All the powers of Nature with which we are acquainted, may be reduced
to two primitive forces; the one which causes weight, and that which
produces heat. The force of impulsion is subordinate to them; it
depends on the first for its particular, and on the latter for its
general effects. As impulsion cannot exercise itself but by the means
of a spring, and the spring only acts by virtue of the force which
approximates the remote parts, it is clear, that to perform its
power it has need of the concurrence of attraction: for if matter
ceased to attract, if bodies lost their coherence, every spring would
be destroyed, every motion intercepted, and every impulsion void;
since motion cannot transmit itself from one body to another but by
elasticity, it is demonstrable, that one body absolutely hard and
inflexible, would be absolutely immoveable, and entirely incapable
of receiving the action of another. Attraction being a general and
permanent effect, impulsion, which in most bodies is neither constant
nor fixed, depends on it as a particular effect; for, if all impulsion
were destroyed, attraction would still equally subsist and act; it is,
therefore, this essential difference which makes impulsion subordinate
to attraction in all inanimate and purely passive matter.
But this impulsion depends still more immediately, and generally, on
the power which produces heat; for it is principally by the means
of heat, that impulsion penetrates organized bodies; it is by heat
that they are formed, grow, and develope themselves. We may refer to
attraction alone all the effects of inanimate matter; and in this same
power of attraction, joined to that of heat, every phenomena of live
matter. By live matter I understand not only every thing that lives,
or vegetates, but also every living organic molecule, dispersed in the
waste or remains of organized bodies. In it I comprehend also light,
heat, fire, and all matter which appears to be active in itself. Now
this live matter always tends from the centre to the circumference,
whereas brute or inanimate matter tends from the circumference to the
centre. It is an expansive power which animates the live matter, and
it is an attractive force to which the inanimate matter is obedient.
Although the directions of these two powers be diametrically opposite,
yet they balance themselves without ever being destroyed, and from the
combination of these two powers equally active, all the phenomena of
the universe result.
But it may be said, by reducing all the powers of Nature to attraction
and expansion, without giving the cause of either, and by rendering
impulsion, (which is the only force whose cause is known and
demonstrated to our senses) subordinate to both, do you not abandon
a clear idea, and substitute two obscure hypotheses in its place? To
this I answer, that as we know nothing except by comparison, we shall
never have an idea of what general effect will produce, because such
an effect belonging to every thing, we should be unable to compare it
to any, and consequently there is no hope of ever knowing the cause or
reason why all matter attracts, although we are sensible such is the
fact. If, on the contrary, the effect were particular, like that of the
attraction of the loadstone and steel, we might expect to discover the
cause, because it might be compared to other particular effects. To ask
why matter is extended, heavy, and impenetrable, are ill-conceived
propositions, and merit not an answer; it is the same with respect
to every particular property, when it is essential to the subject,
and we might as well be interrogated why red is red? The philosopher
becomes a child when he puts such questions; and however much they may
be forgiven to the last, the former ought to exclude them from his
thoughts.
It is sufficient that the forces of attraction and expansion are two
general, real, and fixed effects, for us to receive them for causes
of particular ones; and impulsion is one of these effects, which we
must not look upon as a general cause, known and demonstrated by our
senses, since we have proved that this force of impulsion cannot exist
nor act, but by the means of attraction, which does not fall upon our
senses. Nothing is more evident, nay, certain, than the communication
of motion by impulsion; it is sufficient for one body to strike another
to produce this effect. But even in this sense, is not the cause of
attraction most evident, and that motion, in all cases, belongs more to
attraction than impulsion?
The first reduction being made, it might perhaps be possible to adduce
a second, and to bring back the power even of expansion to that of
attraction, insomuch that all the forces of matter would depend solely
on a primitive one; at least this idea seems to be worthy of that
sublime simplicity with which nature works. Now cannot we conceive
that this attraction changes into repulsion every time that bodies
approach near enough to rub together, or strike one against the
other? Impenetrability, which we must not regard as a force, but as a
resistance essential to matter, not permitting two bodies to occupy
the same place, what must happen when two molecules, which attract the
more powerfully as they approach nearer, suddenly strike against each
other? Does not then this invincible resistance of impenetrability,
become an active force, which, in the contact, drives the bodies with
as much velocity, as they had acquired at the moment they touched? And
from hence the expansive force will not be a particular force opposed
to the attractive one, but an effect derived therefrom. I own, that we
must suppose a perfect spring in every molecule, and in every atom of
matter, to have a clear conception how this change of attraction into
repulsion is performed. But even this is sufficiently indicated by
facts; the more matter is attenuated, the more it takes a spring. Earth
and water, which are the most gross aggregates, have a less spring
than air; and fire, which is the most subtle of all the elements, is
also that which has the most expansive force. The smallest molecules
of matter, the smallest atoms with which we are acquainted are those
of light, and we are sensible of their being perfectly elastic, since
the angle under which the light is reflected, is always equal to that
under which it comes. We may therefore infer, that all the constitutive
parts of matter in general, are a perfect spring; and that this spring
produces all the effects of the expansive force, every time that bodies
strike by meeting in opposite directions.
We know of no other means of producing fire, but by striking or rubbing
bodies together[B]; since by supposing man without any burning glasses,
and without actual fire, he will have no other means of producing it;
for the fire produced by uniting the rays of light, or by application
of fire already produced, had the same origin.
[B] The fire, which arises from the fermentation of herbs heaped
together, and which manifests itself in effervescences, is not an
exception that can be opposed to me, since this production of fire
depends, like all the rest, from the action of the shock of the parts
of matter one against the other.
Expansive force, therefore, in reality might be only the re-action
of the attractive, a re-action which operates every time that the
primitive molecules of matter, always attracted one by the other,
happen immediately to touch; for then it is necessary, that they be
repelled with as much velocity as they had acquired in a contrary
direction, at the moment of contact; and when these molecules are
absolutely free from all coherence and only obey the motion alone
produced by their attraction, this acquired velocity is immense in the
point of contact. Heat, light, and fire, which are the greatest effects
of expansive force, will be produced every time that bodies are either
artificially or naturally divided into very minute parts, and meet in
opposite directions; and the heat will be so much the more sensible,
the light so much the more bright, the fire so much the more violent,
according as the molecules are precipitated one against the other with
more velocity by their force of mutual attraction.
