(1228); and we can apparently go so far in the analogy, whilst limiting the case to the inductive action only, as to show that amongst insulating dielectrics some lead away the lines of force from others (1229.), as the wire will do from worse conductors, though in it the principal effect is no doubt due to the ready discharge between the particles whilst in a low state of tension.  The retardation is for the time insulation; and it seems to me we may just as fairly compare the air at the interval a, b (fig. 115.) and the wire in the circuit, as two bodies of the same kind and acting upon the same principles, as far as the first inductive phenomena are concerned, notwithstanding the different forms of discharge which ultimately follow[A], as we may compare, according to Coulomb’s investigations[B] different lengths of different insulating bodies required to produce the same amount of insulating effect.

  [A] These will be examined hereafter (1348. &c.).

  [B] Memoires de l’Academie, 1785, p. 612. or Ency.  Britann.  First
  Supp. vol. i. p. 614.

1332.  This comparison is still more striking when we take into consideration the experiment of Mr. Harris, in which he stretched a fine wire across a glass globe, the air within being rarefied[A].  On sending a charge through the joint arrangement of metal and rare air, as much, if not more, electricity passed by the latter as by the former.  In the air, rarefied as it was, there can be no doubt the discharge was preceded by induction (1284.); and to my mind all the circumstances indicate that the same was the case with the metal; that, in fact, both substances are dielectrics, exhibiting the same effects in consequence of the action of the same causes, the only variation being one of degree in the different substances employed.

  [A] Philosophical Transactions, 1834, p, 212.

1333.  Judging on these principles, velocity of discharge through the same wire may be varied greatly by attending to the circumstances which cause variations of discharge through spermaceti or sulphur.  Thus, for instance, it must vary with the tension or intensity of the first urging force (1234. 1240.), which tension is charge and induction.  So if the two ends of the wire, in Professor Wheatstone’s experiment, were immediately connected with two large insulated metallic surfaces exposed to the air, so that the primary act of induction, after making the contact for discharge, might be in part removed from the internal portion of the wire at the first instant, and disposed for the moment on its surface jointly with the air and surrounding conductors, then I venture to anticipate that the middle spark would be more retarded than before; and if these two plates were the inner and outer coating of a large jar or a Leyden battery, then the retardation of that spark would be still greater.