more or less acute, draw on and throw off the electrical fluid with more or
less power, and at greater or less distances, and in larger or smaller
quantities in the same time, we may see how to account for the situation of
the leaf of gold suspended between two plates, the upper one continually
electrified, the under one in a person's hand standing on the floor. When
the upper plate is electrified, the leaf is attracted and raised towards
it, and would fly to that plate were it not for its own points. The corner
that happens to be uppermost when the leaf is rising, being a sharp point,
from the extream thinness of the gold, draws and receives at a distance a
sufficient quantity of the electrical fluid to give itself an electrical
atmosphere, by which its progress to the upper plate is stopt, and it
begins to be repelled from that plate, and would be driven back to the
under plate, but that its lowest corner is likewise a point, and throws off
or discharges the overplus of the leaf's atmosphere, as fast as the upper
corner draws it on. Were these two points perfectly equal in acuteness, the
leaf would take place exactly in the middle space, for its Weight is a
trifle, compared to the power acting on it: But it is generally nearest the
unelectrified plate, because, when the leaf is offered to the electrified
plate at a distance, the sharpest point is commonly first affected and
raised towards it; so that point, from its greater acuteness, receiving the
fluid faster than its opposite can discharge it at equal distances, it
retires from the electrified plate, and draws nearer to the unelectrified
plate, till it comes to a distance where the discharge can be exactly equal
to the receipt, the latter being lessened, and the former encreased; and
there it remains as long as the globe continues to supply fresh electrical
matter. This will appear plain, when the difference of acuteness in the
corners is made very great. Cut a piece of _Dutch_ gold (which is fittest
for these experiments on account of its greater strength) into the form of
FIG. 10 the upper corner a right angle, the two next obtuse angles, and the
lowest a very acute one; and bring this on your plate under the electrified
plate, in such a manner as that the right-angled part may be first raised
(which is done by covering the acute part with the hollow of your hand) and
you will see this leaf take place much nearer to the upper than to the
under plate; because, without being nearer, it cannot receive so fast at
its right-angled point, as it can discharge at its acute one. Turn this
leaf with the acute part uppermost, and then it takes place nearest the
unelectrified plate, because otherwise it receives faster at its acute
point than it can discharge at its right-angled one. Thus the difference of
distance is always proportioned to the difference of acuteness. Take care
in cutting your leaf to leave no little ragged particles on the edges,
which sometimes form points where you would not have them. You may make
this figure so acute below and blunt above, as to need no under plate, it
discharging fast enough into the air. When it is made narrower, as the
figure between the pricked lines, we call it the _Golden Fish_, from its
manner of acting. For if you take it by the tail, and hold it at a foot or
greater horizontal distance from the prime conductor, it will, when let go,
fly to it with a brisk but wavering motion, like that of an eel through the
water; it will then take place under the prime conductor, at perhaps a
quarter or half an inch distance, and keep a continual shaking of its tail
like a fish, so that it seems animated. Turn its tail towards the prime
conductor, and then it flies to your finger, and seems to nibble it. And if
you hold a plate under it at six or eight inches distance, and cease
turning the Globe, when the electrical atmosphere of the conductor grows
small, it will descend to the plate and swim back again several times with
the same fish-like motion, greatly to the entertainment of spectators. By a
little practice in blunting or sharpening the heads or tails of these
figures, you may make them take place as desired, nearer, or farther from
the electrified plate.
27. It is said in section 8, of this paper, that all kinds of common matter
are supposed not to attract the electrical fluid with equal strength; and
that those called electrics _per se_, as glass, &c. attract and retain it
strongest, and contain the greatest quantity. This latter position may seem
a paradox to some, being contrary to the hitherto received opinion; and
therefore I shall now endeavour to explain it.
28. In order to this, let it first be considered, _that we cannot, by any
means we are yet acquainted with, force the electrical fluid thro' glass_.
