polar regions by the clouds, which are condensed there, and fall in
snow, which electricity would enter the, earth, but cannot penetrate
the ice; may it not, I say, (_as a bottle overcharged_) break through
that low atmosphere, and run along in the vacuum over the air towards
the equator; diverging as the degrees of longitude enlarge; strongly
visible where densest, and becoming less visible as it more diverges;
till it finds a passage to the earth in more temperate climates, or
is mingled with their upper air?
25. If such an operation of nature were really performed, would it
not give all the appearances of an aurora borealis?
26. And would not the auroras become more frequent _after the
approach of winter_: not only because more visible in longer nights;
but also because in summer the long presence of the sun may soften
the surface of the great ice cake, and render it a conductor, by
which the accumulation of electricity in the polar regions will be
prevented?
27. The _atmosphere of the polar regions_ being made more dense by
the extreme cold, and all the moisture in that air being frozen; may
not any great light arising therein, and passing, through it, render
its density in some degree visible, during the night time, to those
who live in the rarer air of more southern latitudes; and would it
not in that case, although in itself a complete and full circle,
extending perhaps ten degrees from the pole, appear to spectators so
placed (who could see only a part of it) _in the form of a segment_;
its chord resting on the horizon, and its arch elevated more or less
above it as seen from latitudes more or less distant; _darkish in
colour_, but yet _sufficiently transparent_ to permit some stars to
be seen through it.
28. The _rays_ of electric matter issuing out of a body, diverge by
mutually repelling each other, unless there be some conducting body
near, to receive them: and if that conducting body be at a greater
distance, they will _first diverge_, and then _converge_ in order
to enter it. May not this account for some of the varieties of
figure seen at times in the _motions_ of the luminous matter of the
auroras: since it is possible, that in passing over the atmosphere,
from the north in all directions or meridians, towards the equator,
the rays of that matter may find, in many places, portions of cloudy
region, or moist atmosphere under them, which (being in the natural
or negative state) may be fit to receive them, and towards which
they may therefore converge: and when one of those receiving bodies
is more than saturated, they may _again_ diverge from it, towards
other surrounding masses of such humid atmosphere, and thus form the
_crowns_, as they are called, and other figures mentioned in the
histories of this meteor?
29. If it be true that the clouds which go to the polar regions, and
carry thither the vapours of the equatorial and temperate regions,
[have their] vapours condensed by the extreme cold of the polar
regions, and fall in snow or hail; the winds which come from those
regions ought to be generally dry, unless they gain some humidity
by sweeping the ocean in their way. And if I mistake not, the winds
between the north east and the north west, are for the most part dry,
when they have continued for some time.
[In the Philosophical Transactions for 1774, p. 122, is a letter from
Mr. I. S. Winn to Dr. Franklin, stating, that since he had first made
the observation concerning the south or south west winds succeeding
an aurora, he had found it invariably obtaining in twenty-three
instances; and he adds in a note a fresh confirming instance. In
reply, Dr. Franklin makes the following conjecture.]
The _Auroræ Boreales_, though visible almost every night of clear
weather in the more northern regions and very high in the atmosphere,
can scarce be visible in England, but when the atmosphere is pretty
clear of clouds for the whole space between us and those regions;
and therefore are seldom visible here. This extensive clearness may
have been produced by a long continuance of northerly winds. When
the winds have long continued in one quarter, the return is often
violent. Allowing the fact so repeatedly observed by Mr. Winn,
perhaps this may account for the violence of the southerly winds,
that soon follow the appearance of the aurora on our coasts.
FOOTNOTES:
[12] If I mistake not, this paper was read to the Royal Academy of
Sciences, at Paris, at the meeting held immediately after Easter,
1779. B. V[13].
[13] For an explanation of the signature B. V. see the note in page
399 of Vol. I. _Editor._
TO DR. L.[14] AT CHARLES-TOWN, SOUTH-CAROLINA.
_On Cold produced by Evaporation._
_New-York, April_ 14, 1757.
SIR,
It is a long time since I had the pleasure of a line from you; and,
indeed, the troubles of our country, with the hurry of business
I have been engaged in on that account, have made me so bad a
correspondent, that I ought not to expect punctuality in others.
But being about to embark for England, I could not quit the continent
without paying my respects to you, and, at the same time, taking
leave to introduce to your acquaintance a gentleman of learning and
merit, colonel Henry Bouquet, who does me the favour to present you
this letter, and with whom I am sure you will be much pleased.
