In my former paper on this subject, written first in 1747, enlarged
and sent to England in 1749, I considered the sea as the grand source
of lightning, imagining its luminous appearance to be owing to
electric fire, produced by friction between the particles of water
and those of salt. Living far from the sea, I had then no opportunity
of making experiments on the sea-water, and so embraced this opinion
too hastily.
For in 1750, and 1751, being occasionally on the sea-coast, I found,
by experiments, that sea-water in a bottle, though at first it would
by agitation appear luminous, yet in a few hours it lost that virtue:
_hence and from this_, that I could not by agitating a solution of
sea-salt in water produce any light, I first began to doubt of my
former hypothesis, and to suspect that the luminous appearance in
sea-water must be owing to some other principles.
I then considered whether it were not possible, that the particles
of air, being electrics _per se_, might, in hard gales of wind,
by their friction against trees, hills, buildings, &c. as so many
minute electric globes, rubbing against non-electric cushions, draw
the electric fire from the earth, and that the rising vapours might
receive that fire from the air, and by such means the clouds become
electrified.
If this were so, I imagined that by forcing a constant violent stream
of air against my prime conductor, by bellows, I should electrify it
_negatively_; the rubbing particles of air, drawing from it part of
its natural quantity of the electric fluid. I accordingly made the
experiment, but it did not succeed.
In September 1752, I erected an iron rod to draw the lightning
down into my house, in order to make some experiments on it, with
two bells to give notice when the rod should be electrified: a
contrivance obvious to every electrician.
I found the bells rang sometimes when there was no lightning or
thunder, but only a dark cloud over the rod; that sometimes after
a flash of lightning they would suddenly stop; and at other times,
when they had not rang before, they would, after a flash, suddenly
begin to ring; that the electricity was sometimes very faint, so
that when a small spark was obtained, another could not be got for
some time after; at other times the sparks would follow extremely
quick, and once I had a continual stream from bell to bell, the size
of a crow-quill: even during the same gust there were considerable
variations.
In the winter following I conceived an experiment, to try whether the
clouds were electrified _positively_ or _negatively_; but my pointed
rod, with its apparatus, becoming out of order, I did not refit it
till towards the spring, when I expected the warm weather would bring
on more frequent thunder-clouds.
The experiment was this: To take two phials; charge one of them with
lightning from the iron rod, and give the other an equal charge by
the electric glass globe, through the prime conductor: when charged,
to place them on a table within three or four inches of each other,
a small cork ball being suspended by a fine silk thread from the
cieling, so as it might play between the wires. If both bottles then
were electrified _positively_, the ball being attracted and repelled
by one, must be also repelled by the other. If the one _positively_,
and the other _negatively_; then the ball would be attracted and
repelled alternately by each, and continue to play between them as
long as any considerable charge remained.
Being very intent on making this experiment, it was no small
mortification to me, that I happened to be abroad during two of the
greatest thunder-storms we had early in the spring, and though I had
given orders in my family, that if the bells rang when I was from
home, they should catch some of the lightning for me in electrical
phials, and they did so, yet it was mostly dissipated before my
return, and in some of the other gusts, the quantity of lightning
I was able to obtain was so small, and the charge so weak, that I
could not satisfy myself: yet I sometimes saw what heightened my
suspicions, and inflamed my curiosity.
At last, on the 12th of April, 1753, there being a smart gust of some
continuance, I charged one phial pretty well with lightning, and the
other equally, as near as I could judge, with electricity from my
glass globe; and, having placed them properly, I beheld, with great
surprize and pleasure, the cork ball play briskly between them; and
was convinced that one bottle was electrised _negatively_.
I repeated this experiment several times during the gust, and in
eight succeeding gusts, always with the same success; and being
of opinion (for reasons I formerly gave in my letter to Mr.
Kinnersley, since printed in London) that the glass globe electrises
_positively_, I concluded that the clouds are _always_ electrised
_negatively_, or have always in them less than their natural quantity
of the electric fluid.