I know it is commonly thought that it easily pervades glass, and the
experiment of a feather suspended by a thread in a bottle hermetically
sealed, yet moved by bringing a nibbed tube near the outside of the bottle,
is alledged to prove it. But, if the electrical fluid so easily pervades
glass, how does the vial become _charged_ (as we term it) when we hold it
in our hands? Would not the fire thrown in by the wire pass through to our
hands, and so escape into the floor? Would not the bottle in that case be
left just as we found it, uncharged, as we know a metal bottle so attempted
to be charged would be? Indeed, if there be the least crack, the minutest
solution of continuity in the glass, though it remains so tight that
nothing else we know of will pass, yet the extremely subtile electrical
fluid flies through such a crack with the greatest freedom, and such a
bottle we know can never be charged: What then makes the difference between
such a bottle and one that is sound, but this, that the fluid can pass
through the one, and not through the other?[8]
29. It is true there is an experiment that at first sight would be apt to
satisfy a slight observer, that the fire thrown into the bottle by the
wire, does really pass thro' the glass. It is this: place the bottle on a
glass stand, under the prime conductor; suspend a bullet by a chain from
the prime conductor, till it comes within a quarter of an inch right over
the wire of the bottle; place your knuckle on the glass stand, at just the
same distance from the coating of the bottle, as the bullet is from its
wire. Now let the globe be turned, and you see a spark strike from the
bullet to the wire of the bottle, and the same instant you see and feel an
exactly equal spark striking from the coating on your knuckle, and so on
spark for spark. This looks as if the whole received by the bottle was
again discharged from it. And yet the bottle by this means is charged![9]
And therefore the fire that thus leaves the bottle, though the same in
quantity, cannot be the very same fire that entered at the wire; for if it
were, the bottle would remain uncharged.
30. If the fire that so leaves the bottle be not the same that is thrown in
through the wire, it must be fire that subsisted in the bottle, (that is,
in the glass of the bottle) before the operation began.
31. If so, there must be a great quantity in glass, because a great
quantity is thus discharged even from very thin glass.
32. That this electrical fluid or fire is strongly attracted by glass, we
know from the quickness and violence with which it is resumed by the part
that had been deprived of it, when there is an opportunity. And by this,
that we cannot from a mass of glass draw a quantity of electrical fire, or
electrify the whole mass _minus_, as we can a mass of metal. We cannot
lessen or increase its whole quantity, for the quantity it has it holds;
and it has as much as it can hold. Its pores are filled with it as full as
the mutual repellency of the particles will admit; and what is already in,
refuses, or strongly repels, any additional quantity. Nor have we any way
of moving the electrical fluid in glass, but one; that is, by covering part
of the two surfaces of thin glass with non-electrics, and then throwing an
additional quantity of this fluid on one surface, which spreading in the
non-electric, and being bound by it to that surface, acts by its repelling
force on the particles of the electrical fluid contained in the other
surface, and drives them out of the glass into the non-electric on that
side, from whence they are discharged, and then those added on the charged
side can enter. But when this is done, there is no more in the glass, nor
less than before, just as much having left it on one side as it received on
the other.
[Illustration]
33. I feel a want of terms here, and doubt much whether I shall be able to
make this part intelligible. By the word _surface_, in this case, I do not
mean mere length and breadth without thickness; but when I speak of the
upper or under surface of a piece of glass, the outer or inner surface of
the vial, I mean length, breadth, and half the thickness, and beg the
favour of being so understood. Now, I suppose, that glass in its first
principles, and in the Furnace, has no more of this electrical fluid than
other common matter: That when it is blown, as it cools, and the particles
of common fire leave it, its pores become a vacuum: That the component
parts of glass are extremely small and fine, I guess from its never showing
a rough face when it breaks, but always a polish; and from the smallness of
its particles I suppose the pores between them must be exceeding small,
which is the reason that Aqua-fortis, nor any other menstruum we have, can
enter to separate them and dissolve the substance; nor is any fluid we know
of, fine enough to enter, except common fire, and the electrical fluid. Now
the departing fire leaving a vacuum, as aforesaid, between these pores,
which air nor water are fine enough to enter and fill, the electrical fluid
(which is every where ready in what we call the non-electrics, and in the
non-electric Mixtures that are in the air,) is attracted in: yet does not
become fixed with the substance of the glass, but subsists there as water
in a porous stone, retained only by the attraction of the fixed parts,
itself still loose and a fluid. But I suppose farther, that in the cooling
of the glass, its texture becomes closest in the middle, and forms a kind
of partition, in which the pores are so narrow, that the particles of the
electrical fluid, which enter both surfaces at the same time, cannot go
through, or pass and repass from one surface to the other, and so mix
together; yet, though the particles of electrical fluid, imbibed by each
surface, cannot themselves pass through to those of the other, their
repellency can, and by this means they act on one another. The particles of
the electrical fluid have a mutual repellency, but by the power of
attraction in the glass they are condensed or forced nearer to each other.
