SIR,

I received your ingenious letter of the 12th of March last, and thank

you cordially for the account you give me of the new experiments you

have lately made in electricity.--It is a subject that still affords

me pleasure, though of late I have not much attended to it.

Your second experiment, in which you attempted, without success, to

communicate positive electricity by vapour ascending from electrised

water, reminds me of one I formerly made, to try if negative

electricity might be produced by evaporation only. I placed a large

heated brass plate, containing four or five square feet on an

electric stand; a rod of metal, about four feet long, with a bullet

at its end, extended from the plate horizontally. A light lock of

cotton, suspended a fine thread from the cieling, hung opposite to,

and within an inch of the bullet. I then sprinkled the heated plate

with water, which arose fast from it in vapour. If vapour should be

disposed to carry off the electrical, as it does the common fire from

bodies, I expected the plate would, by losing some of its natural

quantity, become negatively electrised. But I could not perceive, by

any motion in the cotton, that it was at all affected: nor by any

separation of small cork-balls suspended from the plate, could it be

observed that the plate was in any manner electrified.

Mr. Canton here has also found, that two tea-cups, set on electric

stands, and filled, one with boiling, the other with cold water,

and equally electrified, continued equally so, notwithstanding the

plentiful evaporation from the hot water. Your experiment and his

agreeing, show another remarkable difference between electric and

common fire. For the latter quits most readily the body that contains

it, where water, or any other fluid, is evaporating from the surface

of that body, and escapes with the vapour. Hence the method, long in

use in the east, of cooling liquors, by wrapping the bottles round

with a wet cloth, and exposing them to the wind. Dr. Cullen, of

Edinburgh, has given some experiments of cooling by evaporation; and

I was present at one made by Dr. Hadley, then professor of chemistry

at Cambridge, when, by repeatedly wetting the ball of a thermometer

with spirit, and quickening the evaporation by the blast of a

bellows, the mercury fell from 65, the state of warmth in the common

air, to 7, which is 22 degrees below freezing; and, accordingly,

from some water mixed with the spirit, or from the breath of the

assistants, or both, ice gathered in small spicula round the ball,

to the thickness of near a quarter of an inch. To such a degree did

the mercury lose the fire it before contained, which, as I imagine,

took the opportunity of escaping, in company with the evaporating

particles of the spirit, by adhering to those particles.

Your experiment of the Florence flask, and boiling water, is very

curious. I have repeated it, and found it to succeed as you describe

it, in two flasks out of three. The third would not charge when

filled with either hot or cold water. I repeated it, because I

remembered I had once attempted to make an electric bottle of a

Florence flask, filled with cold water, but could not charge it at

all; which I then imputed to some imperceptible cracks in the small,

extremely thin bubbles, of which that glass is full, and I concluded

none of that kind would do. But you have shown me my mistake.--Mr.

Wilson had formerly acquainted us, that red hot glass would conduct

electricity; but that so small a degree of heat, as that communicated

by boiling water, would so open the pores of extremely thin glass, as

to suffer the electric fluid freely to pass, was not before known.

Some experiments similar to yours, have, however, been made here,

before the receipt of your letter, of which I shall now give you an

account.

I formerly had an opinion that a Leyden bottle, charged and then

sealed hermetically, might retain its electricity for ever; but

having afterwards some suspicion that possibly that subtle fluid

might, by slow imperceptible degrees, soak through the glass, and in

time escape, I requested some of my friends, who had conveniences for

doing it, to make trial, whether, after some months, the charge of a

bottle so sealed would be sensibly diminished. Being at Birmingham,

in September, 1760, Mr. Bolton of that place opened a bottle that

had been charged, and its long tube neck hermetically sealed in

the January preceding. On breaking off the end of the neck, and

introducing a wire into it, we found it possessed of a considerable

quantity of electricity, which was discharged by a snap and spark.