From the above it must be concluded, that all matter may become
light, heat, and fire; and that this matter of fire and light is
not a substance different from every other, but preserves all its
essential qualities; and even most of the attributes of common matter,
is evidently proved by, first, light, though composed of particles
almost infinitely minute, is, nevertheless, still divisible, since
with the prism we separate the rays, or different coloured atoms one
from another. Secondly, light, though in appearance endowed with a
quality quite opposite to that of weight, that is, with a volatility
which we might think essential, is, nevertheless, heavy like all
matter, since it bends every time it passes near other bodies, and
finds itself inclined to their sphere of attraction. It is very
heavy, relatively to its volume, which is very minute, since the
immense velocity with which light moves in a direct line, does not
prevent it from feeling sufficient attraction near other bodies, for
its direction to incline and change in a manner very sensible to our
eyes. Thirdly, the substance of light is not more simple than all
other matter, since it is composed of parts of unequal weight; the red
rays are much heavier than the blue; and between these two extremes
there are an infinity of intermediate rays, which approach more or
less the weight of the red, or the lightness of the blue according to
their shades. All these consequences are necessarily derived from the
phenomena of the inflection of light, and of its refraction, which, in
reality, is only an inflexion which operates when light passes across
transparent bodies. Fourthly, it may be demonstrated, that light is
massive, and that it acts, in some cases, as all other bodies act;
for, independently of its ordinary effect, which is to shine before
our eyes, and by its own action, always accompanied with lustre, and
often with heat, it acts by its mass when it is condensed, and it acts
to the point of putting in motion heavy bodies placed in the focus of a
good burning glass: it turns a needle on a pivot placed in its focus:
it displaces leaves of gold or silver before it melts or even sensibly
heats them. This action, produced by its mass, precedes that of heat:
it operates between the condensed light and the leaves of metal in
the same manner as it operates between two other bodies which become
contiguous, and, consequently, have still this property in common with
all other matter. Fifthly, light is a mixture, like common matter, not
only of more gross and minute parts, more or less heavy or moveable,
but also differently shaped. Whoever has observed the phenomena which
Newton calls _the access of easy reflection_, and of _easy transmission
of light_; and on the effects of double refraction of rock and Iceland
chrystal, must have perceived that the atoms of light have many sides,
many different surfaces, which, according as they present themselves,
constantly produce different effects.
This, therefore, is sufficient to demonstrate that light is neither
particular nor different from common matter; that its essence, and
its essential properties are the same; and that it differs only from
having undergone, in the point of contact, the repulsion whence its
volatility proceeds; and in the same manner as the effect of the force
of attraction extends, always decreasing as the space augments, the
effects of repulsion extend and decrease the more, but in an inverted
order, insomuch that we can apply to the expansive force all that is
known of the attractive. These are two instruments of the same nature,
or rather the same instrument, only managed in two opposite directions.
All matter will become light, for if all coherence were destroyed
it would be divided into molecules sufficiently minute, and these
molecules, being at liberty, will be determined by their mutual
attraction to rush one against the other. In the moment of the shock
the repulsive force will be exercised, the molecules will fly in all
directions with an almost infinite volatility, which, nevertheless,
is not equal to their velocity acquired in the moment of contact, for
the law of attraction being augmented as the space diminishes, it is
evident, that at the contact the space is always proportionable till
the square of the distance becomes nil, and, consequently, the velocity
acquired by virtue of the attraction must at this point become almost
infinite: and it would be perfectly so if the contact were immediate,
and, consequently, the distance between the two bodies void; but there
is nothing in nature entirely nil, and nothing truly infinite; and all
that I have observed of the _infinite_ minuteness of the atoms which
constitute light, of their _perfect_ spring, and of the _nil_ distance
in the moment of contact, must be understood only relatively. If this
metaphysical truth were doubted, a physical demonstration may be given.
It is pretty generally known that light employs seven minutes and a
half to come from the sun to the earth; supposing, therefore, the sun
at thirty-six millions of miles, light darts through this enormous
distance in that short space, that is (supposing its motion uniform),
80,000 miles in one second. But this velocity, although prodigious,
is yet far from being infinite, since it is determinable by numbers.
It will even cease to appear so prodigious, when we reflect on the
celerity of the motion of the comets to their perihelia, or even that
of the planets, and by computing that, we shall find that the velocity
of those immense masses may pretty nearly be compared to that of the
atoms of light.
So, likewise, as all matter can be converted into light by the
division and expulsion of its parts, when they feel a shock one against
another, we shall find that all the elements are convertible; and if it
have been doubted whether light, which appears to be the most simple
element, may be converted into a solid substance, it is because we have
not paid sufficient attention to every phenomena, and were infected
with the prejudice, that being essentially volatile it can never become
fixed. But it is plain that the fixity and volatility depend on the
same attractive force in the first case, and become repulsive in the
second; and from thence are we led to think that this change of matter
into light, and from light into matter, is one of the most frequent
operations of Nature.
Having shewn that impulsion depends on attraction; that the expansive
force, like the attractive, becomes negative; that light, heat, and
fire, are only modes of the common existing matter; in one word, that
there exists but one sole force, and one sole matter, ever ready to
attract or repel, according to circumstances; let us see how, with this
single spring, and this single subject, Nature can vary her works, _ad
infinitum_. In a general point of view, light, heat, and fire, only
make one object, but in a particular point of view they are three
distinct objects, which, although resembling in a great number of
properties, differ nevertheless in a few others, sufficiently essential
for us to consider them as three distinct things.