When the glass has received and, by its attraction, forced closer together
so much of this electrified fluid, as that the power of attracting and
condensing in the one, is equal to the power of expansion in the other, it
can imbibe no more, and that remains its constant whole quantity; but each
surface would receive more, if the repellency of what is in the opposite
surface did not resist its entrance. The quantities of this fluid in each
surface being equal, their repelling action on each other is equal; and
therefore those of one surface cannot drive out those of the other: but, if
a greater quantity is forced into one surface than the glass would
naturally draw in; this increases the repelling power on that side, and
overpowering the attraction on the other, drives out part of the fluid that
had been imbibed by that surface, if there be any non-electric ready to
receive it: such there is in all cases where glass is electrified to give a
shock. The surface that has been thus emptied by having its electrical
fluid driven out, resumes again an equal quantity with violence, as soon as
the glass has an opportunity to discharge that over-quantity more than it
could retain by attraction in its other surface, by the additional
repellency of which the vacuum had been occasioned. For experiments
favouring (if I may not say confirming) this hypothesis, I must, to avoid
repetition, beg leave to refer you back to what is said of the electrical
phial in my former papers.
34. Let us now see how it will account for several other
appearances.--Glass, a body extremely elastic (and perhaps its elasticity
may be owing in some degree to the subsisting of so great a quantity of
this repelling fluid in its pores) must, when rubbed, have its rubbed
surface somewhat stretched, or its solid parts drawn a little farther
asunder, so that the vacancies in which the electrical fluid resides,
become larger, affording room for more of that fluid, which is immediately
attracted into it from the cushion or hand rubbing, they being supply'd
from the common stock. But the instant the parts of the glass so open'd and
fill'd have pass'd the friction, they close again, and force the additional
quantity out upon the surface, where it must rest till that part comes
round to the cushion again, unless some non electric (as the prime
conductor) first presents to receive it.[10] But if the inside of the globe
be lined with a non-electric, the additional repellency of the electrical
fluid, thus collected by friction on the rubb'd part of the globe's outer
surface, drives an equal quantity out of the inner surface into that
non-electric lining, which receiving it, and carrying it away from the
rubb'd part into the common mass, through the axis of the globe and frame
of the machine, the new collected electrical fluid can enter and remain in
the outer surface, and none of it (or a very little) will be received by
the prime conductor. As this charg'd part of the globe comes round to the
cushion again, the outer surface delivers its overplus fire into the
cushion, the opposite inner surface receiving at the same time an equal
quantity from the floor. Every electrician knows that a globe wet within
will afford little or no fire, but the reason has not before been attempted
to be given, that I know of.