Professor Simpson, of Glasgow, lately communicated to me some
curious experiments of a physician of his acquaintance, by which it
appeared, that an extraordinary degree of cold, even to freezing,
might be produced by evaporation. I have not had leisure to repeat
and examine more than the first and easiest of them, _viz._--Wet the
ball of a thermometer by a feather dipt in spirit of wine, which
has been kept in the same room, and has, of course, the same degree
of heat or cold. The mercury sinks presently three or four degrees,
and the quicker, if, during the evaporation, you blow on the ball
with bellows; a second wetting and blowing, when the mercury is
down, carries it yet lower. I think I did not get it lower than five
or six degrees from where it naturally stood, which was, at that
time, sixty. But it is said, that a vessel of water being placed in
another somewhat larger, containing spirit, in such a manner that
the vessel of water is surrounded with the spirit, and both placed
under the receiver of an air-pump; on exhausting the air, the spirit,
evaporating, leaves such a degree of cold as to freeze the water,
though the thermometer, in the open air, stands many degrees above
the freezing point.
I know not how this phenomenon is to be accounted for, but it gives
me occasion to mention some loose notions relating to heat and cold,
which I have for some time entertained, but not yet reduced into any
form. Allowing common fire, as well as electrical, to be a fluid
capable of permeating other bodies, and seeking an equilibrium, I
imagine some bodies are better fitted by nature to be conductors of
that fluid than others; and that, generally, those which are the best
conductors of the electrical fluid, are also the best conductors of
this; and _e contra_.
Thus a body which is a good conductor of fire readily receives
it into its substance, and conducts it through the whole to all
the parts, as metals and water do; and if two bodies, both good
conductors, one heated, the other in its common state, are brought
into contact with each other, the body which has most fire readily
communicates of it to that which had least, and that which had least
readily receives it, till an equilibrium is produced. Thus, if you
take a dollar between your fingers with one hand, and a piece of
wood, of the same dimensions, with the other, and bring both at the
same time to the flame of a candle, you will find yourself obliged
to drop the dollar before you drop the wood, because it conducts the
heat of the candle sooner to your flesh. Thus, if a silver tea-pot
had a handle of the same metal, it would conduct the heat from the
water to the hand, and become too hot to be used; we therefore give
to a metal tea-pot a handle of wood, which is not so good a conductor
as metal. But a china or stone tea-pot being in some degree of the
nature of glass, which is not a good conductor of heat, may have a
handle of the same stuff. Thus, also, a damp moist air shall make a
man more sensible of cold, or chill him more, than a dry air that is
colder, because a moist air is fitter to receive and conduct away
the heat of his body. This fluid, entering bodies in great quantity,
first expands them, by separating their parts a little, afterwards,
by farther separating their parts, it renders solids fluid, and at
length dissipates their parts in air. Take this fluid from melted
lead, or from water, the parts cohere again, the first grows solid,
the latter becomes ice: and this is sooner done by the means of good
conductors.
[Illustration: (of wooden former for molten lead)]
Thus, if you take, as I have done, a square bar of lead, four inches
long, and one inch thick, together with three pieces of wood planed
to the same dimensions, and lay them, as in the margin, on a smooth
board, fixt so as not to be easily separated or moved, and pour
into the cavity they form, as much melted lead as will fill it,
you will see the melted lead chill, and become firm, on the side
next the leaden bar, some time before it chills on the other three
sides in contact with the wooden bars, though before the lead was
poured in, they might all be supposed to have the same degree of
heat or coldness, as they had been exposed in the same room to the
same air. You will likewise observe, that the leaden bar, as it has
cooled the melted lead more than the wooden bars have done, so it is
itself more heated by the melted lead. There is a certain quantity
of this fluid called fire, in every living human body, which fluid,
being in due proportion, keeps the parts of the flesh and blood at
such a just distance from each other, as that the flesh and nerves
are supple, and the blood fit for circulation. If part of this due
proportion of fire be conducted away, by means of a contact with
other bodies, as air, water, or metals, the parts of our skin and
flesh that come into such contact first draw more near together
than is agreeable, and give that sensation which we call cold; and
if too much be conveyed away, the body stiffens, the blood ceases
to flow, and death ensues. On the other hand, if too much of this
fluid be communicated to the flesh, the parts are separated too far,
and pain ensues, as when they are separated by a pin or lancet. The