This bottle had lain near seven months on a shelf, in a closet, in

contact with bodies that would undoubtedly have carried off all its

electricity, if it could have come readily through the glass. Yet

as the quantity manifested by the discharge was not apparently so

great as might have been expected from a bottle of that size well

charged, some doubt remained whether part had escaped while the neck

was sealing, or had since, by degrees, soaked through the glass. But

an experiment of Mr. Canton's, in which such a bottle was kept under

water a week, without having its electricity in the least impaired,

seems to show, that when the glass is cold, though extremely thin,

the electric fluid is well retained by it. As that ingenious and

accurate experimenter made a discovery, like yours, of the effect

of heat in rendering thin glass permeable by that fluid, it is but

doing him justice to give you his account of it, in his own words,

extracted from his letter to me, in which he communicated it, dated

Oct. 31, 1760, _viz_.

"Having procured some thin glass balls, of about an inch and a

half in diameter, with stems, or tubes, of eight or nine inches in

length, I electrified them, some positively on the inside, and others

negatively, after the manner of charging the Leyden bottle, and

sealed them hermetically. Soon after I applied the naked balls to my

electrometer, and could not discover the least sign of their being

electrical, but holding them, before the fire, at the distance of

six or eight inches, they became strongly electrical in a very short

time, and more so when they were cooling. These balls will, every

time they are heated, give the electrical fluid to, or take it from

other bodies, according to the _plus_ or _minus_ state of it within

them. Heating them frequently, I find will sensibly diminish their

power; but keeping one of them under water a week did not appear

in the least degree to impair it. That which I kept under water,

was charged on the 22d of September last, was several times heated

before it was kept in water, and has been heated frequently since,

and yet it still retains its virtue to a very considerable degree.

The breaking two of my balls accidentally gave me an opportunity of

measuring their thickness, which I found to be between seven and

eight parts in a thousand of an inch.

A down feather, in a thin glass ball, hermetically sealed, will not

be affected by the application of an excited tube, or the wire of a

charged phial, unless the ball be considerably heated; and if a glass

pane be heated till it begins to grow soft, and in that state be held

between the wire of a charged phial, and the discharging wire, the

course of the electrical fluid will not be through the glass, but on

the surface, round by the edge of it."

By this last experiment of Mr. Canton's, it appears, that though by

a moderate heat, thin glass becomes, in some degree, a conductor of

electricity, yet, when of the thickness of a common pane, it is not,

though in a state near melting, so good a conductor as to pass the

shock of a discharged bottle. There are other conductors which suffer

the electric fluid to pass through them gradually, and yet will not

conduct a shock. For instance, a quire of paper will conduct through

its whole length, so as to electrify a person, who, standing on wax,

presents the paper to an electrified prime conductor; but it will

not conduct a shock even through its thickness only; hence the shock

either fails, or passes by rending a hole in the paper. Thus a sieve

will pass water gradually, but a stream from a fire engine would

either be stopped by it, or tear a hole through it.

It should seem, that to make glass permeable to the electric fluid,

the heat should be proportioned to the thickness. You found the heat

of boiling water, which is but 210, sufficient to render the extreme

thin glass in a Florence flask permeable even to a shock.--Lord

Charles Cavendish, by a very ingenious experiment, has found the heat

of 400 requisite to render thicker glass permeable to the common

current.

"A glass tube, (See _Plate_ III.) of which the part C B was solid,

had wire thrust in each end, reaching to B and C.

"A small wire was tied on at D, reaching to the floor, in order to

carry off any electricity that might run along upon the tube.

"The bent part was placed in an iron pot, filled with iron filings;

a thermometer was also put into the filings; a lamp was placed under

the pot; and the whole was supported upon glass.

"The wire A being electrified by a machine, before the heat was

applied, the corks at E separated, at first upon the principle of the

Leyden phial.

"But after the part C B of the tube was heated to 600, the corks

continued to separate, though you discharged the electricity by

touching the wire at E, the electrical machine continuing in motion.

"Upon letting the whole cool, the effect remained till the

thermometer was sunk to 400."

[Illustration: (of the experiment above)

_Plate III._ _Vol. I. page 348._

]

It were to be wished, that this noble philosopher would communicate

more of his experiments to the world, as he makes many, and with

great accuracy.

