The Botanic Garden

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NOTE VII.--ELEMENTARY HEAT.

_Or sphere on sphere in widening waves expand,
And glad with genial warmth the incumbent land._

CANTO I. l. 143.

A certain quantity of heat seems to be combined with all bodies besides
the sensible quantity which gravitates like the electric fluid amongst
them. This combined heat or latent heat of Dr. Black, when set at
liberty by fermentation, inflammation, crystallization, freezing, or
other chemical attractions producing new _combinations_, passes as a
fluid element into the surrounding bodies. And by thawing, diffusion of
neutral salts in water, melting, and other chemical _solutions_, a
portion of heat is attracted from the bodies in vicinity and enters into
or becomes combined with the new solutions.

Hence a _combination_ of metals with acids, of essential oils and acids,
of alcohol and water, of acids and water, give out heat; whilst a
_solution_ of snow in water or in acids, and of neutral salts in water,
attract heat from the surrounding bodies. So the acid of nitre mixed
with oil of cloves unites with it and produces a most violent flame; the
same acid of nitre poured on snow instantly dissolves it and produces
the greatest degree of cold yet known, by which at Petersburgh
quicksilver was first frozen in 1760.

Water may be cooled below 32 without being frozen, if it be placed on a
solid floor and secured from agitation, but when thus cooled below the
freezing point the least agitation turns part of it suddenly into ice,
and when this sudden freezing takes place a thermometer placed in it
instantly rises as some heat is given out in the act of congelation, and
the ice is thus left with the same _sensible_ degree of cold as the
water had possessed before it was agitated, but is nevertheless now
combined with less _latent_ heat.

A cubic inch of water thus cooled down to 32 deg. mixed with an equal
quantity of boiling water at 212 deg. will cool it to the middle number
between these two, or to 122. But a cubic inch of ice whose sensible
cold also is but 32, mixed with an equal quantity of boiling water, will
cool it six times as much as the cubic inch of cold water
above-mentioned, as the ice not only gains its share of the sensible or
gravitating heat of the boiling water but attracts to itself also and
combines with the quantity of latent heat which it had lost at the time
of its congelation.

So boiling water will acquire but 212 deg. of heat under the common pressure
of the atmosphere, but the steam raised from it by its expansion or by
its solution in the atmosphere combines with and carries away a
prodigious quantity of heat which it again parts with on its
condensation; as is seen in common distillation where the large quantity
of water in the worm-tub is so soon heated. Hence the evaporation of
ether on a thermometer soon sinks the mercury below freezing, and hence
a warmth of the air in winter frequently succeeds a shower.

When the matter of heat or calorique is set at liberty from its
combinations, as by inflammation, it passes into the surrounding bodies,
which possess different capacities of acquiring their share of the loose
or sensible heat; thus a pint measure of cold water at 48 deg. mixed with a
pint of boiling water at 212 deg. will cool it to the degree between these
two numbers, or to 154 deg., but it requires two pint measures of
quicksilver at 48 deg. of heat to cool one pint of water as above. These and
other curious experiments are adduced by Dr. Black to evince the
existance of combined or latent heat in bodies, as has been explained by
some of his pupils, and well illustrated by Dr. Crawford. The world has
long been in expectation of an account of his discoveries on this
subject by the celebrated author himself.

As this doctrine of elementary heat in its fluid and combined state is
not yet universally received, I shall here add two arguments in support
of it drawn from different sources, viz. from the heat given out or
absorbed by the mechanical condensation or expansion of the air, and
perhaps of other bodies, and from the analogy of the various phenomena
of heat with those of electricity.

I. If a thermometer be placed in the receiver of an air-pump, and the
air hastily exhausted, the thermometer will sink some degrees, and the
glass become steamy; the same occurs in hastily admitting a part of the
air again. This I suppose to be produced by the expansion of part of the
air, both during the exhaustion and re-admission of it; and that the air
so expanded becomes capable of attracting from the bodies in its
vicinity a part of their heat, hence the vapours contained in it and the
glass receiver are for a time colder and the steam is precipitated. That
the air thus parts with its moisture from the cold occasioned by its
rarefaction and not simply by the rarefaction itself is evident, because
in a minute or two the same rarefied air will again take up the dew
deposited on the receiver; and because water will evaporate sooner in
rare than in dense air.