Light, and elementary fire, compose, it is said, only one and the same
thing. This may be, but as we have not yet a clear idea of elementary
fire we shall desist from pronouncing on this first point. Light and
fire, such as we are acquainted with, are two distinct substances,
differently composed. Fire is, in fact, very often luminous, but it
sometimes also exists without any appearance of light. Fire, whether
luminous or obscure, never exists without a great heat, whereas light
often burns with a noise without the least sensible heat. Light appears
to be the work of nature while fire is only the produce of the industry
of man. Light subsists of itself, and is found diffused in the immense
space of the whole universe. Fire cannot subsist without food, and
is only found in some parts of this space where man preserves it,
and in some parts of the profundity of the earth, where it is also
supported by suitable food. Light when condensed and united by the art
of man, may produce fire, but it is only as much as it lets fall on
combustible matters. Light is therefore no more, and in this single
instance, only the principle of fire and not the fire itself: even this
principle is not immediate, for it supposes the intermediate one of
heat, and which appears to appertain more than light to the essence of
fire. Now heat exists as often without light as light exists without
heat: these two principles might, therefore, appear not to bind them
necessarily together; their effects are not contemporary, since in
certain circumstances we feel heat long before light appears, and in
others we see light long before we feel any heat. Hence is not heat
a mode of being, a modification of matter, which, in fact, differs
less than all the rest from that of light, but which can be considered
apart, and still more easily conceived? It is, nevertheless, certain,
that much fewer discoveries have been made on the nature of heat than
on that of light; whether man better catches what he sees than what
he feels; whether light, presenting itself generally as a distinct
and different substance from all the rest, has appeared worthy of a
particular consideration; whereas heat, the effect of which is the most
obscure, and presents itself as a less detached and less simple object,
has not been regarded as a distinct substance but as an attribute of
light and fire.
The first thing worthy of remark, is, that the seat of heat is quite
different from that of light: the latter occupies and runs through
the void space of the universe; heat, on the contrary, is diffused
through all solid matter. The globe of the earth, and the whole matter
of which it is composed, have a considerable degree of heat. Water has
its degree of heat which it does not lose but by losing its fluidity.
The air has also heat, which we call its temperature, and which varies
much, but is never entirely lost, since its springs subsist even in
the greatest cold. Fire has also its different degrees of heat, which
appear to depend less on its own nature, than on that of the aliments
which feed it. Thus all known matter possesses warmth; and, hence, heat
is a much more general affection than that of light.
Heat penetrates every body without exception which is exposed to it,
while light passes through transparent bodies only, and is stopped and
in part repelled, by every opaque one. Heat, therefore acts in a much
more general and palpable manner than light, and although the molecules
of heat are excessively minute, since they penetrate the most compact
bodies, it seems, however, demonstrable, that they are much more gross
than those of light; for we make heat with light, by collecting it in
a great quantity. Besides, heat acting on the sense of feeling, it is
nececssary that its action be proportionate to the grossness of this
sense, the same as the delicacy of the organs of sight appears to be to
the extreme fineness of the parts of light; these parts move with the
greatest velocity, and act in the instant at immense distances, whereas
those of heat have but a slow progressive motion, and only extend to
small intervals from the bodies whence they emanate.
The principle of all heat seems to be the attrition of bodies; all
friction, that is, all contrary motion between solid matters produces
heat; and if the same effect do not happen to fluids, it is because
their parts do not touch close enough to rub one against the other;
and that, having little adherence between them, their resistance to
the shock of other bodies is too weak for the heat to be produced to
a sensible degree; but we often see light produced by an attrition of
a fluid, without feeling any heat. All bodies whether great or little
become heated as soon as they meet in a contrary direction; heat is,
therefore, produced by the motion of all palpable matter; while the
production of light, which is also made by motion, but in a contrary
direction, supposes also the division of matter into very minute parts:
and as this operation of Nature is the same with respect to both, we
must conclude, that the atoms of light are solid of themselves, and are
hot at the moment of their birth. But we cannot be equally certain,
that they preserve their heat in the same degree as their light, nor
that they cease to be hot before they cease to be luminous.
It is well known, that heat grows less, or cold becomes greater, the
higher we ascend on the mountains. It is true that the heat which
proceeds from the terrestrial globe, is of course sensibly less on
those advanced points, than it is on the plains; but this cause is not
proportionable to the effect; the action of heat, which emanates from
the terrestrial globe, not being able to diminish but by the square of
the distance, it does not appear that at the height of half a mile,
which is only the three thousandth part of the semi-diameter of the
globe, whose centre must be taken for the focus of heat, that this
difference, which in this supposition is only a unit and nine millions,
can produce a diminution of heat nearly so considerable; for the
thermometer lowers at that height, at all times of the year, to the
freezing point. It is not probable, that this great difference of heat
simply proceeds from the difference of the earth; and of that we must
be fully convinced, if we consider, that at the mouth of the volcanos,
where the earth is hotter than in any other part on the surface of the
globe, the air is nearly as cold as on other mountains of the same
height.
It may then be supposed that the atoms of light, though very hot at the
moment of quitting the sun, are greatly cooled during the seven minutes
and a half in which they pass from that body to the earth; and this
in fact would be the case if they were detached; but, as they almost
immediately succeed each other, and are the more confined as they are
nearer the place of their origin, the heat lost by each atom falls on
the neighbouring ones; and this reciprocal communication supports the
general heat of light a longer time; and as their constant direction is
in divergent rays, their distance from each other increases according
to the space they run over; and as the heat which flies from each
atom, as a centre, diminishes also in the same ratio, it follows, that
the light of the solar rays, decreasing in an inverted ratio from the
square of the distance, that of their heat decreases in an inverted
ratio of the square of the same distance.
