I have received your very obliging and very ingenious letter by
Captain Kearney. Your observations upon the electricity of fogs and
the air in Ireland, and upon different circumstances of storms,
appear to me very curious, and I thank you for them. There is not,
in my opinion, any part of the earth whatever which is, or can be,
naturally in a state of negative electricity: and though different
circumstances may occasion an inequality in the distribution of
the fluid, the equilibrium is immediately restored by means of its
extreme subtilty, and of the excellent conductors with which the
humid earth is amply provided. I am of opinion, however, that when
a cloud, well charged positively, passes near the earth, it repels
and forces down into the earth that natural portion of electricity,
which exists near its surface, and in buildings, trees, &c. so as
actually to reduce them to a negative state before it strikes them.
I am of opinion too, that the negative state in which you have
frequently found the balls, which are suspended from your apparatus,
is not always occasioned by clouds in a negative state; but more
commonly by clouds positively electrified, which have passed over
them, and which in their passage have repelled and driven off a part
of the electrical matter, which naturally existed in the apparatus;
so that what remained after the passing of the clouds, diffusing
itself uniformly through the apparatus, the whole became reduced to a
negative state.
If you have read my experiments made in continuation of those of Mr.
Canton, you will readily understand this; but you may easily make
a few experiments, which will clearly demonstrate it. Let a common
glass be warmed before the fire that it may continue very dry for
some time; set it upon a table, and place upon it the small box made
use of by Mr. Canton, so that the balls may hang a little beyond
the edge of the table. Rub another glass, which has previously been
warmed in a similar manner, with a piece of black silk or a silk
handkerchief, in order to electrify it. Hold then the glass above
the little box, at about the distance of three or four inches from
that part, which is most distant from the balls; and you will see
the balls separate from each other; being positively electrified
by the natural portion of electricity, which was in the box, and
which is driven to the further part of it by the repulsive power of
the atmosphere in the excited glass. Touch the box near the little
balls (the excited glass continuing in the same state) and the balls
will again unite; the quantity of electricity which had been driven
to this part being drawn off by your finger. Withdraw then both
your finger and the glass at the same instant, and the quantity of
electricity which remained in the box, uniformly diffusing itself,
the balls will again be separated; being now in a negative state.
While things are in this situation, begin once more to excite your
glass, and hold it above the box, but not too near, and you will
find, that when brought within a certain distance, the balls will
at first approach each other, being then in a natural state. In
proportion as the glass is brought nearer, they, will again separate,
being positive. When the glass is moved beyond them, and at some
little farther distance, they will unite again, being in a natural
state. When it is entirely removed, they will separate again,
being then made negative. The excited glass in this experiment may
represent a cloud positively charged, which you see is capable of
producing in this manner all the different changes in the apparatus,
without the least necessity for supposing any negative cloud.
I am nevertheless fully convinced, that these are negative clouds;
because they sometimes absorb, through the medium of the apparatus,
the positive electricity of a large jar, the hundredth part of
which the apparatus itself would have not been able to receive or
contain at once. In fact, it is not difficult to conceive, that a
large cloud, highly charged positively, may reduce smaller clouds to
a negative state, when it passes above or near them, by forcing a
part of their natural portion of the fluid either to their inferior
surfaces, whence it may strike into the earth, or to the opposite
side, whence it may strike into the adjacent clouds; so that when
the large cloud has passed off to a distance, the small clouds
shall remain in a negative state, exactly like the apparatus; the
former (like the latter) being frequently insulated bodies, having
communication neither with the earth nor with other clouds. Upon the
same principle it may easily be conceived, in what manner a large
negative cloud may render others positive.
The experiment which you mention, of filing your glass, is analogous
to one which I made in 1751, or 1752. I had supposed in my preceding
letters, that the pores of glass were smaller in the interior parts
than near the surface, and that on this account they prevented the
passage of the electrical fluid. To prove whether this was actually
the case or not, I ground one of my phials in a part where it was
extremely thin, grinding it considerably beyond the middle, and very
near to the opposite superficies, as I found, upon breaking it after
the experiment. It was charged nevertheless after being ground,
equally well as before, which convinced me, that my hypothesis on
this subject was erroneous. It is difficult to conceive where the
immense superfluous quantity of electricity on the charged side of a
glass is deposited.
I send you my paper concerning meteors, which was lately published
here in the Philosophical Transactions, immediately after a paper by
Mr. Hamilton on the same subject.