There is a curious phenomenon similar to this observed in the fountain
of Hiero constructed on a large scale at the Chemnicensian mines in
Hungary. In this machine the air in a large vessel is compressed by a
column of water 260 feet high, a stop-cock is then opened, and as the
air issues out with great vehemence, and thus becomes immediately
greatly expanded, so much cold is produced that the moisture from this
stream of air is precipitated in the form of snow, and ice is formed
adhering to the nosel of the cock. This remarkable circumstance is
described at large with a plate of the machine in Philos. Trans. Vol.
LII. for 1761.

The following experiment is related by Dr. Darwin in the Philos. Trans.
Vol. LXXVIII. Having charged an air-gun as forcibly as he well could the
air-cell and syringe became exceedingly hot, much more so than could be
ascribed to the friction in working it; it was then left about half an
hour to cool down to the temperature of the air, and a thermometer
having been previously fixed against a wall, the air was discharged in a
continual stream on its bulb, and it sunk many degrees. From these three
experiments of the steam in the exhausted receiver being deposited and
re-absorbed, when a part of the air is exhausted or re-admitted, and the
snow produced by the fountain of Hiero, and the extraordinary heat given
out in charging, and the cold produced in discharging an air-gun, there
is reason to conclude that when air is mechanically compressed the
elementary fluid heat is pressed out of it, and that when it is
mechanically expanded the same fluid heat is re-absorbed from the common
mass.

It is probable all other bodies as well as air attract heat from their
neighbours when they are mechanically expanded, and give it out when
they are mechanically condensed. Thus when a vibration of the particles
of hard bodies is excited by friction or by percussion, these particles
mutually recede from and approach each other reciprocally; at the times
of their recession from each other, the body becomes enlarged in bulk,
and is then in a condition to attract heat from those in its vicinity
with great and sudden power; at the times of their approach to each
other this heat is again given out, but the bodies in contact having in
the mean while received the heat they had thus lost, from other bodies
behind them, do not so suddenly or so forcibly re-absorb the heat again
from the body in vibration; hence it remains on its surface like the
electric fluid on a rubbed glass globe, and for the same reason, because
there is no good conductor to take it up again. Hence at every vibration
more and more heat is acquired and stands loose upon the surface; as in
filing metals or rubbing glass tubes; and thus a smith with a few
strokes on a nail on his anvil can make it hot enough to light a
brimstone-match; and hence in striking flint and steel together heat
enough is produced to vitrify the parts thus strucken off, the quantity
of which heat is again probably increased by the new chemical
combination.

II. The analogy between the phenomena of the electric fluid and of heat
furnishes another argument in support of the existence of heat as a
gravitating fluid. 1. They are both accumulated by friction on the
excited body. 2. They are propagated easily or with difficalty along the
same classes of bodies; with ease by metals, with less ease by water;
and with difficulty by resins, bees-wax, silk, air, and glass. Thus
glass canes or canes of sealing-wax may be melted by a blow-pipe or a
candle within a quarter of an inch of the fingers which hold them,
without any inconvenient heat, while a pin or other metallic substance
applyed to the flame of a candle so readily conducts the heat as
immediately to burn the fingers. Hence clothes of silk keep the body
warmer than clothes of linen of equal thickness, by confining the heat
upon the body. And hence plains are so much warmer than the summits of
mountains by the greater density of the air confining the acquired heat
upon them. 3. They both give out light in their passage through air,
perhaps not in their passage through a vacuum. 4. They both of them fuse
or vitrify metals. 5. Bodies after being electrized if they are
mechanically extended will receive a greater quantity of electricity, as
in Dr. Franklin's experiment of the chain in the tankard; the same seems
true in respect to heat as explained above. 6. Both heat and electricity
contribute to suspend steam in the atmosphere by producing or increasing
the repulsion of its particles. 7. They both gravitate, when they have
been accumulated, till they find their equilibrium.