Taking therefore the semi-diameter of the sun for a unit, and supposing
the action of light to be as 1000 to the distance of a demi-diameter
of the surface of this planet, it will not be more than as 1000/4
to the distance of two demi-diameters; as 1000/9 to that of three
demi-diameters, as 1000/16 to the distance of four demi-diameters; and
finally, when it arrives at us, who are distant from the sun thirty-six
millions of leagues, that is about two hundred and twenty-four of its
demi-diameters, the action of light will be no more than as 1000/50625,
that is, more than 50,000 times weaker than at its issuing from the
sun; and the heat of each atom of light being also supposed 1000 at its
issuing from the sun, will not be more than as 1000/16 1000/81 1000/256
to the successive of 1, 2, 3, demi-diameters, and, when arrived at
us, as 1000/2562890625 that is, more than two thousand five hundred
millions of times weaker than at issuing from the sun.
If even this diminution of the heat of light should not be admitted
by reason of the squared square of the distance to the sun, it will
still be evident that heat, in its propagation, diminishes more than
light. If we excite a very strong heat, by kindling a large fire, we
shall only feel it at a moderate distance but we shall see the light
at a very great one. If we bring our hands by degrees nearer and nearer
a body excessively hot, we shall perceive that the heat increases
much more in proportion than as the space diminishes; for we may warm
ourselves with pleasure at a distance which differs only by a few
inches from that at which we should be burnt. Every thing, therefore,
appears to indicate, that heat diminishes in a greater ratio than
light, in proportion as both are removed from the focus whence they
issued.
This might lead us to imagine, that the atoms of light would be very
cold when they came to the surface of our atmosphere; but that by
traversing the great extent of this transparent mass, they receive a
new heat by friction. The infinite velocity with which the particles
of light rub against those of the air, must produce a heat so much the
stronger as the friction is more multiplied: and it is, probably, for
this reason, that the heat of the solar rays is found much stronger in
the lower parts of the atmosphere, and that the coldness of the air
appears to augment as we are elevated. Perhaps, likewise, as light
receives heat only by uniting, a great number of atoms of light is
required to constitute a single atom of heat, and this may be the
cause why the feeble light of the moon, although in the atmosphere,
like that of the sun, does not receive any sensible degree of heat.
If, as M. Bouguer says, the intensity of the light of the sun to
the surface of the earth is 300,000 times stronger than that of the
moon, the latter must be almost insensible, even by uniting it in the
focus of the most powerful burning glasses, which cannot condense it
more than 2000 times; subtracting the half of which for the loss by
reflexion or refraction, there remains only a 300dth part intensity to
the focus of the glass.
Thus, we must not infer that light can exist without any heat, but
only that the degrees of this heat are very different, according to
different circumstances, and always insensible when light is very weak.
Heat, on the contrary, seems to exist habitually, and even to cause
itself to be strongly felt without light; for in general it is only
when it becomes excessive, that light accompanies it. But the very
essential difference between these two modifications of matter is,
that heat, which penetrates all bodies, does not appear to fix in any
one, whereas light incorporates and extinguishes in all those which do
not reflect, or permit it to pass freely; heat bodies of all kinds
to any degree, in a very short time they will lose the acquired heat,
and return to the general temperature. If we receive light on black
or white bodies, rude or polished, it will easily be perceived, that
some admit, and others repel it; and that instead of being affected in
a uniform manner as they are by heat, they are only so relatively to
their nature, colour, and polish. Black will absorb more light than
white, and the rough more than the smooth. Light once absorbed remains
fixed in the body which received it, nor quits it like heat; whence
we must conclude, that atoms of light may become constituent parts of
bodies by uniting with the matter which composes them; whereas heat
not fixing at all, seems to prevent the union of every part of matter,
and only acts to keep them separate. Nevertheless, there are instances
where heat remains fixed in bodies, and others where the light they
have absorbed re-appears, and goes out like heat.
After all there appear to be two kinds of heat, the one luminous, of
which the sun is the focus; the other obscure, of which the grand
reservoir is the terrestrial globe. Our body, as making part of the
globe, participates of this obscure heat; and it is for this reason,
that it is still obscure to us, because we do not perceive it by any
one of our senses. It is with respect to this heat of the globe, as
with its motion, we are subject to and participate thereof without
feeling or doubting of it: from hence it happened that physicians at
first carried all their views and enquiries on the heat of the sun,
without suspecting that it makes but a very small part of what we
really feel; but having made instruments to discover the difference of
the immediate heat of the rays of the sun, they with astonishment found
that the heat of them was sixty-six times stronger in summer than in
winter, notwithstanding the strongest heat of our summer differs only
a seventh from the strongest cold of our winter; from whence they have
concluded, that, independent of the heat we receive from the sun, there
emanates another, even from this terrestrial globe, which is much more
considerable; insomuch, that it is at present demonstrable, that this
heat, which escapes from the bowels of the earth, is in our climate at
least twenty-nine times in summer, and four hundred times in winter,
stronger than the heat which comes to us from the sun.
This strong heat which resides in the interior part of the globe, and
which, without ceasing to emanate externally, must, like an element,
enter into the combination of all the other elements. If the sun is
the parent of Nature, the heat of the earth must be the mother; they
both unite to produce, support, and animate organized beings, and to
assimilate and compose inanimate substances. This internal heat of the
globe, which tends always from the centre to the circumference, is, in
my opinion, a great agent in nature. We can scarcely doubt but it is
the principal influence on the perpendicularity of the trunks of trees,
on the phenomena of electricity, on the effects of magnetism, &c. But
as I do not pretend to make a physical treatise here, I shall confine
myself to the effects of this heat on the other elements. It is alone
sufficient to maintain the rarefaction of the air to the degree that
we breathe in: it is more than sufficient to keep water in its state
of fluidity, for we have lowered the thermometers to the depth of 120
fathoms, and have found the temperature of the water was there nearly
the same as at the like depth in the earth, namely, ten degrees two
thirds. We must not, therefore, be surprized, especially as salt acts
as a prevention, that the sea in general does not freeze, that fresh
water freezes but to a certain thickness, and that the water at bottom
always remains liquid, even in the most intense frosts.