sensation that the separation by fire occasions, we call heat, or
burning. My desk on which I now write, and the lock of my desk, are
both exposed to the same temperature of the air, and have therefore
the same degree of heat or cold; yet if I lay my hand successively
on the wood and on the metal, the latter feels much the coldest, not
that it is really so, but being a better conductor, it more readily
than the wood takes away and draws into itself the fire that was in
my skin. Accordingly if I lay one hand, part on the lock, and part
on the wood, and after it had lain so some time, I feel both parts
with my other hand, I find the part that has been in contact with
the lock, very sensibly colder to the touch, than the part that lay
on the wood. How a living animal obtains its quantity of this fluid
called fire, is a curious question. I have shown, that some bodies
(as metals) have a power of attracting it stronger than others; and
I have sometimes suspected, that a living body had some power of
attracting out of the air, or other bodies, the heat it wanted. Thus
metals hammered, or repeatedly bent, grow hot in the bent or hammered
part. But when I consider that air, in contact with the body, cools
it; that the surrounding air is rather heated by its contact with
the body; that every breath of cooler air drawn in, carries off part
of the body's heat when it passes out again; that therefore there
must be in the body a fund for producing it, or otherwise the animal
would soon grow cold; I have been rather inclined to think, that the
fluid _fire_, as well as the fluid _air_, is attracted by plants in
their growth, and becomes consolidated with the other materials of
which they are formed, and makes a great part of their substance:
that when they come to be digested, and to suffer in the vessels a
kind of fermentation, part of the fire, as well as part of the air,
recovers its fluid active state again, and diffuses itself in the
body digesting and separating it: that the fire so reproduced, by
digestion and separation continually leaving the body, its place is
supplied by fresh quantities, arising from the continual separation.
That whatever quickens the motion of the fluids in an animal quickens
the separation, and reproduces more of the fire; as exercise. That
all the fire emitted by wood, and other combustibles, when burning,
existed in them before, in a solid state, being only discovered when
separating. That some fossils, as sulphur, sea-coal, &c. contain a
great deal of solid fire; and that, in short, what escapes and is
dissipated in the burning of bodies, besides water and earth, is
generally the air and fire that before made parts of the solid. Thus
I imagine that animal heat arises by or from a kind of fermentation
in the juices of the body, in the same manner as heat arises in the
liquors preparing for distillation, wherein there is a separation
of the spirituous, from the watry and earthy parts. And it is
remarkable, that the liquor in a distiller's vat, when in its highest
and best state of fermentation, as I have been informed, has the same
degree of heat with the human body; that is, about 94 or 96.
Thus, as by a constant supply of fuel in a chimney, you keep a warm
room, so, by a constant supply of food in the stomach, you keep a
warm body; only where little exercise is used, the heat may possibly
be conducted away too fast; in which case such materials are to be
used for cloathing and bedding, against the effects of an immediate
contact of the air, as are, in themselves, bad conductors of heat,
and, consequently, prevent its being communicated through their
substance to the air. Hence what is called _warmth_ in wool, and
its preference on that account, to linen; wool not being so good a
conductor: and hence all the natural coverings of animals, to keep
them warm, are such as retain and confine the natural heat in the
body, by being bad conductors, such as wool, hair, feathers, and the
silk by which the silk-worm, in its tender embrio state, is first
cloathed. Cloathing, thus considered, does not make a man warm by
_giving_ warmth, but by _preventing_ the too quick dissipation of the
heat produced in his body, and so occasioning an accumulation.
There is another curious question I will just venture to touch
upon, viz. Whence arises the sudden extraordinary degree of cold,
perceptible on mixing some chemical liquors, and even on mixing salt
and snow, where the composition appears colder than the coldest of
the ingredients? I have never seen the chemical mixtures made, but
salt and snow I have often mixed myself, and am fully satisfied
that the composition feels much colder to the touch, and lowers the
mercury in the thermometer more than either ingredient would do
separately. I suppose, with others, that cold is nothing more than
the absence of heat or fire. Now if the quantity of fire before
contained or diffused in the snow and salt was expelled in the
uniting of the two matters, it must be driven away either through
the air or the vessel containing them. If it is driven off thro' the
air, it must warm the air, and a thermometer held over the mixture,
without touching it, would discover the heat, by the rising of the
mercury, as it must, and always does in warm air.