If we add to the above the many chemical experiments which receive an
easy and elegant explanation from the supposed matter of heat, as
employed in the works of Bergman and Lavoisier, I think we may
reasonably allow of its existence as an element, occasionally combined
with other bodies, and occasionally existing as a fluid, like the
electric fluid gravitating amongst them, and that hence it may be
propagated from the central fires of the earth to the whole mass, and
contribute to preserve the mean heat of the earth, which in this country
is about 48 degrees but variable from the greater or less effect of the
sun's heat in different climates, so well explained in Mr. Kirwan's
Treatise on the Temperature of different Latitudes. 1787, Elmsly.
London.

NOTE VIII.--MEMNON'S LYRE.

_So to the sacred Sun in Memnon's fane
Spontaneous concords quired the matin strain._

CANTO I. l. 183.

The gigantic statue of Memnon in his temple at Thebes had a lyre in his
hands, which many credible writers assure us, sounded when the rising
sun shone upon it. Some philosophers have supposed that the sun's light
possesses a mechanical impulse, and that the sounds abovementioned might
be thence produced. Mr. Michell constructed a very tender horizontal
balance, as related by Dr. Priestley in his history of light and
colours, for this purpose, but some experiments with this balance which
I saw made by the late Dr. Powel, who threw the focus of a large
reflector on one extremity of it, were not conclusive either way, as the
copper leaf of the balance approached in one experiment and receded in
another.

There are however methods by which either a rotative or alternating
motion may be produced by very moderate degrees of heat. If a straight
glass tube, such as are used for barometers, be suspended horizontally
before a fire, like a roasting spit, it will revolve by intervals; for
as glass is a bad conductor of heat the side next the fire becomes
heated sooner than the opposite side, and the tube becomes bent into a
bow with the external part of the curve towards the fire, this curve
then falls down and produces a fourth part of a revolution of the glass
tube, which thus revolves with intermediate pauses.

Another alternating motion I have seen produced by suspending a glass
tube about eight inches long with bulbs at each end on a centre like a
scale beam. This curious machine is filled about one third part with
purest spirit of wine, the other two thirds being a vacuum, and is
called a pulse-glass, if it be placed in a box before the fire, so that
either bulb, as it rises, may become shaded from the fire, and exposed
to it when it descends, an alternate libration of it is produced. For
spirit of wine in vacuo emits steam by a very small degree of heat, and
this steam forces the spirit beneath it up into the upper bulb, which
therefore descends. It is probable such a machine on a larger scale
might be of use to open the doors or windows of hot-houses or mellon-
frames, when the air within them should become too much heated, or might
be employed in more important mechanical purposes.

On travelling through a hot summer's day in a chaise with a box covered
with leather on the fore-axle-tree, I observed, as the sun shone upon
the black leather, the box began to open its lid, which at noon rose
above a foot, and could not without great force be pressed down; and
which gradually closed again as the sun declined in the evening. This I
suppose might with still greater facility be applied to the purpose of
opening melon-frames or the sashes of hot-houses.

The statue of Memnon was overthrown and sawed in two by Cambyses to
discover its internal structure, and is said still to exist. See
Savary's Letters on Egypt. The truncated statue is said for many
centuries to have saluted the rising sun with chearful tones, and the
setting sun with melancholy ones.

NOTE IX.--LUMINOUS INSECTS.

_Star of the earth, and diamond of the night._

CANTO I. l. 196.

There are eighteen species of Lampyris or glow-worm, according to
Linneus, some of which are found in almost every part of the world. In
many of the species the females have no wings, and are supposed to be
discovered by the winged males by their shining in the night. They
become much more lucid when they put themselves in motion, which would
seem to indicate that their light is owing to their respiration; in
which process it is probable phosphoric acid is produced by the
combination of vital air with some part of the blood, and that light is
given out through their transparent bodies by this slow internal
combustion.

There is a fire-fly of the beetle-kind described in the Dict. Raisonne
under the name of Acudia, which is said to be two inches long, and
inhabits the West-Indies and South America; the natives use them instead
of candles, putting from one to three of them under a glass. Madam
Merian says, that at Surinam the light of this fly is so great, that she
saw sufficiently well by one of them to paint and finish one of the
figures of them in her work on insects. The largest and oldest of them
are said to become four inches long, and to shine like a shooting star
as they fly, and are thence called Lantern-bearers. The use of this
light to the insect itself seems to be that it may not fly against
objects in the night; by which contrivance these insects are enabled to
procure their sustenance either by night or day, as their wants may
require, or their numerous enemies permit them; whereas some of our
beetles have eyes adapted only to the night, and if they happen to come
abroad too soon in the evening are so dazzled that they fly against
every thing in their way. See note on Phosphorus, No. X.