34. So if a tube lined with a [11]non-electric, be rubb'd, little or no
fire is obtained from it. What is collected from the hand in the downward
rubbing stroke, entering the pores of the glass, and driving an equal
quantity out of the inner surface into the non-electric lining: and the
hand in passing up to take a second stroke, takes out again what had been
thrown into the outer surface, and then the inner surface receives back
again what it had given to the non-electric lining. Thus the particles of
electrical fluid belonging to the inside surface go in and out of their
pores every stroke given to the tube. Put a wire into the tube, the inward
end in contact with the non-electric lining, so it will represent the
_Leyden_ bottle. Let a second person touch the wire while you rub, and the
fire driven out of the inward surface when you give the stroke, will pass
through him into the common mass, and return through him when the inner
surface resumes its quantity, and therefore this new kind of _Leyden_
bottle cannot so be charged. But thus it may: after every stroke, before
you pass your hand up to make another, let the second person apply his
finger to the wire, take the spark, and then withdraw his finger; and so on
till he has drawn a number of sparks; thus will the inner surface be
exhausted, and the outer surface charged; then wrap a sheet of gilt paper
close round the outer surface, and grasping it in your hand you may receive
a shock by applying the finger of the other hand to the wire: for now the
vacant pores in the inner surface resume their quantity, and the
overcharg'd pores in the outer surface discharge that overplus; the
equilibrium being restored through your body, which could not be restored
through the glass.[12] If the tube be exhausted of air, a non electric
lining in contact with the wire is not necessary; for _in vacuo_, the
electrical fire will fly freely from the inner surface, without a
non-electric conductor: but air resists its motion; for being itself an
electric _per se_, it does not attract it, having already its quantity. So
the air never draws off an electric atmosphere from any body, but in
proportion to the non-electrics mix'd with it: it rather keeps such an
atmosphere confin'd, which from the mutual repulsion of its particles,
tends to dissipation, and would immediately dissipate _in vacuo_.--And thus
the experiment of the feather inclosed in a glass vessel hermetically
sealed, but moving on the approach of the rubbed tube, is explained: When
an additional quantity of the electrical fluid is applied to the side of
the vessel by the atmosphere of the tube, a quantity is repelled and driven
out of the inner surface of that side into the vessel, and there affects
the feather, returning again into its pores, when the tube with its
atmosphere is withdrawn; not that the particles of that atmosphere did
themselves pass through the glass to the feather.----And every other
appearance I have yet seen, in which glass and electricity are concern'd,
are, I think, explain'd with equal ease by the same hypothesis. Yet,
perhaps, it may not be a true one, and I shall be obliged to him that
affords me a better.
35. Thus I take the difference between non electrics and glass, an electric
_per se_, to consist in these two particulars. 1st, That a non-electric
easily suffers a change in the quantity of the electrical fluid it
contains. You may lessen its whole quantity by drawing out a part, which
the whole body will again resume; but of glass you can only lessen the
quantity contain'd in one of its surfaces; and not that, but by supplying
an equal quantity at the same time to the other surface; so that the whole
glass may always have the same quantity in the two surfaces, their two
different quantities being added together. And this can only be done in
glass that is thin; beyond a certain thickness we have yet no power that
can make this change. And, 2dly, that the electrical fire freely removes
from place to place, in and through the substance of a non-electric, but
not so through the substance of glass. If you offer a quantity to one end
of a long rod of metal, it receives it, and when it enters, every particle
that was before in the rod, pushes its neighbour quite to the further end,
where the overplus is discharg'd; and this instantaneously where the rod is
part of the circle in the experiment of the shock. But glass, from the
smallness of its pores, or stronger attraction of what it contains, refuses
to admit so free a motion; a glass rod will not conduct a shock, nor will
the thinnest glass suffer any particle entring one of its surfaces to pass
thro' to the other.
36. Hence we see the impossibility of success, in the experiments propos'd,
to draw out the effluvial virtues of a non-electric, as cinnamon for
instance, and mixing them with the electrical fluid, to convey them with
that into the body, by including it in the globe, and then applying
friction, etc. For though the effluvia of cinnamon, and the electrical
fluid should mix within the globe, they would never come out together
through the pores of the glass, and so go to the prime conductor; for the
electrical fluid itself cannot come through; and the prime conductor is
always supply'd from the cushion, and that from the floor. And besides,
when the globe is filled with cinnamon, or other non-electric, no
electrical fluid can be obtain'd from its outer surface, for the reason
before-mentioned. I have try'd another way, which I thought more likely to
obtain a mixture of the electrical and other effluvia together, if such a
mixture had been possible. I placed a glass plate under my cushion, to cut
off the communication between the cushion and floor; then brought a small
chain from the cushion into a glass of oil of turpentine, and carried
another chain from the oil of turpentine to the floor, taking care that the
chain from the cushion to the glass touch'd no part of the frame of the
machine. Another chain was fix'd to the prime conductor, and held in the
hand of a person to be electrised. The ends of the two chains in the glass
were near an inch distant from each other, the oil of turpentine between.
