272.  Whilst in the state described, no decomposition took place at the point a, but when the side of a spirit-lamp flame was applied to the two platina extremities at e, so as to make them bright red-hot, decomposition occurred; iodine soon appeared at the point a, and the transference of electricity through the heated air was established.  On raising the temperature of the points e by a blowpipe, the discharge was rendered still more free, and decomposition took place instantly.  On removing the source of heat, the current immediately ceased.  On putting the ends of the wires very close by the side of and parallel to each other, but not touching, the effects were perhaps more readily obtained than before.  On using a larger voltaic battery (270.), they were also more freely obtained.

273.  On removing the decomposing apparatus and interposing a galvanometer instead, heating the points e as the needle would swing one way, and removing the heat during the time of its return (302.), feeble deflections were soon obtained:  thus also proving the current through heated air; but the instrument used was not so sensible under the circumstances as chemical action.

274.  These effects, not hitherto known or expected under this form, are only cases of the discharge which takes place through air between the charcoal terminations of the poles of a powerful battery, when they are gradually separated after contact.  Then the passage is through heated air exactly as with common electricity, and Sir H. Davy has recorded that with the original battery of the Royal Institution this discharge passed through a space of at least four inches[A].  In the exhausted receiver the electricity would strike through nearly half an inch of space, and the combined effects of rarefaction and heat were such upon the inclosed air us to enable it to conduct the electricity through a space of six or seven inches.

  [A] Elements of Chemical Philosophy, p. 153

275.  The instantaneous charge of a Leyden battery by the poles of a voltaic apparatus is another proof of the tension, and also the quantity, of electricity evolved by the latter.  Sir H. Davy says[A], “When the two conductors from the ends of the combination were connected with a Leyden battery, one with the internal, the other with the external coating, the battery instantly became charged; and on removing the wires and making the proper connexions, either a shock or a spark could be perceived:  and the least possible time of contact was sufficient to renew the charge to its full intensity.”

  [A] Elements of Chemical Philosophy, p. 154.

276. In motion: i. Evolution of Heat.—­The evolution of heat in wires and fluids by the voltaic current is matter of general notoriety.

277. ii. Magnetism.—­No fact is better known to philosophers than the power of the voltaic current to deflect the magnetic needle, and to make magnets according to certain laws; and no effect can be more distinctive of an electrical current.