SIR,

THE inclosed papers were designed for the perusal of the Royal Society;

being an endeavour to confute the opinion of Mr. Eeles about the ascent

of vapours, published in the last volume of their Transactions. But the

author, having no electrical friend, whose sagacity he could confide

in, has at length prevailed upon himself to be so free to send them to

Mr. Watson; to whom the world is so much indebted for the advancement

of their knowlege in electricity.

Whence, Sir, if you should think that these papers have truth, the

great Diana of real philosophers, to patronize them, you will confer

a favour upon me, by laying them before that learned Body. If, on

the contrary, you should deem this confutation trifling or futil,

I hope you will be humane enough to suppress them, and give me your

objections; and by that means lay a still greater obligation on one,

who has not the pleasure to be personally acquainted with you. From,

SIR,

Your very humble Servant,

Erasmus Darwin,

_Physician at Litchfield, Staffordshire_.

March 23. 1757.

+LETTER I.+

_To the very honourable and learned the_ PRESIDENT _and_ MEMBERS _of

the_ Royal Society.

[Read May 5, 1757.]

Gentlemen,

THERE is ever such a charm attendant upon novelty, that be it in

philosophy, medicine, or religion, the gazing world are too often led

to adore, what they ought only to admire: whilst this vehemence of

enthusiasm has generally soon rendered that object contemptible, that

would otherwise have long laid claim to a more sober esteem. This was

once the fate of chemistry: the vain and pompous boasts of her adepts

brought the whole art into disrespect; and I should be sorry, if her

sister electricity should share the same misfortunes. It is hence the

ingenious Mr. Eeles will excuse me, for endeavouring to lay before you

my opinion on the ascent of vapours, tho’ it by no means coincides with

that he is so strenuous to establish, and plucks a plume from his idol

goddess electricity.

The probability, supporting the hypothesis of Mr. Eeles, according to

his own expressions, rests on this: “That every particle of vapour is

endued with a portion of electric fire; and that there is no other

sufficient cause assigned for their ascending.” (_Phil. Trans. vol._

xlix. _part._ i. _p._ 134.). My design is therefore first to attempt

to shew, that another theory, founded on principles better known, will

sufficiently explain the ascent of vapours: and then, that some kinds

of vapours are not endued with a more or less than their natural share

of electric æther.

The immense rarefaction of explosive bodies by heat, depends either

on the escape of air before condensed in them, or on the expansion

of the constituent parts of those bodies. This distinction has not

been sufficiently considered by any one to my knowlege; nor shall I

at present amuse the Society upon this head; it being enough for my

present purpose to observe, that they may be thus distinguished: where

air is emitted, it cannot be condensed again into the same bulk by

cold; but the expansion of heated parts of bodies, as soon as that heat

is withdrawn, ceases to exist.

Nitre comes under the first of these classes: in detonation it emits

great quantities of air, not afterwards condensible to the like space.

This may be seen by firing a few grains of gunpowder in an unblown

bladder, or in a vessel nearly full of water with its mouth inverted.

The same is true of all the solid parts of animals and vegetables, when

subjected to fire; as appears from the experiments of that learned

philanthropist, Dr. Hales.

But of water the contrary is evident. In the steam-engine, a jet of

cold water, we find, instantly condenses that immense rarefaction;

which I apprehend could not be, if it was constituted of escaped

elastic air. And altho’ this steam must be acknowledged to put on some

properties of air; such as ventilating a fire; or that a taper blown

out by it, is capable of being again lighted immediately, and that

without a crackling noise, which occurs when touched with water; this

does not in the least invalidate our opinion, tho’ it has certainly

conduced very much to propagate the former one: since from this way of

reasoning, the whole must be air, and we should have no water at all in

vapour.

