1357.  I have described in the last paper, cases, where shell-lac was rendered a conductor by absorption of ammonia (1294.).  The same effect happens with muriatic acid; yet both these substances, when gaseous, are non-conductors; and the ammonia, also when in strong solution (718.).  Mr. Harris has mentioned instances[A] in which the conducting power of metals is seriously altered by a very little alloy.  These may have no relation to the former cases, but nevertheless should not be overlooked in the general investigation which the whole question requires.

  [A] Philosophical Transactions, 1827, p. 22.

1358.  Nothing is perhaps more striking in that class of dielectrics which we call electrolytes, than the extraordinary and almost complete suspension of their peculiar mode of effecting discharge when they are rendered solid (380, &c.), even though the intensity of the induction acting through them may be increased a hundredfold or more (419.).  It not only establishes 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.