1744.  It seems highly probable, that excitement by friction may very frequently be of the same character.  Wollaston endeavoured to refer such excitement to chemical action[A]; but if by chemical action ultimate union of the acting particles is intended, then there are plenty of cases which are opposed to such a view.  Davy mentions some such, and for my own part I feel no difficulty in admitting other means of electrical excitement than chemical action, especially if by chemical action is meant a final combination of the particles.

  [A] Philosophical Transactions, 1801, p. 427.

1745.  Davy refers experimentally to the opposite states which two particles having opposite chemical relations can assume when they are brought into the close vicinity of each other, but not allowed to combine[A].  This, I think, is the first part of the action already described (1743.); but in my opinion it cannot give rise to a continuous current unless combination take place, so as to allow other particles to act successively in the same manner, and not even then unless one set of the particles be present as an element of an electrolyte (923. 963.); i.e. mere quiescent contact alone without chemical action does not in such cases produce a current.

  [A] Philosophical Transactions, 1807, p. 31.

1746.  Still it seems very possible that such a relation may produce a high charge, and thus give rise to excitement by friction.  When two bodies are rubbed together to produce electricity in the usual way, one at least must be an insulator.  During the act of rubbing, the particles of opposite kinds must be brought more or less closely together, the few which are most favourably circumstanced being in such close contact as to be short only of that which is consequent upon chemical combination.  At such moments they may acquire by their mutual induction (1740.) and partial discharge to each other, very exalted opposite states, and when, the moment after, they are by the progress of the rub removed from each other’s vicinity, they will retain this state if both bodies be insulators, and exhibit them upon their complete separation.

1747.  All the circumstances attending friction seem to me to favour such a view.  The irregularities of form and pressure will cause that the particles of the two rubbing surfaces will be at very variable distances, only a few at once being in that very close relation which is probably necessary for the development of the forces; further, those which are nearest at one time will be further removed at another, and others will become the nearest, and so by continuing the friction many will in succession be excited.  Finally, the lateral direction of the separation in rubbing seems to me the best fitted to bring many pairs of particles, first of all into that close vicinity necessary for their assuming the opposite states by relation to each other, and then to remove them from each other’s influence whilst they retain that state.