You know I have always looked upon and mentioned the equal repulsion

in cases of positive and of negative electricity, as a phenomenon

difficult to be explained. I have sometimes, too, been inclined,

with you, to resolve all into attraction; but besides that

attraction seems in itself as unintelligible as repulsion, there are

some appearances of repulsion that I cannot so easily explain by

attraction; this for one instance. When the pair of cork balls are

suspended by flaxen threads, from the end of the prime conductor,

if you bring a rubbed glass tube near the conductor, but without

touching it, you see the balls separate, as being electrified

positively; and yet you have communicated no electricity to the

conductor, for, if you had, it would have remained there, after

withdrawing the tube; but the closing of the balls immediately

thereupon, shows that the conductor has no more left in it than its

natural quantity. Then again approaching the conductor with the

rubbed tube, if, while the balls are separated, you touch with a

finger that end of the conductor to which they hang, they will come

together again, as being, with that part of the conductor, brought

to the same state with your finger, _i. e._ the natural state. But

the other end of the conductor, near which the tube is held, is not

in that state, but in the negative state, as appears on removing the

tube; for then part of the natural quantity left at the end near

the balls, leaving that end to supply what is wanting at the other,

the whole conductor is found to be equally in the negative state.

Does not this indicate that the electricity of the rubbed tube had

repelled the electric fluid, which was diffused in the conductor

while in its natural state, and forced it to quit the end to which

the tube was brought near, accumulating itself on the end to which

the balls were suspended? I own I find it difficult to account for

its quitting that end, on the approach of the rubbed tube, but on the

supposition of repulsion; for, while the conductor was in the same

state with the air, _i. e._ the natural state, it does not seem to

me easy to suppose, that an attraction should suddenly take place

between the air and the natural quantity of the electric fluid in the

conductor, so as to draw it to, and accumulate it on the end opposite

to that approached by the tube; since bodies, possessing only their

natural quantity of that fluid, are not usually seen to attract each

other, or to affect mutually the quantities of electricity each

contains.

There are likewise appearances of repulsion in other parts of nature.

Not to mention the violent force with which the particles of water,

heated to a certain degree, separate from each other, or those of

gunpowder, when touched with the smallest spark of fire, there is

the seeming repulsion between the same poles of the magnet, a body

containing a subtle moveable fluid in many respects analagous to the

electric fluid. If two magnets are so suspended by strings, as that

their poles of the same denomination are opposite to each other, they

will separate, and continue so; or if you lay a magnetic steel bar

on a smooth table, and approach it with another parallel to it, the

poles of both in the same position, the first will recede from the

second, so as to avoid the contact, and may thus be pushed (or at

least appear to be pushed) off the table. Can this be ascribed to the

attraction of any surrounding body or matter drawing them asunder,

or drawing the one away from the other? If not, and repulsion exists

in nature, and in magnetism, why may it not exist in electricity? We

should not, indeed, multiply causes in philosophy without necessity;

and the greater simplicity of your hypothesis would recommend it to

me, if I could see that all appearances would be solved by it. But

I find, or think I find, the two causes more convenient than one of

them alone. Thus I might solve the circular motion of your horizontal

stick, supported on a pivot, with two pins at their ends, pointing

contrary ways, and moving in the same direction when electrified,

whether positively or negatively: when positively, the air opposite

to the points being electrised positively, repels the points; when

negatively, the air opposite the points being also, by their means,

electrised negatively, attraction takes place between the electricity

in the air behind the heads of the pins, and the negative pins, and

so they are, in this case, drawn in the same direction that in the

other they were driven.--You see I am willing to meet you half way,

a complaisance I have not met with in our brother Nollet, or any

other hypothesis-maker, and therefore may value myself a little upon

it, especially as they say I have some ability in defending even the

wrong side of a question, when I think fit to take it in hand.