From considering this power of expansion, which the constituent parts

of some bodies acquire by heat; and withal, that some bodies have

a greater affinity to heat, that is, acquire it sooner and retain

it longer than others; which affinity appears from experiments, and

which, I apprehend, is in some ratio of their specific gravities and

their powers of refraction, reflexion, or absorption of light; or at

least in some ratio much greater than that of their specific gravities

alone. From considering these, I say, many things, before utterly

inexplicable, became easily understood by me. Such as, Why when bismuth

and zinc are fused together, and set to cool, the zinc, which is

specifically heavier, is found above the bismuth? Why the buff covering

of inflammatory blood, the skum of heated milk, the sedative salt of

borax, which are all specifically heavier than the liquids in which

they are formed, are still formed at the surface of them? How benzoin,

sulphur, and even the ponderous body mercury, may be raised into

vapour, again to be condensed unaltered? And, lastly, how water, whose

parts appear from the æolipile to be capable of immeasurable expansion,

should by heat alone become specifically lighter than the common

atmosphere, without having recourse to a shell inclosing air, or other

assistant machinery? and when raised, I am persuaded we shall find,

that to support them floating, perhaps many days, in the atmosphere, is

not a knot so intricate, as to oblige us to conjure up a new divinity

to unravel it.

But before we proceed to this second part of our task, it will be

necessary previously to consider, first, how small a degree of heat is

required to detach or raise the vapour of water from its parent-fluid.

In the coldest day, I might say the coldest night, of winter, when the

weather is not frosty or very damp, wet linen or paper will become dry

in the course of a few hours. A greater degree of heat must indeed

cause a quicker evaporation. But I am persuaded, that was it not for

the pressure of the superincumbent fluid, greatly less than that of

boiling water would instantly disperse the whole so heated into vapour.

Secondly, That in the opinion of Sir Isaac Newton, well illustrated by

the late lamented Mr. Melvil, the sun-beams appear only to communicate

heat to bodies by which they are refracted, reflected, or obstructed;

whence, by their impulse, a reaction or vibration is caused in the

parts of such impacted bodies.

This is supported by the experiment of approaching some light body,

or blowing smoke near the focus of the largest glasses; and from

observing, that these do not ascend, it is evident the air is not

so much as warmed by the passage of those beams thro’ it, yet would

instantly calcine or vitrify every opake body in nature. And from

this we may collect, that transparent bodies are only heated at

their surfaces, and that perhaps in proportion to their quantity of

refraction: which will further give and receive illustration from

those very curious experiments, of producing cold by the evaporation

of liquors, published by the learned Dr. Cullen, in the late volume

of Essays Physical and Literary, at Edinburgh. In these experiments a

spirit-thermometer was immersed in spirit of wine, and being suddenly

retracted, was again exposed to the air; and as the spirit of wine

adhering to the glass evaporated, the spirit contained within the

thermometer was observed to subside. Now as the difference of the

refraction of spirit of wine and glass is exceedingly minute, compared

with the difference of refraction of spirit of wine and air; we may

consider, in the above experiment, the heat to be communicated to

the thermometer only at its surface: but here the adherent fluid

escapes as soon as heated; by which means the glass, and its contents,

are deprived of that constant addition of heat, which other bodies

perpetually enjoy either from the sun-beams immediately, or from the

emanations of other contiguous warmer bodies; and must thence, in a few

minutes, become colder than before.

The ingenious Mr. Eeles, I dare say, has already foreseen the use I

am going to make of this principle; _viz._ “That the little spherules

of vapour will thus, by refracting the solar rays, acquire a constant

heat, tho’ the surrounding atmosphere remain cold.” And as from the

minuteness of their diameters, if they are allowed to be globules,

they must do this to a very great degree, I apprehend none of those

objections will take place against us, with which Mr. Eeles has so

sensibly confuted the former received theories on this subject.

If we are asked, how clouds come to be supported in the absence of

the sun? It must be remembered, that large masses of vapour must for

a considerable time retain much of the heat they have acquired in

the day; at the same time reflecting, how small a quantity of heat

was necessary to raise them; and that doubtless even a less will be

sufficient to support them, as from the diminished pressure of the

atmosphere at a given height, a less power may be able to continue

them in their present state of rarefaction; and, lastly, that clouds

of particular shapes will be sustained or elevated by the motion they

acquire from winds.