Yet notwithstanding so many experiments, it seems I concluded too
soon; for at last, June the 6th, in a gust which continued from five
o'clock, P. M. to seven, I met with one cloud that was electrised
positively, though several that passed over my rod before, during the
same gust, were in the negative state. This was thus discovered:
I had another concurring experiment, which I often repeated, to prove
the negative state of the clouds, viz. while the bells were ringing,
I took the phial charged from the glass globe, and applied its wire
to the erected rod, considering, that if the clouds were electrised
_positively_, the rod which received its electricity from them must
be so too; and then the additional _positive_ electricity of the
phial would make the bells ring faster:--But, if the clouds were in a
_negative_ state, they must exhaust the electric fluid from my rod,
and bring that into the same negative state with themselves, and then
the wire of a positively charged phial, supplying the rod with what
it wanted (which it was obliged otherwise to draw from the earth by
means of the pendulous brass ball playing between the two bells) the
ringing would cease till the bottle was discharged.
In this manner I quite discharged into the rod several phials that
were charged from the glass globe, the electric fluid streaming from
the wire to the rod, till the wire would receive no spark from the
finger; and, during this supply to the rod from the phial, the bells
stopped ringing; but by continuing the application of the phial wire
to the rod, I exhausted the natural quantity from the inside surface
of the same phials, or, as I call it, charged them _negatively_.
At length, while I was charging a phial by my glass globe, to repeat
this experiment, my bells, of themselves, stopped ringing, and, after
some pause, began to ring again.--But now, when I approached the
wire of the charged phial to the rod, instead of the usual stream
that I expected from the wire to the rod, there was no spark; not
even when I brought the wire and the rod to touch; yet the bells
continued ringing vigorously, which proved to me, that the rod was
then _positively_ electrified, as well as the wire of the phial, and
equally so; and, consequently, that the particular cloud then over
the rod was in the same positive state. This was near the end of the
gust.
But this was a single experiment, which, however, destroys my first
too general conclusion, and reduces me to this: _That the clouds of
a thunder-gust are most commonly in a negative state of electricity,
but sometimes in a positive state._
The latter I believe is rare; for though I soon after the last
experiment set out on a journey to Boston, and was from home most
part of the summer, which prevented my making farther trials and
observations; yet Mr. Kinnersley returning from the Islands just as I
left home, pursued the experiments during my absence, and informs me
that he always found the clouds in the _negative_ state.
So that, for the most part, in thunder-strokes, _it is the earth that
strikes into the clouds, and not the clouds that strike into the
earth_.
Those who are versed in electric experiments, will easily conceive,
that the effects and appearances must be nearly the same in either
case; the same explosion, and the same flash between one cloud and
another, and between the clouds and mountains, &c. the same rending
of trees, walls, &c. which the electric fluid meets with in its
passage, and the same fatal shock to animal bodies; and that pointed
rods fixed on buildings, or masts of ships, and communicating with
the earth or sea, must be of the same service in restoring the
equilibrium silently between the earth and clouds, or in conducting a
flash or stroke, if one should be, so as to save harmless the house
or vessel: for points have equal power to throw off, as to draw on
the electric fire, and, rods will conduct up as well as down.
But though the light gained from these experiments makes no
alteration in the practice, it makes a considerable one in the
theory. And now we as much need an hypothesis to explain by what
means the clouds become negatively, as before to shew how they
became positively electrified.
I cannot forbear venturing some few conjectures on this occasion:
they are what occur to me at present, and though future discoveries
should prove them not wholly right, yet they may in the mean time be
of some use, by stirring up the curious to make more experiments, and
occasion more exact disquisitions.
I conceive then, that this globe of earth and water, with its plants,
animals, and buildings, have diffused throughout their substance, a
quantity of the electric fluid, just as much as they can contain,
which I call the _natural quantity_.
That this natural quantity is not the same in all kinds of common
matter under the same dimensions, nor in the same kind of common
matter in all circumstances; but a solid foot, for instance, of one
kind of common matter, may contain more of the electric fluid than a
solid foot of some other kind of common matter; and a pound weight of
the same kind of common matter may, when in a rarer state, contain
more of the electric fluid than when in a denser state.
