University of Virginia Library

XVIII.
What is the Cause of Thunder?

ASERIES of observations, and a single experiment,
would throw some light upon this important question.
Take, for instance, a summer afternoon when the
air is close and sultry, and the atmosphere rarefied, when
respiration is laborious, and no wind stirring among the
leaves. But, on the distant horizon, there are indications
of vapor; not rolling clouds, but thin exhalations from
the earth, drawn up by the heat of the sun. Suddenly
this humid veil is illuminated by flashes, and people call
it heat lightning, summer lightning, sheet lightning. I
wish particularly to direct attention to the fact, that this
exhibition of electricity is not often accompanied with
other phenomena peculiar to thunder storms. No rain
follows the flash, nor is any report heard; and, furthermore,
these illuminated vapors are always much elevated.

It is idle to say that on account of distance from the
earth the report is not audible; for few persons, familiar
with mountain heights, can fail to remember that some

time or other they were in the midst of such an atmosphere,
when the lightning appeared to surround them,
apparently within a few feet of them, flashing on every
side, yet without rain or detonation. In this condition
the atmosphere is said to be highly charged with electricity.
But surely we cannot accept this as equivalent to
the same meaning applied to a Leyden jar, fresh from
contact with the knob of the electric machine. Indeed,
is not the contrary very possible? Would not the data
show that, in such a condition the atmosphere, instead
of being highly charged, had not its usual percentage of
electric stimulus? Experiments with the electrometer
might prove this supposition to be correct, and, on the
other hand, it might prove it to be incorrect. But one
thing cannot be disproved nor denied—that air, highly
rarefied by heat, and humid, is air, plus water; and also
that in this condition air is susceptible of being silently
illuminated by electricity. This point being settled, we
will proceed to the next—which is, “What is the cause
of Thunder?”

The learned, down to the latest moment of going to
press, have advanced no further than this, that “thunder
is a noise produced by THE EXPLOSION OF LIGHTNING, or
by the passage of lightning from one cloud to another! or
from a cloud to the ground.” Whoever has read the celebrated
treatise of John Conrad Francis de Hatzfield
upon the subject, will find a far more plausible theory
advanced by that sagacious philosopher, and quite as

amusing as the modern idea, that the sound of thunder is
analogous to the snap produced by holding the knuckle
of one's forefinger to the brass bulb of an electrical
machine!—an explanation that has never satisfied any
reasonable mind. Let us see if there be not a rational
solution of the mystery.

The phenomena of thunder storms are: first, heavy
clouds; then lightning; then the report, and then a
fall of rain! Now, let us trace the consequence to its
source. The rain is produced by two causes, either sudden
condensation of watery vapors or clouds, by colder
temperature, or the formation of water by the action
of the electric fluid. The first explains itself; the latter
is linked with the subject of this paper. Let us, therefore,
confine ourselves to that rain only which follows
the thunder. Rain water is composed of two elements,
oxygen and hydrogen. Hydrogen is a combustible gas,
and oxygen supports combustion. A stream of pure
hydrogen, ejected from a pipe into pure oxygen, burns
brightly in perfect silence. But, mixed with oxygen, it
explodes upon taking fire; just as a young man, having his
own fortune to make, goes quietly to work until he gets
a partner with a tremendous capital. The relative aspects
of silent lightning and noisy lightning may be compared
by a simple apparatus sold at any chemists; it is a tin
lamp filled with inflammatory gas. So long as the gas is
allowed to burn in small quantities it is taciturn, but, exposed
to a larger mixture of oxygen, it goes off with a

loud report. This is a lamp that any spark of electricity
can ignite. And then again the product of the flame is
water! The union of hydrogen and oxygen is water.
What meteoric phenomenon is so simple as this, that
thunder is caused by the electric spark uniting with rarefied
air plus oxygen, and rarefied vapor plus hydrogen,
detonating, recompounding, and forming rain!