In some seas, as particularly about the coast of Malabar, as a ship
floats along, it seems during the night to be surrounded with fire, and
to leave a long tract of light behind it. Whenever the sea is gently
agitated it seems converted into little stars, every drop as it breaks
emits light, like bodies electrified in the dark. Mr. Bomare says, that
when he was at the port of Cettes in Languedoc, and bathing with a
companion in the sea after a very hot day, they both appeared covered
with fire after every immersion, and that laying his wet hand on the arm
of his companion, who had not then dipped himself, the exact mark of his
hand and fingers was seen in characters of fire. As numerous microscopic
insects are found in this shining water, its light has been generally
ascribed to them, though it seems probable that fish-slime in hot
countries may become in such a state of incipient putrefaction as to
give light, especially when by agitation it is more exposed to the air;
otherwise it is not easy to explain why agitation should be necessary to
produce this marine light. See note on Phosphorus No. X.

NOTE X.--PHOSPHORUS.

_Or mark in shining letters Kunckel's name
In the pale phosphor's self-consuming flame._

CANTO I. l. 231.

Kunckel, a native of Hamburgh, was the first who discovered to the world
the process for producing phosphorus; though Brandt and Boyle were
likewise said to have previously had the art of making it. It was
obtained from sal microcosmicum by evaporation in the form of an acid,
but has since been found in other animal substances, as in the ashes of
bones, and even in some vegetables, as in wheat flour. Keir's chemical
Dict. This phosphoric acid is like all other acids united with vital
air, and requires to be treated with charcoal or phlogiston to deprive
it of this air, it then becomes a kind of animal sulphur, but of so
inflammable a nature, that on the access of air it takes fire
spontaneously, and as it burns becomes again united with vital air, and
re-assumes its form of phosphoric acid.

As animal respiration seems to be a kind of slow combustion, in which it
is probable that phosphoric acid is produced by the union of phosphorus
with the vital air, so it is also probable that phosphoric acid is
produced in the excretory or respiratory vessels of luminous insects, as
the glow-worm and fire-fly, and some marine insects. From the same
principle I suppose the light from putrid fish, as from the heads of
hadocks, and from putrid veal, and from rotten wood in a certain state
of their putrefaction, is produced, and phosphorus thus slowly combined
with air is changed into phosphoric acid. The light from the Bolognian
stone, and from calcined shells, and from white paper, and linen after
having been exposed for a time to the sun's light, seem to produce
either the phosphoric or some other kind of acid from the sulphurous or
phlogistic matter which they contain. See note on Beccari's shells. l.
180.

There is another process seems similar to this slow combustion, and that
is _bleaching_. By the warmth and light of the sun the water sprinkled
upon linen or cotton cloth seems to be decomposed, (if we credit the
theory of M. Lavoisier,) and a part of the vital air thus set at liberty
and uncombined and not being in its elastic form, more easily dissolves
the colouring or phlogistic matter of the cloth, and produces a new
acid, which is itself colourless, or is washed out of the cloth by
water. The new process of bleaching confirms a part of this theory, for
by uniting much vital air to marine acid by distilling it from
manganese, on dipping the cloth to be bleached in water repleat with
this super-aerated marine acid, the colouring matter disappears
immediately, sooner indeed in cotton than in linen. See note XXXIV.

There is another process which I suspect bears analogy to these above-
mentioned, and that is the rancidity of animal fat, as of bacon; if
bacon be hung up in a warm kitchen, with much salt adhering on the
outside of it, the fat part of it soon becomes yellow and rancid; if it
be washed with much cold water after it has imbibed the salt, and just
before it is hung up, I am well informed, that it will not become
rancid, or in very slight degrees. In the former case I imagine the salt
on the surface of the bacon attracts water during the cold of the night,
which is evaporated during the day, and that in this evaporation a part
of the water becomes decomposed, as in bleaching, and its vital air
uniting with greater facility in its unelastic state with the animal
fat, produces an acid, perhaps of the phosphoric kind, which being of a
fixed nature lies upon the bacon, giving it the yellow colour and rancid
taste. It is remarkable that the super-aerated marine acid does not
bleach living animal substances, at least it did not whiten a part of my
hand which I for some minutes exposed to it.