What you give as an established law of the electric fluid, "That

quantities of different densities mutually attract each other, in

order to restore the equilibrium," is, I think, not well founded,

or else not well expressed. Two large cork balls, suspended by

silk strings, and both well and equally electrified, separate to a

great distance. By bringing into contact with one of them another

ball of the same size, suspended likewise by silk, you will take

from it half its electricity. It will then, indeed, hang at a less

distance from the other, but the full and the half quantities will

not appear to attract each other, that is, the balls will not come

together. Indeed, I do not know any proof we have, that one quantity

of electric fluid is attracted by another quantity of that fluid,

whatever difference there may be in their densities. And, supposing

in nature, a mutual attraction between two parcels of any kind

of matter, it would be strange if this attraction should subsist

strongly while those parcels were unequal, and cease when more matter

of the same kind was added to the smallest parcel, so as to make it

equal to the biggest. By all the laws of attraction in matter, that

we are acquainted with, the attraction is stronger in proportion to

the increase of the masses, and never in proportion to the difference

of the masses. I should rather think the law would be, "That the

electric fluid is attracted strongly by all other matter that we

know of, while the parts of that fluid mutually repel each other."

Hence its being equally diffused (except in particular circumstances)

throughout all other matter. But this you jokingly call "electrical

orthodoxy." It is so with some at present, but not with all; and,

perhaps, it may not always be orthodoxy with any body. Opinions are

continually varying, where we cannot have mathematical evidence of

the nature of things; and they must vary. Nor is that variation

without its use, since it occasions a more thorough discussion,

whereby error is often dissipated, true knowledge is encreased, and

its principles become better understood and more firmly established.

Air should have, as you observe, "its share of the common stock of

electricity, as well as glass, and, perhaps, all other electrics

_per se_." But I suppose, that, like them, it does not easily part

with what it has, or receive more, unless when mixed with some

non-electric, as moisture for instance, of which there is some in our

driest air. This, however, is only a supposition; and your experiment

of restoring electricity to a negatively electrised person, by

extending his arm upwards into the air, with a needle between his

fingers, on the point of which light may be seen in the night, is,

indeed, a curious one. In this town the air is generally moister than

with us, and here I have seen Mr. Canton electrify the air in one

room positively, and in another, which communicated by a door, he has

electrised the air negatively. The difference was easily discovered

by his cork balls, as he passed out of one room into another.--Pere

Beccaria, too, has a pretty experiment, which shows that air may

be electrised. Suspending a pair of small light balls, by flaxen

threads, to the end of his prime conductor, he turns his globe some

time, electrising positively, the balls diverging and continuing

separate all the time. Then he presents the point of a needle to

his conductor, which gradually drawing off the electric fluid, the

balls approach each other, and touch, before all is drawn from the

conductor; opening again as more is drawn off, and separating nearly

as wide as at first, when the conductor is reduced to the natural

state. By this it appears, that when the balls came together, the air

surrounding the balls was just as much electrised as the conductor

at that time; and more than the conductor, when that was reduced to

its natural state. For the balls, though in the natural state, will

diverge, when the air that surrounds them is electrised _plus_ or

_minus_, as well as when that is in its natural state and they are

electrised _plus_ or _minus_ themselves. I foresee that you will

apply this experiment to the support of your hypothesis, and I think

you may make a good deal of it.

It was a curious enquiry of yours, Whether the electricity of the

air, in clear dry weather, be of the same density at the height of

two or three hundred yards, as near the surface of the earth; and

I am glad you made the experiment. Upon reflection, it should seem

probable, that whether the general state of the atmosphere at any

time be positive or negative, that part of it which is next the earth

will be nearer the natural state, by having given to the earth in one

case, or having received from it in the other. In electrising the

air of a room, that which is nearest the walls, or floor, is least

altered. There is only one small ambiguity in the experiment, which

may be cleared by more trials; it arises from the supposition that

bodies may be electrised positively by the friction of air blowing

strongly on them, as it does on the kite and its string. If at some

times the electricity appears to be negative, as that friction is the

same, the effect must be from a negative state of the upper air.

I am much pleased with your electrical thermometer, and the

experiments you have made with it. I formerly satisfied myself by an

experiment with my phial and syphon, that the elasticity of the air

was not increased by the mere existence of an electric atmosphere

within the phial; but I did not know, till you now inform me, that

heat may be given to it by an electric explosion. The continuance

of its rarefaction, for some time after the discharge of your glass

jar and of your case of bottles, seem to make this clear. The

other experiments on wet paper, wet thread, green grass, and green

wood, are not so satisfactory; as possibly the reducing part of

the moisture to vapour, by the electric fluid passing through it,

might occasion some expansion which would be gradually reduced by

the condensation of such vapour. The fine silver thread, the very

small brass wire, and the strip of gilt paper, are also subject to a

similar objection, as even metals, in such circumstances, are often

partly reduced to smoke, particularly the gilding on paper.

