a tube of insulating matter closed at one extremity; or the _electric
condition_ of the point itself may be altered by the relation of other
parts in its neighbourhood, also rendered electric, as when the point is in
a metal tube, by the metal itself, or when it is in the glass tube, by a
similar action of the charged parts of the glass, or even by the
surrounding air which has been charged, and which cannot escape.

1575. Whenever it is intended to observe such inductive phenomena in a
fluid dielectric as have a direct relation to, and dependence upon, the
fluidity of the medium, such, for instance, as discharge from points, or
attractions and repulsions, &c., then the mass of the fluid should be
great, and in such proportion to the distance between the inductric and
inducteous surfaces as to include all the _lines of inductive force_
(1369.) between them; otherwise, the effects of currents, attraction, &c.,
which are the resultants of all these forces, cannot be obtained. The
phenomena, which occur in the open air, or in the middle of a globe filled
with oil of turpentine, will not take place in the same media if confined
in tubes of glass, shell-lac, sulphur, or other such substances, though
they be excellent insulating dielectrics; nor can they be expected: for in
such cases, the polar forces, instead of being all dispersed amongst fluid
particles, which tend to move under their influence, are now associated in
many parts with particles that, notwithstanding their tendency to motion,
are constrained by their solidity to remain quiescent.

1576. The varied circumstances under which, with conductors differently
formed and constituted, currents can occur, all illustrate the same
simplicity of production. A _ball_, if the intensity be raised sufficiently
on its surface, and that intensity be greatest on a part consistent with
the production of a current of air up to and off from it, will produce the
effect like a point (1537); such is the case whenever the glow occurs upon
a ball, the current being essential to that phenomenon. If as large a
sphere as can w