1334.  Cavendish was perhaps the first to show distinctly that discharge was not always by one channel[A], but, if several are present, by many at once.  We may make these different channels of different bodies, and by proportioning their thicknesses and lengths, may include such substances as air, lac, spermaceti, water, protoxide of iron, iron and silver, and by one discharge make each convey its proportion of the electric force.  Perhaps the air ought to be excepted, as its discharge by conduction is questionable at present (1336.); but the others may all be limited in their mode of discharge to pure conduction.  Yet several of them suffer previous induction, precisely like the induction through the air, it being a necessary preliminary to their discharging action.  How can we therefore separate any one of these bodies from the others, as to the principles and mode of insulating and conducting, except by mere degree?  All seem to me to be dielectrics acting alike, and under the same common laws.

  [A] Philosophical Transactions, 1776, p. 197.

1335.  I might draw another argument in favour of the general sameness, in nature and action, of good and bad conductors (and all the bodies I refer to are conductors more or less), from the perfect equipoise in action of very different bodies when opposed to each other in magneto-electric inductive action, as formerly described (213.), but am anxious to be as brief as is consistent with the clear examination of the probable truth of my views.

1336.  With regard to the possession by the gases of any conducting power of the simple kind now under consideration, the question is a very difficult one to determine at present.  Experiments seem to indicate that they do insulate certain low degrees of tension perfectly, and that the effects which may have appeared to be occasioned by conduction have been the result of the carrying power of the charged particles, either of the air or of dust, in it.  It is equally certain, however, that with higher degrees of tension or charge the particles discharge to one another, and that is conduction.  If the gases possess the power of insulating a certain low degree of tension continuously and perfectly, such a result may be due to their peculiar physical state, and the condition of separation under which their particles are placed.  But in that, or in any case, we must not forget the fine experiments of Cagniard de la Tour[A], in which he has shown that liquids and their vapours can be made to pass gradually into each other, to the entire removal of any marked distinction of the two states.  Thus, hot dry steam and cold water pass by insensible gradations into each other; yet the one is amongst the gases as an insulator, and the other a comparatively good conductor.  As to conducting power, therefore, the transition from metals even up to gases is gradual; substances make but one series in this respect, and the various cases must come under one condition and law (444.).  The specific differences of bodies as to conducting power only serves to strengthen the general argument, that conduction, like insulation, is a result of induction, and is an action of contiguous particles.