But your subsequent beautiful experiment on the wire, which you made

hot by the electric explosion, and in that state fired gunpowder

with it, puts it out of all question, that heat is produced by our

artificial electricity, and that the melting of metals in that way,

is not by what I formerly called a cold fusion. A late instance here,

of the melting a bell-wire, in a house struck by lightning, and parts

of the wire burning holes in the floor on which they fell, has proved

the same with regard to the electricity of nature. I was too easily

led into that error by accounts given, even in philosophical books,

and from remote ages downwards, of melting money in purses, swords

in scabbards, &c. without burning the inflammable matters that were

so near those melted metals. But men are, in general, such careless

observers, that a philosopher cannot be too much on his guard in

crediting their relations of things extraordinary, and should never

build an hypothesis on any thing but clear facts and experiments, or

it will be in danger of soon falling, as this does, like a house of

cards.

How many ways there are of kindling fire, or producing heat in

bodies! By the sun's rays, by collision, by friction, by hammering,

by putrefaction, by fermentation, by mixtures of fluids, by mixtures

of solids with fluids, and by electricity. And yet the fire when

produced, though in different bodies it may differ in circumstances,

as in colour, vehemence, &c. yet in the same bodies is generally the

same. Does not this seem to indicate that the fire existed in the

body, though in a quiescent state, before it was by any of these

means excited, disengaged, and brought forth to action and to view?

May it not constitute a part, and even a principal part, of the

solid substance of bodies? If this should be the case, kindling fire

in a body would be nothing more than developing this inflammable

principle, and setting it at liberty to act in separating the parts

of that body, which then exhibits the appearances of scorching,

melting, burning, &c. When a man lights an hundred candles from the

flame of one, without diminishing that flame, can it be properly

said to have _communicated_ all that fire? When a single spark from

a flint, applied to a magazine of gunpowder, is immediately attended

with this consequence, that the whole is in flame, exploding with

immense violence, could all this fire exist first in the spark? We

cannot conceive it. And thus we seem led to this supposition, that

there is fire enough in all bodies to singe, melt, or burn them,

whenever it is, by any means, set at liberty, so that it may exert

itself upon them, or be disengaged from them. This liberty seems to

be afforded it by the passage of electricity through them, which

we know can and does, of itself, separate the parts even of water;

and perhaps the immediate appearances of fire are only the effects

of such separations? If so, there would be no need of supposing that

the electric fluid _heats itself_ by the swiftness of its motion, or

heats bodies by the resistance it meets with in passing through them.

They would only be heated in proportion as such separation could be

more easily made. Thus a melting heat cannot be given to a large wire

in the flame of a candle, though it may to a small one; and this not

because the large wire resists _less_ that action of the flame which

tends to separate its parts, but because it resists it _more_ than

the smaller wire; or because the force being divided among more parts

acts weaker on each.

This reminds me, however, of a little experiment I have frequently

made, that shows, at one operation, the different effects of the same

quantity of electric fluid passing through different quantities of

metal. A strip of tinfoil, three inches long, a quarter of an inch

wide at one end, and tapering all the way to a sharp point at the

other, fixed between two pieces of glass, and having the electricity

of a large glass jar sent through it, will not be discomposed in the

broadest part; towards the middle will appear melted in spots; where

narrower, it will be quite melted; and about half an inch of it next

the point will be reduced to smoke.