This, indeed, I have not tried, but I should guess it would rather
be driven off through the vessel, especially if the vessel be metal,
as being a better conductor than air; and so one should find the
bason warmer after such mixture. But, on the contrary, the vessel
grows cold, and even water, in which the vessel is sometimes placed
for the experiment, freezes into hard ice on the bason. Now I know
not how to account for this, otherwise than by supposing, that the
composition is a better conductor of fire than the ingredients
separately, and, like the lock compared with the wood, has a stronger
power of attracting fire, and does accordingly attract it suddenly
from the fingers, or a thermometer put into it, from the bason that
contains it, and from the water in contact with the outside of the
bason; so that the fingers have the sensation of extreme cold, by
being deprived of much of their natural fire; the thermometer sinks,
by having part of its fire drawn out of the mercury; the bason grows
colder to the touch, as, by having its fire drawn into the mixture,
it is become more capable of drawing and receiving it from the hand;
and through the bason, the water loses its fire that kept it fluid;
so it becomes ice. One would expect, that from all this attracted
acquisition of fire to the composition, it should become warmer; and,
in fact, the snow and salt dissolve at the same time into water,
without freezing.
I am, Sir, &c.
B. FRANKLIN.
FOOTNOTE:
[14] Dr. Lining. _Editor_.
TO THE SAME ON THE SAME SUBJECT.
_London, June 17, 1758._
DEAR SIR,
In a former letter I mentioned the experiment for cooling bodies by
evaporation, and that I had, by repeatedly wetting the thermometer
with common spirits, brought the mercury down five or six degrees.
Being lately at Cambridge, and mentioning this in conversation with
Dr. Hadley, professor of chemistry there, he proposed repeating the
experiments with ether, instead of common spirits, as the ether is
much quicker in evaporation. We accordingly went to his chamber,
where he had both ether and a thermometer. By dipping first the
ball of the thermometer into the ether, it appeared that the ether
was precisely of the same temperament with the thermometer, which
stood then at 65; for it made no alteration in the height of the
little column of mercury. But when the thermometer was taken out
of the ether, and the ether, with which the ball was wet, began to
evaporate, the mercury sunk several degrees. The wetting was then
repeated by a feather that had been dipped into the ether, when
the mercury sunk still lower. We continued this operation, one of
us wetting the ball, and another of the company blowing on it with
the bellows, to quicken the evaporation, the mercury sinking all
the time, till it came down to 7, which is 25 degrees below the
freezing point, when we left off. Soon after it passed the freezing
point, a thin coat of ice began to cover the ball. Whether this was
water collected and condensed by the coldness of the ball, from the
moisture in the air, or from our breath; or whether the feather,
when dipped into the ether, might not sometimes go through it, and
bring up some of the water that was under it, I am not certain;
perhaps all might contribute. The ice continued increasing till we
ended the experiment, when it appeared near a quarter of an inch
thick all over the ball, with a number of small spicula, pointing
outwards. From this experiment one may see the possibility of
freezing a man to death on a warm summer's day, if he were to stand
in a passage through which the wind blew briskly, and to be wet
frequently with ether, a spirit that is more inflammable than brandy,
or common spirits of wine.
It is but within these few years, that the European philosophers seem
to have known this power in nature, of cooling bodies by evaporation.
But in the east they have long been acquainted with it. A friend
tells me, there is a passage in Bernier's Travels through Indostan,
written near one hundred years ago, that mentions it as a practice
(in travelling over dry deserts in that hot climate) to carry water
in flasks wrapt in wet woollen cloths, and hung on the shady side of
the camel, or carriage, but in the free air; whereby, as the cloths
gradually grow drier, the water contained in the flasks is made cool.
They have likewise a kind of earthen pots, unglazed, which let the
water gradually and slowly ooze through their pores, so as to keep
the outside a little wet, notwithstanding the continual evaporation,
which gives great coldness to the vessel, and the water contained
in it. Even our common sailors seem to have had some notion of this
property; for I remember, that being at sea, when I was a youth,
I observed one of the sailors, during a calm in the night, often
wetting his finger in his mouth, and then holding it up in the air,
to discover, as he said, if the air had any motion, and from which
side it came; and this he expected to do, by finding one side of his
finger grow suddenly cold, and from that side he should look for the
next wind; which I then laughed at as a fancy.