NOTE XI.--STEAM-ENGINE.

_Quick moves the balanced beam, of giant-birth,
Wields his large limbs, and nodding shakes the earth._

CANTO I. l. 261.

The expansive force of steam was known in some degree to the antients,
Hero of Alexandria describes an application of it to produce a rotative
motion by the re-action of steam issuing from a sphere mounted upon an
axis, through two small tubes bent into tangents, and issuing from the
opposite sides of the equatorial diameter of the sphere, the sphere was
supplied with steam by a pipe communicating with a pan of boiling water,
and entering the sphere at one of its poles.

A french writer about the year 1630 describes a method of raising water
to the upper part of a house by filling a chamber with steam, and
suffering it to condense of itself, but it seems to have been mere
theory, as his method was scarcely practicable as he describes it. In
1655 the Marquis of Worcester mentions a method of raising water by fire
in his Century of Inventions, but he seems only to have availed himself
of the expansive force and not to have known the advantages arising from
condensing the steam by an injection of cold water. This latter and most
important improvement seems to have been made by Capt. Savery sometime
prior to 1698, for in that year his patent for the use of that invention
was confirmed by act of parliament. This gentleman appears to have been
the first who reduced the machine to practice and exhibited it in an
useful form. This method consisted only in expelling the air from a
vessel by steam and condensing the steam by an injection of cold water,
which making a vacuum, the pressure of the atmosphere forced the water
to ascend into the steam-vessel through a pipe of 24 to 26 feet high,
and by the admission of dense steam from the boiler, forcing the water
in the steam-vessel to ascend to the height desired. This construction
was defective because it required very strong vessels to resist the
force of the steam, and because an enormous quantity of steam was
condensed by coming in contact with the cold water in the steam-vessel.

About or soon after that time M. Papin attempted a steam-engine on
similar principles but rather more defective in its construction.

The next improvement was made very soon afterwards by Messrs. Newcomen
and Cawley of Dartmouth, it consisted in employing for the steam-vessel
a hollow cylinder, shut at bottom and open at top, furnished with a
piston sliding easily up and down in it, and made tight by oakum or
hemp, and covered with water. This piston is suspended by chains from
one end of a beam, moveable upon an axis in the middle of its length, to
the other end of this beam are suspended the pump-rods.

The danger of bursting the vessels was avoided in this machine, as
however high the water was to be raised it was not necessary to increase
the density of the steam but only to enlarge the diameter of the
cylinder.

Another advantage was, that the cylinder not being made so cold as in
Savary's method, much less steam was lost in filling it after each
condensation.

The machine however still remained imperfect, for the cold water thrown
into the cylinder acquired heat from the steam it condensed, and being
in a vessel exhausted of air it produced steam itself, which in part
resisted the action of the atmosphere on the piston; were this remedied
by throwing in more cold water the destruction of steam in the next
filling of the cylinder would be proportionally increased. It has
therefore in practice been found adviseable not to load these engines
with columns of water weighing more than seven pounds for each square
inch of the area of the piston. The bulk of water when converted into
steam remained unknown until Mr. J. Watt, then of Glasgow, in 1764,
determined it to be about 1800 times more rare than water. It soon
occurred to Mr. Watt that a perfect engine would be that in which no
steam should be condensed in filling the cylinder, and in which the
steam should be so perfectly cooled as to produce nearly a perfect
vacuum.

Mr. Watt having ascertained the degree of heat in which water boiled in
vacuo, and under progressive degrees of pressure, and instructed by Dr.
Black's discovery of latent heat, having calculated the quantity of cold
water necessary to condense certain quantities of steam so far as to
produce the exhaustion required, he made a communication from the
cylinder to a cold vessel previously exhausted of air and water, into
which the steam rushed by its elasticity, and became immediately
condensed. He then adapted a cover to the cylinder and admitted steam
above the piston to press it down instead of air, and instead of
applying water he used oil or grease to fill the pores of the oakum and
to lubricate the cylinder.

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