For the electric fluid, being attracted by any portion of common
matter, the parts of that fluid, (which have among themselves a
mutual repulsion) are brought so near to each other by the attraction
of the common matter that absorbs them, as that their repulsion is
equal to the condensing power of attraction in common matter; and
then such portion of common matter will absorb no more.
Bodies of different kinds having thus attracted and absorbed what I
call their _natural quantity, i. e._ just as much of the electric
fluid as is suited to their circumstances of density, rarity, and
power of attracting, do not then show any signs of electricity among
each other.
And if more electric fluid be added to one of these bodies, it does
not enter, but spreads on the surface, forming an atmosphere; and
then such body shews signs of electricity.
I have in a former paper compared common matter to a spunge, and the
electric fluid to water: I beg leave once more to make use of the
same comparison, to illustrate farther my meaning in this particular.
When a spunge is somewhat condensed by being squeezed between the
fingers, it will not receive and retain so much water as when in its
more loose and open state.
If _more_ squeezed and condensed, some of the water will come out of
its inner parts, and flow on the surface.
If the pressure of the fingers be entirely removed, the spunge will
not only resume what was lately forced out, but attract an additional
quantity.
As the spunge in its rarer state will _naturally_ attract and absorb
_more_ water, and in its denser state will _naturally_ attract and
absorb _less_ water; we may call the quantity it attacks and absorbs
in either state, its _natural quantity_, the state being considered.
Now what the spunge is to water, the same is water to the electric
fluid.
When a portion of water is in its common dense state, it can hold no
more electric fluid than it has: if any be added, it spreads on the
surface.
When the same portion of water is rarefied into vapour, and forms
a cloud, it is then capable of receiving and absorbing a much
greater quantity; there is room for each particle to have an electric
atmosphere.
Thus water, in its rarefied state, or in the form of a cloud, will
be in a negative state of electricity; it will have less than its
_natural quantity_; that is, less than it is naturally capable of
attracting and absorbing in that state.
Such a cloud, then, coming so near the earth as to be within the
striking distance, will receive from the earth a flash of the
electric fluid; which flash, to supply a great extent of cloud, must
sometimes contain a very great quantity of that fluid.
Or such a cloud, passing over woods of tall trees, may from the
points and sharp edges of their moist top leaves, receive silently
some supply.
A cloud being by any means supplied from the earth, may strike into
other clouds that have not been supplied, or not so much supplied;
and those to others, till an equilibrium is produced among all the
clouds that are within striking distance of each other.
The cloud thus supplied, having parted with much of what it first
received, may require and receive a fresh supply from the earth,
or from some other cloud, which by the wind is brought into such a
situation as to receive it more readily from the earth.
Hence repeated and continual strokes and flashes till the clouds have
all got nearly their natural quantity as clouds, or till they have
descended in showers, and are united again with this terraqueous
globe, their original.
Thus, thunder-clouds are generally in a negative state of electricity
compared with the earth, agreeable to most of our experiments; yet
as by one experiment we found a cloud electrised positively, I
conjecture that, in that case, such cloud, after having received
what was, in its rare state, only its _natural quantity_, became
compressed by the driving winds, or some other means, so that part
of what it had absorbed was forced out, and formed an electric
atmosphere around it in its denser state. Hence it was capable of
communicating positive electricity to my rod.