But of all the elements the earth is that on which this internal
heat must necessarily have produced, and still produces the greatest
effects. This heat originally was doubtless much greater than it is
at present; therefore we must refer to it, as to the first cause, all
the sublimations, precipitations, aggregations, and separations, which
have been, and still continue to be made in the internal part of the
globe, especially in the external layer which we have penetrated, and
the matter of which has been removed by the convulsions of Nature, or
by the hands of man. The whole mass of the globe having been melted, or
liquefied, by fire, the internal is only a concrete or discreet glass,
whose simple substance cannot receive any alteration by heat alone;
there is, therefore, only an upper and superficial layer, which being
exposed to the action of external causes united to that of the internal
heat, will have undergone all the modifications, differences, and
forms, in one word, of Mineral Substances, which their combined actions
were enabled to produce.
Fire, which at first sight appears to be only a compound of heat and
light, might also be a modification of the matter, though it does
not essentially differ from either, and still less from both taken
together. Fire never exists without heat, but it can exist without
light. Heat alone, deprived of all appearance of light, can produce the
same effects as the most violent fire; so can also light, when it is
united. Light seems to carry a substance in itself which has no need
of fuel; but fire cannot subsist without absorbing the air, and it
becomes more violent in proportion to the quantity it absorbs; whereas
light, concentrated and received into a vessel exhausted of air, acts
as fire in air; and heat, confined and retained in a narrow space,
subsists and even augments with a very small quantity of food. The most
general difference between fire, heat, and light, appears, therefore,
to consist in the quantity, and perhaps quality, of their food.
Air is the first food of fire; combustible matters are only the second.
It has been demonstrated, by experiments, that a little spark of
fire, placed in a vessel well closed, in a short time absorbs a great
quantity of air, and becomes extinguished as soon as the quantity or
quality, of this food becomes deficient. By other experiments it is
proved, that the most combustible matters will not consume in vessels
well closed, although exposed to the action of the greatest fire. Air
is, therefore, the first and true food of fire, and combustible matters
would not be able to supply it without the assistance and mediation of
this element.
We have observed that heat is the cause of all fluidity, and we find,
by comparing some fluids together, that more heat is requisite to keep
iron in fusion than gold; and more to keep gold than tin; much less
is necessary for wax, for water less than that, and still less for
spirits of wine, and a mere trifle is sufficient for mercury, since
the latter goes 187 degrees below what water can without losing its
fluidity; mercury, therefore, is the most fluid of all matter, air
excepted. Now this superior fluidity in air indicates the least degree
of adherence possible between its constituting parts, and supposes them
of such a figure as only to be touched at one point. It may be also
imagined, that, being endowed with so little apparent energy and mutual
attraction, they are, for that reason, less massive, and more light,
than those of every other body; but that conclusion appears unfounded,
from the comparison of mercury, the next fluid body, but of which the
constituting parts appear to be more massive and heavy than those of
any other matter, excepting gold. The greater or lesser fluidity, does
not, therefore, indicate that the parts of the fluid are more or less
weighty, but only that their adherence is so much the less, and their
separation so much the easier.
Air, therefore, of all known matter, is that which heat divides the
easiest, and is very near the nature of fire, whose property consists
in the expansive motions of its parts; and it is from this similarity
that air so strongly augments the activity of fire, to which it is the
most powerful assistant, and the most intimate and necessary food.
Even combustible matters will not keep it alive if deprived of air,
for under this privation the most intense fire will not burn; but a
single spark of air is sufficient to kindle them, and in proportion as
it is supplied with that element the fire becomes strong, extended, and
devouring.
Artificial phosphorus, and gunpowder, seem, at first, to be an
exception, for they have no need of the assistance of renewed air to
inflame and wholly consume them: their combustion may be performed in
the closest vessels, but that is because those matters, which are also
the most combustible, contain the necessary quantity of air in their
substance, therefore they have no need of the assistance of foreign
air.
This seems to indicate that the most essential difference between
combustible matters and those which are not so, consists in the latter
containing only a few or none of the light, ethereal, and oily matters
susceptible of an expansive motion, or, at least, if they contain them,
that they are fixed, so that they cannot exercise their volatility
whenever the force of the fire is not strong enough to surmount the
force of adhesion which retains them united to the fixed parts of
matter. It may be said that this induction is confirmed by a number of
observations well known to chemists; but what appears to be less so,
and which, nevertheless, is a necessary consequence of it, is, that
all matter may become volatile when the expansive force of the fire
can be rendered superior to the attractive force which holds the parts
of matter united; for though to produce a fire sufficiently strong it
may require better constructed mirrors than any at present known, yet
we are certain that fixity is only a relative quality, and that there
is no matter absolutely so, since heat dilates the most fixed bodies.
Now is not this dilation the index of a beginning separation, that may
be augmented with a degree of heat to fusion, and with a still greater
heat to volatilisation?
Combustion supposes something more than volatilisation; it is not
sufficient that the parts of matter be sufficiently separated to be
carried off by those of heat; they must also be of an analogous nature
to fire; without that, mercury, being the most fluid next to air, would
also be the most combustible, whereas experience demonstrates, that
though very volatile it is not combustible. Matter is, in general,
composed of four principal substances, called _elements_, that is,
earth, water, air, and fire. Those in which earth and water predominate
will be fixed, and will only become volatile by the action of heat;
and those which contain most air and fire will be the only real
combustibles. The great difficulty here is clearly to conceive how air
and fire, both so volatile, can fix and become constituent parts of all
bodies.
Fire, by absorbing air, destroys the spring. Now there are but two
methods of destroying a spring, either by compressing it till it
breaks, or extending it till it loses its effect. It is plain that
fire cannot destroy air by compression, since the least degree of heat
rarefies it; on the contrary, by a very strong heat the rarefaction of
the air will be so great that it will occupy a space thirteen times
more extended than that of its general volume; and by this means the
spring becomes weakened, and it is in this state that it can become
fixed, and unite with other bodies.