You were not mistaken in supposing that your account of the effect of

the pointed rod, in securing Mr. West's house from damage by a stroke

of lightning, would give me great pleasure. I thank you for it most

heartily, and for the pains you have taken in giving me so complete a

description of its situation, form, and substance, with the draft of

the melted point. There is one circumstance, viz. that the lightning

was seen to diffuse itself from the foot of the rod over the wet

pavement, which seems, I think, to indicate, that the earth under

the pavement was very dry, and that the rod should have been sunk

deeper, till it came to earth moister, and therefore apter to receive

and dissipate the electric fluid. And although, in this instance,

a conductor formed of nail rods, not much above a quarter of an

inch thick, served well to convey the lightning, yet some accounts

I have seen from Carolina, give reason to think, that larger may

be sometimes necessary, at least for the security of the conductor

itself, which, when too small, may be destroyed in executing its

office, though it does, at the same time, preserve the house. Indeed,

in the construction of an instrument so new, and of which we could

have so little experience, it is rather lucky that we should at first

be so near the truth as we seem to be, and commit so few errors.

There is another reason for sinking deeper the lower end of the

rod, and also for turning it outwards under ground to some distance

from the foundation; it is this, that water dripping from the eaves

falls near the foundation, and sometimes soaks down there in greater

quantities, so as to come near the end of the rod, though the ground

about it be drier. In such case, this water may be exploded, that is,

blown into vapour, whereby a force is generated, that may damage the

foundation. Water reduced to vapour, is said to occupy 14,000 times

its former space. I have sent a charge through a small glass tube,

that has borne it well while empty, but when filled first with water,

was shattered to pieces and driven all about the room:--Finding no

part of the water on the table, I suspected it to have been reduced

to vapour; and was confirmed in that suspicion afterwards, when I

had filled a like piece of tube with ink, and laid it on a sheet of

clean paper, whereon, after the explosion, I could find neither any

moisture nor any sully from the ink. This experiment of the explosion

of water, which I believe was first made by that most ingenious

electrician, father Beccaria, may account for what we sometimes

see in a tree struck by lightning, when part of it is reduced to

fine splinters like a broom; the sap vessels being so many tubes

containing a watry fluid, which, when reduced to vapour, rends every

tube lengthways. And perhaps it is this rarefaction of the fluids in

animal bodies killed by lightning or electricity, that, by separating

its fibres, renders the flesh so tender, and apt so much sooner to

putrify. I think too, that much of the damage done by lightning to

stone and brick-walls may sometimes be owing to the explosion of

water, found, during showers, running or lodging in the joints or

small cavities or cracks that happen to be in the walls.

Here are some electricians that recommend knobs instead of points

on the upper end of the rods, from a supposition that the points

invite the stroke. It is true that points draw electricity at greater

distances in the gradual silent way; but knobs will draw at the

greatest distance a stroke. There is an experiment that will settle

this. Take a crooked wire of the thickness of a quill, and of such a

length as that one end of it being applied to the lower part of a

charged bottle, the upper may be brought near the ball on the top of

the wire that is in the bottle. Let one end of this wire be furnished

with a knob, and the other may be gradually tapered to a fine point.

When the point is presented to discharge the bottle, it must be

brought much nearer before it will receive the stroke, than the

knob requires to be. Points besides tend to repel the fragments of

an electrised cloud, knobs draw them nearer. An experiment, which I

believe I have shewn you, of cotton fleece hanging from an electrised

body, shows this clearly when a point or a knob is presented under it.

You seem to think highly of the importance of this discovery, as

do many others on our side of the water. Here it is very little

regarded; so little, that though it is now seven or eight years

since it was made public, I have not heard of a single house as

yet attempted to be secured by it. It is true the mischiefs done

by lightning are not so frequent here as with us, and those who

calculate chances may perhaps find that not one death (or the

destruction of one house) in a hundred thousand happens from

that cause, and that therefore it is scarce worth while to be at

any expence to guard against it.--But in all countries there are

particular situations of buildings more exposed than others to

such accidents, and there are minds so strongly impressed with the

apprehension of them, as to be very unhappy every time a little

thunder is within their hearing;--it may therefore be well to render

this little piece of new knowledge as general and as well understood

as possible, since to make us _safe_ is not all its advantage, it is

some to make us _easy_. And as the stroke it secures us from might

have chanced perhaps but once in our lives, while it may relieve us

a hundred times from those painful apprehensions, the latter may

possibly on the whole contribute more to the happiness of mankind

than the former.

Your kind wishes and congratulations are very obliging. I return them

cordially;--being, with great regard and esteem,

My dear Sir,

Your affectionate friend,

And most obedient humble servant,