To show that a body in different circumstances of dilatation and
contraction is capable of receiving and retaining more or less of
the electric fluid on its surface, I would relate the following
experiment: I placed a clean wine glass on the floor, and on it a
small silver can. In the can I put about three yards of brass chain;
to one end of which I fastened a silk thread, which went right up to
the cieling, where it passed over a pulley, and came down again to
my hand, that I might at pleasure draw the chain up out of the can,
extending it till within a foot of the cieling, and let it gradually
sink into the can again.--From the cieling, by another thread of
fine raw silk, I suspended a small light lock of cotton, so as that
when it hung perpendicularly, it came in contact with the side of
the can.--Then approaching the wire of a charged phial to the can,
I gave it a spark, which flowed round in an electric atmosphere;
and the lock of cotton was repelled from the side of the can to the
distance of about nine or ten inches. The can would not then receive
another spark from the wire of the phial; but as I gradually drew
up the chain, the atmosphere of the can diminished by flowing over
the rising chain, and the lock of cotton accordingly drew nearer and
nearer to the can; and then, if I again brought the phial wire near
the can, it would receive another spark, and the cotton fly off
again to its first distance; and thus, as the chain was drawn higher,
the can would receive more sparks; because the can and extended chain
were capable of supporting a greater atmosphere than the can with
the chain gathered up into its belly.--And that the atmosphere round
the can was diminished by raising the chain, and increased again by
lowering it, is not only agreeable to reason, since the atmosphere
of the chain, must be drawn from that of the can, when it rose, and
returned to it again when it fell; but was also evident to the eye,
the lock of cotton always approaching the can when the chain was
drawn up, and receding when it was let down again.
Thus we see that increase of surface makes a body capable of
receiving a greater electric atmosphere: but this experiment does
not, I own, fully demonstrate my new hypothesis; for the brass and
silver still continue in their solid state, and are not rarefied into
vapour, as the water is in clouds. Perhaps some future experiments on
vapourized water may set this matter in a clearer light.
One seemingly material objection arises to the new hypothesis, and it
is this: If water, in its rarefied state, as a cloud, requires, and
will absorb more of the electric fluid than when in its dense state
as water, why does it not acquire from the earth all it wants at the
instant of its leaving the surface, while it is yet near, and but
just rising in vapour? To this difficulty I own I cannot at present
give a solution satisfactory to myself: I thought, however, that I
ought to state it in its full force, as I have done, and submit the
whole to examination.
And I would beg leave to recommend it to the curious in this branch
of natural philosophy, to repeat with care and accurate observation
the experiments I have reported in this and former papers relating to
_positive_ and _negative_ electricity, with such other relative ones
as shall occur to them, that it may be certainly known whether the
electricity communicated by a glass globe, be _really positive_. And
also I would request all who may have an opportunity of observing the
recent effects of lightning on buildings, trees, &c. that they would
consider them particularly with a view to discover the direction. But
in these examinations, this one thing is always to be understood,
viz. that a stream of the electric fluid passing through wood,
brick, metal, &c. while such fluid passes in _small quantity_, the
mutually repulsive power of its parts is confined and overcome by the
cohesion of the parts of the body it passes through, so as to prevent
an explosion; but when the fluid comes in a quantity too great to
be confined by such cohesion, it explodes, and rends or fuses the
body that endeavoured to confine it. If it be wood, brick, stone,
or the like, the splinters will fly off on that side where there is
least resistance. And thus, when a hole is struck through pasteboard
by the electrified jar, if the surfaces of the pasteboard are not
confined or compressed, there will be a bur raised all round the hole
on both sides the pasteboard; but if one side be confined, so that
the bur cannot be raised on that side, it will be all raised on the
other, which way soever the fluid was directed. For the bur round the
outside of the hole, is the effect of the explosion every way from
the centre of the stream, and not an effect of the direction.
In every stroke of lightning, I am of opinion that the stream of the
electric fluid, moving to restore the equilibrium between the cloud
and the earth, does always previously find its passage, and mark out,
as I may say, its own course, taking in its way all the conductors
it can find, such as metals, damp walls, moist wood, &c. and will go
considerably out of a direct course, for the sake of the assistance
of good conductors; and that, in this course, it is actually moving,
though silently and imperceptibly, before the explosion, in and among
the conductors; which explosion happens only when the conductors
cannot discharge it as fast as they receive it, by reason of their
being incomplete, dis-united, too small, or not of the best materials
for conducting. Metalline rods, therefore, of sufficient thickness,
and extending from the highest part of an edifice to the ground,
being of the best materials and complete conductors, will, I think,
secure the building from damage, either by restoring the equilibrium
so fast as to prevent a stroke, or by conducting it in the substance
of the rod as far as the rod goes, so that there shall be no
explosion but what is above its point, between that and the clouds.