Light, which falls on bodies, is not merely reflected, but remains in
quantities on the small thickness of the surface which it strikes;
consequently it loses its motion, extends, is fixed, and becomes a
constituent part of all that it penetrates. Let us add this light,
transformed and fixed in bodies, to the above air, and to both, the
constant and actual heat of the terrestrial globe, whose sum is much
greater than that which comes from the sun, and then it will appear to
be not only one of the greatest springs of the mechanism of Nature, but
an element with which the whole matter of the globe is penetrated.
If we consider more particularly the nature of combustible matters,
we shall find, that they all proceed originally from vegetables and
animals; in a word, from bodies placed on the surface of the globe,
which the sun enlightens, heats, and vivifies. Wood, bitumen, resins,
coals, fat and oil, by expression, wax, and suet, are substances
proceeding immediately from animals and vegetables. Turf, fossil, coal,
amber, liquid, or concrete bitumens, are the productions of their
mixture, and their decomposition, whose ulterior waste forms sulphurs,
and the combustible parts of iron, tin, pyrites, and every inflammable
mineral. I know, that this last assertion will be rejected by those
who have studied nature only by the mode of chemistry; but I must
request them to consider, that their method is not that of nature, and
that it cannot even approach it without banishing all those precarious
principles, those fictitious beings which they play upon, without being
acquainted with them.
But, without pressing longer on those general considerations, let
us pursue in a more direct and particular manner the examination of
fire and its effects. The action of fire depends much on the manner
in which it is applied; and the effects of its motion, on similar
substances, will appear different according to the mode in which it
is administered. I conceive that fire should be considered in three
different states, first relative to its velocity; secondly, as to its
volume; and thirdly, as to its mass. Under each of these points of
view, this element, so simple, and so uniform to all appearance, will
appear extremely different. The velocity of fire is augmented without
the apparent volume being increased, every time that in a given space
and filled with combustible matters, its action and expansion is
pressed by augmenting the velocity of the air by bellows, caverns,
ventilators, aspirative tubes, &c. all of which accelerate more or less
the rapidity of the air directed on the fire. The action of fire is
augmented by its volume, when a great quantity of combustible matters
is accumulated, and the heat and fire are driven into the reverberatory
furnaces, which comprehend those of our glass, porcelain, and pottery
manufactories, and all those wherein metals and minerals are melted,
iron excepted. Fire acts here by its volume, and has only its own
velocity, since the rapidity is not augmented by the bellows, or other
instruments which carry air to the fire.
There are many modes of augmenting the action of fire by its velocity
or volume; but there is only one way of augmenting its mass; namely,
by uniting it in the focus of a burning glass. When we receive on the
refracting, or reflecting mirror, the rays of the sun, or even those of
a well-kindled fire, we unite them in so much the less space, as the
mirror is longer, and the focus shorter; for example, by a mirror of
four feet diameter, and one inch focus, it is clear, that the quantity
of light, or fire, which falls on the four-feet mirror, will be united
in the space of one inch, that is, it will be 2304 times denser than
it was, if all the incident matter arrived to this focus without any
loss, and when even the loss is two thirds or three fourths, the mass
of fire concentrated in the focus of this mirror, will always be six
or seven hundred times denser than on the surface. In this, as in all
other cases, the mass goes by the contraction of the volume; and the
fire which we thus augment the density of, has all the properties of a
mass of matter; for, independently of the action of heat, by which it
penetrates bodies, it impels and displaces them as a solid moving body
which strikes another would do.
Each of these modes of administering fire, and increasing either the
velocity, volume, or mass, often produce very different effects on the
same substances; insomuch, that no reliance is to be placed on any
thing that cannot be worked at the same time, or successively, by all
three. In the like manner, as I divide into three general proceedings
the administration of this element, I divide every matter that can
be submitted to its action into three classes. Passing over for the
present those which are purely combustible, and which immediately
proceed from animals and vegetables; we proceed to minerals, in the
first class of which we reckon those mineral matters, which this
action, continued for a long time, renders lighter, as iron; in the
second, such as it renders heavier, as lead; and in the third class,
are those matters on which, as gold, this action of fire does not
appear to produce any sensible effect, since it does not at all alter
their weight. All existing matters, that is, all substances simple and
compounded, will necessarily be comprized under one of these three
classes; and experiments on them by the three proceedings, which are
not difficult to be made, and only require exactness and time, might
develope many useful discoveries, and prove very necessary to build
on real principles the theory of chemistry, which has hitherto been
carried on by a precarious nomenclatura, and on words the more vague as
they are the more general.
Fire is the lightest of all bodies, notwithstanding which it has
weight, and it may be demonstrated, that even in a small volume it is
really heavy, as it obeys, like all other matters, the general law of
gravity, and consequently must have connections or affinities with
other bodies. All matters it renders more weighty will be those with
which it has the greatest affinity. One of the effects of this affinity
in the matters is to retain the substance even of fire, with which it
is incorporated, and this incorporation supposes that fire not only
loses its heat and elasticity, but even all its motion, since it fixes
itself in these bodies, and becomes a constituent part. From which it
may be imagined that there is fire under a fixed and concrete form in
almost every body.
It is evident, that all matters, whose weight increases by the
action of fire, are endowed with an attractive force superior to the
expansive, the fiery particles of which are animated; this being
extinguished the motion ceases, and the elastic and fugitive particles
become fixed, and take a concrete form. Thus matters, whose weight is
increased by fire, as tin, lead, &c. are substances which, by their
affinity with fire, attract and incorporate. All matters, on the
contrary, which, like iron, copper, &c. become lighter in proportion as
they are calcined, are substances whose attractive forces, relative to
the igneous particles, is less than the expansive force of fire; and
hence the fire, instead of fixing in these matters, carries off and
drives away the least adherent parts which cannot resist its impulsion.