Now the globe being turn'd, could draw no fire from the floor through the
machine, the communication that way being cut off by the thick glass plate
under the cushion: it must then draw it through the chains whose ends were
dipt in the oil of turpentine. And as the oil of turpentine being an
electric _per se_, would not conduct what came up from the floor, was
obliged to jump from the end of one chain, to the end of the other, through
the substance of that oil, which we could see in large sparks; and so it
had a fair opportunity of seizing some of the finest particles of the oil
in its passage, and carrying them off with it: but no such effect followed,
nor could I perceive the least difference in the smell of the electrical
effluvia thus collected, from what it has when collected otherwise; nor
does it otherwise affect the body of a person electrised. I likewise put
into a phial, instead of water, a strong purgative liquid, and then charged
the phial, and took repeated shocks from it, in which case every particle
of the electrical fluid must, before it went through my body, have first
gone through the liquid when the phial is charging, and returned through it
when discharging, yet no other effect followed than if it had been charged
with water. I have also smelt the electrical fire when drawn through gold,
silver, copper, lead, iron, wood, and the human body, and could perceive no
difference; the odour is always the same where the spark does not burn what
it strikes; and therefore I imagine it does not take that smell from any
quality of the bodies it passes through. And indeed, as that smell so
readily leaves the electrical matter, and adheres to the knuckle receiving
the sparks, and to other things; I suspect that it never was connected with
it, but arises instantaneously from something in the air acted upon by it.
For if it was fine enough to come with the electrical fluid through the
body of one person, why should it stop on the skin of another?
But I shall never have done, if I tell you all my conjectures, thoughts,
and imaginations, on the nature and operations of this electrical fluid,
and relate the variety of little experiments we have try'd. I have already
made this paper too long, for which I must crave pardon, not having now
time to make it shorter. I shall only add, that as it has been observed
here that spirits will fire by the electrical spark in the summer time,
without heating them, when _Fahrenheit_'s thermometer is above 70; so, when
colder, if the operator puts a small flat bottle of spirits in his bosom,
or a close pocket, with the spoon, some little time before he uses them,
the heat of his body will communicate warmth more than sufficient for the
purpose.
ADDITIONAL EXPERIMENT, _proving that the_ Leyden Bottle _has no more
electrical Fire in it when charged, than before; nor less when
discharged: That in discharging, the Fire does not issue from the Wire
and the Coating at the same Time, as some have thought, but that the
Coating always receives what is discharged by the Wire, or an equal
Quantity; the outer Surface being always in a negative State of
Electricity, when the inner Surface is in a positive State_.
Place a thick plate of glass under the rubbing cushion, to cut off the
communication of electrical fire from the floor to the cushion; then, if
there be no fine points or hairy threads sticking out from the cushion, or
from the parts of the machine opposite to the cushion, (of which you must
be careful) you can get but a few sparks from the prime conductor, which
are all the cushion will part with.
Hang a phial then on the prime conductor, and it will not charge, tho' you
hold it by the coating.----But
Form a communication by a chain from the coating to the cushion, and the
phial will charge.
For the globe then draws the electrical fire out of the outside surface of
the phial, and forces it, through the prime conductor and wire of the
phial, into the inside surface.
Thus the bottle is charged with its own fire, no other being to be had
while the glass plate is under the cushion.
Hang two cork balls by flaxen threads to the prime conductor; then touch
the coating of the bottle, and they will be electrified and recede from
each other.
For just as much fire as you give the coating, so much is discharged
through the wire upon the prime conductor, whence the cork balls receive an
electrical atmosphere. But