erposing 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.

278. iii. _Chemical decomposition._--The chemical powers of the voltaic
current, and their subjection to _certain laws_, are also perfectly well
known.

279. iv. _Physiological effects._--The power of the voltaic current, when
strong, to shock and convulse the whole animal system, and when weak to
affec