I should here have concluded this paper, perhaps already too long; but

upon revising it, I find, where the affinity of some bodies with heat

is mentioned, that the deductions made from thence are not sufficiently

explained to be intelligible. First then, If the power of expansion

of any two bodies, by heat, be in a greater proportion than their

specific gravities, then will there be a certain degree of heat, in

which their specific gravities will be equal; and another, in which the

gravity of that, which was lighter when cold, will exceed the gravity

of that, which was heavier when cold. Hence zinc and bismuth alter

their specific gravities in fusion; some urine, and many solutions of

solids, grow turbid as they cool; others alter their colours. Secondly,

If (the power of expansion by heat being equal) the power of retaining

heat be in a greater ratio than the specific gravities; then, during

the time of cooling after being sufficiently heated, there will be an

instant, when the heavier body will become the lighter, and swim upon

the other. This seems the case in the buff covering of inflamed blood,

the skum of heated milk, and the cristallization of some salts: for

if these effects were from the evaporation of the thinner parts at

the surface, they should happen during the greatest evaporation, or

when boiling; but, on the contrary, they are all done in the greatest

degree when the liquor has for some time began to cool. Lastly, If

the quickness of acquiring heat be in a greater proportion than their

specific gravities (the power of expansion being equal), then, during

the time of their acquiring heat, there will be an instant, when the

body, that was heavier when cold, will now become the lighter. From one

or more of which principles, I apprehend, the volatility or fixity of

all minerals, and many other bodies, takes its origin.

It is no part of my design to account to you, gentlemen, in what manner

such an expansion of the parts of bodies can be brought about by the

action of fire. Tho’ perhaps a rotatory motion only of each particle

on its own center might be sufficient to produce such a rarefaction;

and the more so, if such parts were any other figures than spheres,

as by the percussion of their angles they must result further from

each other. Nor is the existence of such a rotatory motion without

some probability, when we observe the verticillary motion given to

charcoal-dust thrown on nitre in fusion, or the wonderful agitation of

the parts of burning phosphorus, or even of a common red letter-wafer

touched by the flame of a candle. But as in this paper I have laboured

(and I hope not without success) to shew you, that some properties of

solar heat are sufficient to account for the elevation and support of

vapours; so in another letter I propose nearly to demonstrate to you,

that the electric æther is far from having any share in the production

of this important phænomenon.

From,

GENTLEMEN,

Your very humble Servant,

Erasmus Darwin.

Litchfield, Mar. 20, 1757.

+LETTER II.+

_To the very honourable and learned the_ PRESIDENT _and_ MEMBERS _of

the_ Royal Society.

[Read May 5, 1757.]

Gentlemen,

EVERY theoretical inquiry, whose basis does not rest upon experiments,

is at once exploded in this well-thinking age; where truth, under

your patronage, has at length broke thro’ those clouds, with which

superstition, policy, or parade, had overwhelmed her. But experiments

themselves, gentlemen, are not exempted from fallacy. A strong

inventive faculty, a fine mechanic hand, a clear unbiassed judgment,

are at once required for the contrivance, conduct, and application, of

experiments; and even where these are joined (such is the condition of

humanity!) error too frequently intrudes herself, and spoils the work.

My very respectable antagonist, Mr. Eeles, to whose ear, I am

convinced, the voice of truth is more agreeable than that of applause,

will forgive me the following critique on his performance; as by that

means, I am persuaded, the probability of his notions will be intirely

destroyed, and the foregoing theory receive additional supports.

For this purpose our first endeavour will be to shew the uncertainty of

some of the most material principles, that support his arguments; and

afterwards, the fallacy of the experiments he has given us.

