CIX
charged the induction ceases all of a sudden, and this rapid
change from the electric state to the neutral state produces the
return-shock.
§ 273. The thunder is not a single report (like the crackling
sound produced by the Leyden jar), but a prolonged rolling
which is due to the different distances of the various points
of the lightning, and to the manifold echoes called forth by the
thunder on account of the zigzag of the lightning.
§ 274. The lightning conductor is an application of what
we have heard about induction and the power of points (§ 262);
it prevents the accumulation of electricity on the surface of the
earth, because through the point of the iron rod electricity flows
into the atmosphere, neutralizing the electricity of an approach
ing cloud. In case the production of electricity is too abun
dant to be discharged by the conductor, the lightning will strike;
but then the conductor and not the house will receive the dis
charge and the building is preserved. 434. The conductive
power of the air is different in different places. After the
lightning has struck we hear only a single thunder-clap. 435.
§ 275. Thunderstorms however are necessary and useful
revolutions in nature. Lightning on its way decomposes the
vapour contained in the atmosphere, Hydrogen and Oxygen are
liberated and unite with the Nitrogen of the air forming Ammo
niac or Nitric acid. Hence the rain water receives its portion
of Nitrate of ammoniac, a substance highly necessary for the
growth of plants. Maimbrai by electrifying two trees caused
them to put forth buds in a time, in which other trees lost their
leaves.
§ 276. St. Elmo's fire (a case of induction). Lightning
without thunder perhaps due to electric discharges below the
horizon, by which the clouds above the horizon are illuminated
