on a London fog, discharging from lightning conductors or captive
balloons carrying flames, but it is premature to say anything about
this matter yet. I have, however, cleared a room of smoke very quickly
with a small hand machine.

It will naturally strike you how closely allied these phenomena must
be to the fact of popular science that "thunder clears the air." Ozone
is undoubtedly generated by the flashes, and may have a beneficial
effect, but the dust-coagulating and dust-expelling power of the
electricity has a much more rapid effect, though it may not act till
the cloud is discharged. Consider a cloud electrified slightly; the
mists and clouds in its vicinity begin to coagulate, and go on till
large drops are formed, which may be held up by electrical action, the
drops dancing from one cloud to another and thus forming the very
dense thunder cloud. The coagulation of charged drops increases the
potential, as Prof. Tait points out, until at length--flash--the cloud
is discharged, and the large drops fall in a violent shower. Moreover,
the rapid excursion to and fro of the drops may easily have caused
them to evaporate so fast as to freeze, and hence we may get hail.

While the cloud was electrified, it acted inductively on the earth
underneath, drawing up an opposite charge from all points, and thus
electrifying the atmosphere. When the discharge occurs this
atmospheric electrification engages with the earth, clearing the air
between, and driving the dust and germs on to all exposed surfaces. In
some such way also it may be that "thunder turns milk sour," and
exerts other putrefactive influences on the bodies which receive the
germs and dust from the air.

But we are now no longer on safe and thoroughly explored territory. I