Those which, like gold, platina, silver, &c. neither lose nor acquire
by the application of fire, are substances which, having no affinity
with fire, and not being able to unite, cannot, consequently, either
retain or accompany it when it is carried off. It is evident that the
matters of the two first classes have a certain degree of affinity with
fire, since those of the second class are loaded with fire, which they
retain; and the fire loads itself with those of the first class, which
it carries off; whereas the matters of the third class, to which it
neither lends nor borrows, have not any affinity or attraction with it,
but are indifferent to its action, which can neither unnaturalize nor
even change them.
This division of every matter into three classes, relative to the
action of fire, does not exclude the more particular and less absolute
division of all matters into two other classes, hitherto regarded as
relative to their own nature, which is said to be always vitrifiable,
or calcareous. Our new division is only a more elevated point of view,
under which we must consider them, to endeavour to deduce therefrom
even the agent that is used by the relations fire can have with every
substance to which it is applied.
We might say, with naturalists, that all is vitrifiable in Nature,
excepting that which is calcareous: that quartz, chrystals, precious
stones, flints, granites, porphyries, agates, gypsums, clays, lava,
pumice stone, with all metals and other minerals, are vitrifiable
either by the fire of our furnaces, or that of mirrors; whereas marble,
alabaster, stones, chalk, marl, and other substances which proceed from
the residue of shells and madrepores, cannot be reduced into fusion
by these means. Nevertheless I am persuaded, that if the power of our
furnaces and mirrors were further increased, we should be enabled to
put these calcareous matters in fusion; since there are a multiplicity
of reasons to conclude, that at the bottom their substance is the same,
and that glass is the common basis of all terrestrial matter.
By my own experiments I have found, that the most powerful glass
furnaces is only a weak fire, compared with that of bellows furnaces;
and that fire produced in the focus of a good mirror, is stronger than
that of the most glowing fire of a furnace. I have kept iron ore for
thirty-six hours in the hottest part of the glass furnace of Rouelle,
in Burgundy, without its being melted, agglutinated, or even in any
manner changed; whereas, in less than twelve hours this ore runs in a
forge furnace. I have also melted, or volatilized, by a mirror many
matters which neither the fire, nor reverberatory furnace, nor the
most powerful bellows furnace could cause to run.
It is commonly supposed, that flame is the hottest part of fire, yet
nothing is more erroneous than this opinion; the contrary may be
demonstrated by the most simple and familiar experiments. Offer to a
straw fire, or even to the flame of a lighted faggot, a cloth to dry or
heat, and treble the time will be required to what would be necessary
if presented to a brasier without flame. Newton very accurately defines
flame to be a burning smoke, and this smoke, or vapour, has never the
same quantity or intensity of heat as the combustible body from which
it escapes. By being carried upwards and extending, it has the property
of communicating fire, and carrying it further than the heat of the
brasier, which alone might not be sufficient to communicate it when
even very near.
The communication of fire merits a particular attention. I found, after
repeated reflections that besides the assistance of facts which appear
to have a relation to it, that experiments were necessary to understand
the manner in which this operation of Nature is made. Let us receive
two or three thousand weight of iron in a mould at its issuing from
the furnace; this metal in a short time loses its incandescence, and
ceases from its redness, according to the thickness of the ingot. If at
the moment its redness leaves it, it is drawn from the mold, the under
parts will be still red, but this colour will fly off. Now so long as
the redness subsists, we can light combustible matters by applying them
to the ingot; but as soon as it has lost its incandescent state, there
are numbers of matters which it will not set fire to, although the heat
which it diffuses is, perhaps a hundred times stronger than that of a
straw fire, which would inflame them. This made me think that flame
being necessary to the communication of fire, there is therefore a
flame in all incandescence. The red colour seems, in fact, to indicate
it; and indeed I am convinced, that combustible, and even the most
fixed matters, such as gold and silver, when in an incandescent state,
are surrounded with a dense flame which extends only to a very short
distance, and which is attached to their surface; and I can easily
conceive, that when flame becomes dense to a certain degree, it ceases
from obeying the fluctuation of the air. This white or red body, which
issues from all bodies in incandescence, and which strikes our eyes,
is the evaporation of this dense flame which surrounds the body by
renewing itself incessantly on its surface; and even the light of the
sun, which emits such an amazing brightness, I presume to be only an
evaporation of the dense state that constantly plays on its surface;
and which we must regard as a true flame, more pure and dense than any
proceeding from our combustible matters.
It is, therefore, by light that fire communicates, and heat alone
cannot produce the same effect as when it becomes very strong to be
luminous. Even water, that destructive element to fire, by which alone
we can prevent its progress, nevertheless communicates when in a
well-closed vessel, such as Papin's digester, where it is penetrated
with a sufficient quantity of fire to render it luminous, and capable
of melting lead and tin, whereas when it is only boiling, far from
communicating fire, it extinguishes it immediately. It is true, that
heat alone is sufficient to prepare and dispose combustible bodies for
inflammation, by driving off the humid parts from bodies; and what is
very remarkable, this heat, which dilates all bodies, does not desist
from hardening them by drying. I have an hundred times discovered, by
examining the stones of my great furnaces, especially the calcareous,
they increased in hardness in proportion to the time they had undergone
the heat, and they also at the same time became specifically heavier.
From this circumstance, I think an induction may be drawn, which
would prove, and fully confirm, that heat, although in appearance
always fugitive and never stable in the bodies which it penetrates,
nevertheless deposits in a positive manner many parts which fixes there
even in greater quantities than the aqueous and other parts which it
has driven off. But what appears very difficult to be reconciled,
this same calcareous stone, which becomes specifically heavier by the
action of a moderate heat a long time continued, becomes near a half
lighter, when submitted to a fire sufficient for its calcination, and,
at the same time, not only loses all the hardness it had acquired by
the action of heat, but even the natural adherence of its constituting
parts.
Calcination generally received, is, with respect to fixed and
incombustible bodies, what combustion is to volatile and inflammable.