First then, in page 130. Mr. Eeles has asserted, that the greatest

possible rarefaction of water is when it boils. I think it might be

said, with equal propriety, that the greatest rarefaction of solids was

when they began to melt: and this may indeed be verbally true, if we

chuse to alter the names of bodies, when they undergo any alteration

by fire; so solids take the name of fluids, when they are in fusion;

and water the name of vapour, when it is greatly rarefied in the

steam-engine. Whence we find this assertion seems to be founded on a

confusion in terms, and the fact far from being existent in nature.

In page 133. the sphere of electrical activity is said to be increased

by heat. If by electrical activity is here meant an increase of its

repulsive power (the thing, which seems to be wanted in Mr. Eeles’s

hypothesis), I know no experiment to show it. If it be meant, that it

is capable of being attracted to a greater distance; I conjecture it

may, as the heat will rarefy the ambient air, and we know the electric

æther is attracted at very great distances in _vacuo_; but this cannot

properly be called an increased activity of electric fire.

We are afterwards told (page _ib._) “that electric fire will not mix

with air:” whence, in the succeeding section, it is argued, “That

as each particle of vapour, with its surrounding electric fluid,

will occupy a greater space than the same weight of air, they will

ascend.” In answer to this, it must be observed, that there are

some bodies, whose parts are fine enough to penetrate the pores of

other bodies, without increasing their bulk; or to pass thro’ them,

without apparently moving or disturbing them. A certain proportion of

alcohol of wine mixed with water, and of copper and tin in fusion, are

instances of the first of these; the existence and passage of light

thro’ air, and, I am persuaded, of electric fire, are instances of the

second.

To illustrate this, the following experiment was instituted. A glass

tube, open at one end, and with a bulb at the other, had its bulb,

and half way from thence to the aperture of the tube, coated on the

inside with gilt paper. The tube was then inverted in a glass of oil

of turpentine, which was placed on a cake of wax, and the tube kept

in that perpendicular situation by a silk line from the cieling of

the room. The bulb was then warmed, so that, when it became cold, the

turpentine rose about half-way up the tube. A bent wire then being

introduced, thro’ the oil into the air above, high electricity was

given. The oil did not appear at all to subside: whence I conclude,

the electric atmosphere flowing round the wire and coating of the tube

above the oil, did not displace the air, but existed in its pores.

This experiment I formerly tried various ways, as I had conceived, if

the electric matter would displace air, it might have been applied

to answer the end of steam in the steam-engine, and many other great

mechanical purposes. But as from the above it appears, that the

contrary is true, it is evident, that electric matter surrounding

particles of vapour must, in fact, increase their specific gravity, and

cannot any-ways be imagined to facilitate their ascent.

I may add further, that if this be true, that it pervades the pores

of air, its specific levity cannot, by any means I know, be compared

with that of air. Its particular attraction to some bodies, at least to

much the greater part of the terraqueous globe, is abundantly greater

than that of air to those bodies: and hence its gravitation to the

whole globe would appear, at first view, to exceed that of air. But the

more I consider this, the more perplexing and amazing it appears to

me: and thence must leave it to the investigation of my very ingenious

antagonist, or some other able philosopher.

I come now to the experiments, that are given us to show all vapour to

be electrifed. In these Mr. Eeles seems to have been led into error,

by not having observed, that many bodies electrifed will retain that

electricity for some time, altho’ in contact with conductors. The

Leyden phial may be touched three or four times by a quick finger

before the whole is discharged. Almost all light dry animal or

vegetable substances, such as feathers and cork, do this in a much

greater degree: and in general I have observed, the more slow any

bodies are to acquire electricity, the more avaritious they are to keep

it.

Part of the plume of a feather, hanging to a green line of silk about

a foot long, which was suspended from the midst of an horizontal line

of the same, about four yards in length, was electrified with a dry

wine-glass, according to the method of Mr. Eeles; and, after being

touched nine times with my finger, at the intervals of two seconds of

time, still manifested signs of electricity, by being attracted at the

tenth approach of it.