If it be asked, what thickness of a metalline rod may be supposed
sufficient? In answer, I would remark, that five large glass jars,
such as I have described in my former papers, discharge a very great
quantity of electricity, which nevertheless will be all conducted
round the corner of a book, by the fine filleting of gold on the
cover, it following the gold the farthest way about, rather than
take the shorter course through the cover, that not being so good
a conductor. Now in this line of gold, the metal is so extremely
thin as to be little more than the colour of gold, and on an octavo
book is not in the whole an inch square, and therefore not the
thirty-sixth part of a grain, according to M. Reaumur; yet it is
sufficient to conduct the charge of five large jars, and how many
more I know not. Now, I suppose a wire of a quarter of an inch
diameter to contain about five thousand times as much metal as there
is in that gold line, and if so, it will conduct the charge of
twenty-five thousand such glass jars, which is a quantity, I imagine,
far beyond what was ever contained in any one stroke of natural
lightning. But a rod of half an inch diameter would conduct four
times as much as one of a quarter.
And with regard to conducting, though a certain thickness of metal
be required to conduct a great quantity of electricity, and, at the
same time, keep its own substance firm and unseparated; and a less
quantity, as a very small wire for instance, will be destroyed by
the explosion; yet such small wire will have answered the end of
conducting that stroke, though it become incapable of conducting
another. And considering the extreme rapidity with which the electric
fluid moves without exploding, when it has a free passage, or
compleat metal communication, I should think a vast quantity would
be conducted in a short time, either to or from a cloud, to restore
its equilibrium with the earth, by means of a very small wire; and
therefore thick rods should seem not so necessary.--However, as
the quantity of lightning discharged in one stroke, cannot well be
measured, and, in different strokes, is certainly very various, in
some much greater than others; and as iron (the best metal for the
purpose, being least apt to fuse) is cheap, it may be well enough
to provide a larger canal to guide that impetuous blast than we
imagine necessary: for, though one middling wire may be sufficient,
two or three can do no harm. And time, with careful observations
well compared, will at length point out the proper size to greater
certainty.
Pointed rods erected on edifices may likewise often prevent a stroke,
in the following manner: An eye so situated as to view horizontally
the under side of a thunder-cloud, will see it very ragged, with a
number of separate fragments, or petty clouds, one under another,
the lowest sometimes not far from the earth. These, as so many
stepping-stones, assist in conducting a stroke between the cloud
and a building. To represent these by an experiment, take two or
three locks of fine loose cotton, connect one of them with the prime
conductor by a fine thread of two inches (which may be spun out of
the same lock by the fingers) another to that, and the third to the
second, by like threads.--Turn the globe and you will see these locks
extend themselves towards the table (as the lower small clouds do
towards the earth) being attracted by it: but on presenting a sharp
point erect under the lowest, it will shrink up to the second, the
second to the first, and all together to the prime conductor, where
they will continue as long as the point continues under them. May
not, in like manner, the small electrised clouds, whose equilibrium
with the earth is soon restored by the point, rise up to the main
body, and by that means occasion so large a vacancy, as that the
grand cloud cannot strike in that place?
These thoughts, my dear friend, are many of them crude and hasty;
and if I were merely ambitious of acquiring some reputation
in philosophy, I ought to keep them by me, till corrected and
improved by time, and farther experience. But since even short
hints and imperfect experiments in any new branch of science,
being communicated, have oftentimes a good effect, in exciting the
attention of the ingenious to the subject, and so become the occasion
of more exact disquisition, and more compleat discoveries, you are
at liberty to communicate this paper to whom you please; it being of
more importance that knowledge should increase, than that your friend
should be thought an accurate philosopher.