stablishes a very general relation between the physical properties of
these bodies and electricity acting by induction through them, but draws
both their physical and chemical relations so near together, as to make us
hope we shall shortly arrive at the full comprehension of the influence
they mutually possess over each other.

P ix. _Disruptive discharge and insulation._

1359. The next form of discharge has been distinguished by the adjective
_disruptive_ (1319.), as it in every case displaces more or less the
particles amongst and across which it suddenly breaks. I include under it,
discharge in the form of sparks, brushes, and glow (1405.), but exclude the
cases of currents of air, fluids, &c., which, though frequently
accompanying the former, are essentially distinct in their nature.

1360. The conditions requisite for the production of an electric spark in
its simplest form are well-known. An insulating dielectric must be
interposed between two conducting surfaces in opposite states of
electricity, and then if the actions be continually increased in strength,
or otherwise favoured, either by exalting the electric state of the two
conductors, or bringing them nearer to each other, or diminishing the
density of the dielectric, a _spark_ at last appears, and the two forces
are for the time annihilated, for _discharge_ has occurred.

1361. The conductors (which may be considered as the termini of the
inductive action) are in ordinary cases most generally metals, whilst the
dielectrics usually employed are common air and glass. In my view of
induction, however, every dielectric becomes of importance, for as the
results are considered essentially dependent on these bodies, it was to be
expected that differences of action never before suspected would be evident
upon close examination, and so at once give fresh confirmation of the
theory, and open new doors of discovery into the extensive and varied
fields of our science. This hope was especially entertained with respect to
the gases, because of their high degree of insulation, their uniformity in
physical condition, and great difference in chemical properties.

1362. All the effects prior to the discharge are inductive; and the degree
of tension which it is necessary to attain before the spark passes is
therefore, in the examination I am now making of the new view of induction,
a very important point. It is the limit of the influence which the
dielectric exerts in r