Calcination, like combustion, needs the assistance of air; it operates
so much the quicker, as it is furnished with a greater quantity of that
element, without which the fiercest fire cannot calcine nor inflame
any thing, except such matters as contain in themselves all the air
necessary for those purposes. This necessity for the concurrence of
air in calcination, as in combustion, indicates, that there are more
things common between them than has been suspected. The application of
fire is the principle of both; that of air is the second cause, and
almost as necessary as the first; but these two causes are equally
combined, according as they act in more or less time, and with more or
less power on different substances.
Combustion operates almost instantaneously; calcination is sometimes
so long, as to be thought impossible; for in proportion as matters
are more incombustible, the calcination is there more slowly made;
and when the constituent parts of a substance, such as gold, are not
only incombustible, but appear so fixed as not to be volatilized,
calcination produces no effect. They must both, therefore, be
considered as effects of the same cause, whose two extremes are
delineated to us by phosphorus, which is the most inflammable of all
bodies, and by gold, which is the most fixed and least combustible.
All substances comprized between these two extremes, will be more or
less subjected to the effects of combustion and calcination, according
as they approach either of them; insomuch, that in the middle points
there will be found substances that endure an almost equal degree
of both; from which we may conclude, that all calcination is always
accopmanied with a little combustion, and all combustion with a little
calcination. Cinders and other residue of the most combustible matters,
demonstrate that fire has calcined all the parts it has not burned,
and consequently, a little calcination is found here with combustion.
The small flame which rises from most matters, that are calcined,
demonstrates also that a slight combustion is made. Thus, we must not
separate these two effects, if we would find out the results of the
action of fire on the different substances to which it is applied.
But it may be said, that combustion always diminishes the volume or
mass, on account of the quantity of matter it consumes; and that, on
the contrary, calcination increases the weight of many substances.
Ought we then to consider these two effects whose results are so
contrary, as effects of the same nature? Such an objection appears
well-founded, and deserves an answer, especially as this is the most
difficult point of the question. For that purpose let us consider a
matter in which we shall suppose one half to be fixed parts, and the
other volatile or combustible. By the application of fire to this,
all the volatile or combustible parts will be raised up or burnt, and
consequently separated from the whole mass; from hence this mass or
quantity of matter will be found diminished one half, as we see it
in calcareous stones, which lose near half their weight in the fire.
But if we continue to apply the fire for a very long time to the other
half, composed of fixed parts, all combustion and volatilization
being ceased, that matter, instead of continuing to lose its mass,
must increase at the expense of the air and fire with which it is
penetrated; and those are matters already calcined, and prepared
by Nature to the degree where combustion ceases, and consequently
susceptible of increasing the weight from the first moment of the
application. We have seen, that light extinguishes on the surface of
all bodies which do not reflect; and that heat, by long residence,
fixes partly in the matters which it penetrates; we know also that
air is necessary for calcination, or combustion, and the more so for
calcination as having more fixity in the external parts of bodies, and
becomes a constituent part: hence, it is natural to imagine, that this
augmentation of weight proceeds only from the addition of the particles
of light, heat, and air, which are at length fixed and united to one
matter, against which they have made so many efforts, without being
able either to raise or burn them. This appears clearly to be the
fact, for if we afterwards present a combustible substance to them they
will quit the fixed matter, to which they were only attached through
force, retake their natural motion, elasticity, and volatility, and
all depart with it; from hence, metal, or calcinized matter, to which
these volatile parts has been rendered, retakes its pristine form,
and its weight is found diminished by the whole quantity of fiery
and airy particles which were fixed in it, and which had been just
raised by this new combustion. All this is performed by the sole law
of affinities; and there seems to be no more difficulty to conceive
how the lime of a metal is reduced, than to understand how it is
precipitated in dissolution; the cause is the same, and the effects are
similar. A metal dissolved by an acid, will precipitate when to this
acid another substance is offered with which it has more affinity than
metal, the acid then quits it and falls to the bottom. So, likewise,
this metal calcines, that is, loaded with parts of air, heat, and
fire, which being fixed, keeps it under the form of a lime, and will
precipitate, or be reduced, when presented to this fire and fixed air,
from the combustible matters with which they have more affinity than
with the metal; the latter will retake its first form as soon as it is
disembarrassed from this superfluous air and fire, at the expence of
the combustible matters offered to it, and the volatile parts it had
lost.
I think I have now demonstrated, that all the little laws of chemical
affinities, which appeared so variable and different, are no other
than the general laws of attraction, common to all matter; that this
great law, always constant and the same, appeared only to vary in
its expression, which cannot be the same when the figure of bodies
enters, like an element, into their distance. With this new key we
can unlock the most profound secrets of Nature; we can attain the
knowledge of the figure of the primitive parts of different substances;
assign the laws and degrees of their affinities; determine the forms
which they take by re-uniting, &c. I think also I have made it appear
that impulsion depends on attraction; and that, although it may be
considered as a different force, it is, notwithstanding, a particular
effect of this sole and general one. I have shewn the communication
of motion to be impossible without a spring, whence I have concluded,
that all bodies in Nature are more or less elastic, and that there
is not one perfectly hard; that is, entirely deprived of a spring,
since all are susceptible of receiving motion. I have endeavoured to
shew how this sole force may change direction, and attraction become
repulsion; and from these grand principles, which are all founded on
rational mechanics, I have sought to deduce the principal operations
of Nature, such as the production of light, heat, and fire, and their
action on different substances; this last object which interests us
the most is a vast field, but of which I can only cultivate a little
spot, yet I presume I may render some assistance, by putting into
more capable and laborious hands the instruments I made use of. These
instruments were the three modes of making use of fire, that is, by its
velocity, volume, and mass; by applying it concurrently to the three
classes of substances, which either lose, gain, or are not affected
by the application of fire. The experiments which I had made on the
refrigeration of bodies, on the real weight of fire, on the nature of
flame, on the progress of heat, or its communication, its diperdition,
its concentration, or its violent action without flame, &c. are also
so many instruments which will spare much labour to those who choose
to avail themselves of them, and will produce an ample harvest of
knowledge.