A cork ball, on the same line and circumstances, after being

electrised, was touched at the intervals of ten seconds repeatedly, for

seven times, before it was exhausted. The fumes of boiling water were

conveyed upon this ball after being electrised; and, after a fumigation

for thirty seconds, it shewed signs of electricity, by being attracted

to the approaching finger; and, after thirty seconds more, without

any fumigation, it again obeyed the finger; and again, after thirty

more, but at less and less distances. The same appearances occurred to

me from the fumes of resin. From whence I apprehend, that Mr. Eeles,

having dipped the electrised down of the _juncus bombycinus_ in vapour

for perhaps half a minute (for no time is mentioned), and finding it

still retained its electric attraction, was not aware, that this same

had happened, if he had by intervals touched it with his finger, or any

other known conductor of electricity.

As Mr. Eeles had here objected, that there was no real opposition in

the electric æther of glass, and that from wax; the common experiment

to shew this was many times repeated with constant success; _viz._

the cork ball, suspended as above, after being electrised by the

wine-glass, and repelled from it, was strongly attracted by a rubbed

stick of sealing-wax; and _vice versâ_. In the same manner I observed

the electric æther from a black silk stocking (which was held

horizontally extended by the top and foot, and, being rubbed in the

midst with an iron poker, was applied to the cork ball), to be similar

to that of glass, and opposite to that of wax. But the following

experiment appears to me to put this matter out of all doubt, and to

demonstrate, that this difference is only a _plus_ and _minus_ of the

same specific æther, and not different qualities of it, as Mr. Eeles

would suppose.

A stick of dry sealing-wax was rubbed on the side of a dry wine-glass,

and a cork ball, suspended as in the former experiments, played for

some time between them: but glass rubbed with glass, or wax with wax,

did not manifest any electric appearance. Whence it would appear, that

in rubbing glass and wax together, the glass accumulated on its surface

the identical æther that the wax lost. Nor is this a digression from

my design: for if this opposition of the electricity of glass and wax

be established, it still contributes to demonstrate the fallacy of Mr.

Eeles’s experiments.

But what alone would intirely destroy this electric hypothesis, is,

that from the experiments of Mr. Franklin and others, the clouds

are sometimes found to be electrised _plus_, sometimes _minus_, and

sometimes manifest no signs of electricity at all. Whence to say

an accumulation of electric æther supports these clouds, seems

an assertion built upon a very unstable foundation, whose whole

superstructure may well enough be termed an air-built castle, the

baseless fabric of a vision.

Add to this, that Mr. Eeles, in page 140. tells us, that himself has

passed thro’ clouds resting on the sides of mountains. Ought not those

clouds to have immediately discharged their electricity, and fallen?

And common experience may remind us, that any cold bodies will condense

vapour, whatever be their electric properties. So mirrors, or the glass

of windows, in damp rooms, are most frequently found covered with dew;

which, of all other bodies, ought most to be exempted from collecting

vapours supported by electricity, as they are the least capable to

attract or draw off that æther.

From all which, well examined, I am persuaded, gentlemen, you will

be induced to conclude, that tho’ clouds may sometimes possess an

accumulation of electricity, yet that this is only an accidental

circumstance, and not a constant one; and thence can have no possible

influence either in the elevation or support of them. I am,

GENTLEMEN,

Your very humble Servant,

Erasmus Darwin.

Litchfield, March 23. 1757.

[Illustration: _Philos. Trans. Vol. L._ TAB. VI. _p. 255_.

_The grey Coot-footed_ Tringa _shot near Halifax in Yorkshire (January

1757) and Presented to me by Mʳ. Thomas Belton Florist of Worley-clough

in Yorkshire. Drawn from nature of the Figures of Life by Geo. Edwards

in Feb.⁴ 1757._]

XXXI. _An Account of new-discovered Species of the Snipe or Tringa:

In a Letter to the Rev._ Tho. Birch, _D.D. F.R.S. from Mr._ George

Edwards, _Librarian of the College of Physicians_.

[Read May 5, 1757.]