OF THE CAUSES AND EFFECTS OF HEAT AND COLD.
_A._ It is a fine day, and pleasant walking through the fields, but that
the sun is a little too hot.
_B._ How know you that the sun is hot?
_A._ I feel it.
_B._ That is to say, you know that yourself, but not that the sun is
hot. But when you find yourself hot, what body do you feel?
_A._ None.
_B._ How then can you infer your heat from the sense of feeling? Your
walking may have made you hot: is motion therefore hot? No. You are to
consider the concomitants of your heat; as, that you are more faint, or
more ruddy, or that you sweat, or feel some endeavour of moisture or
spirits tending outward; and when you have found the causes of those
accidents, you have found the causes of heat, which in a living
creature, and especially in a man, is many times the motion of the parts
within him, such as happen in sickness, anger, and other passions of the
mind; which are not in the sun nor in fire.
_A._ That which I desire now to know, is what motions and of what bodies
without me are the efficient causes of my heat.
_B._ I showed you yesterday, in discoursing of rain, how by this
compounded motion of the sun’s body, the air was every way at once
thrust off west and east; so that where it was contiguous, the small
parts of the water were forced to rise, for the avoiding of vacuum.
Think then that your hand were in the place of water so exposed to the
sun. Must not the sun work upon it as it did upon the water? Though it
break not the skin, yet it will give to the inner fluids and looser
parts of your hand, an endeavour to get forth, which will extend the
skin, and in some climates fetch up the blood, and in time make the skin
black. The fire also will do the same to them that often sit with their
naked skins too near it. Nay, one may sit so near, without touching it,
as it shall blister or break the skin, and fetch up both spirits and
blood mixt into a putrid oily matter, sooner than in a furnace oil can
be extracted out of a plant.
_A._ But if the water be above the fire in a kettle, what then will it
do? Shall the particles of water go toward the fire, as it did toward
the sun?
_B._ No. For it cannot. But the motion of the parts of the kettle which
are caused by the fire, shall dissipate the water into vapour till it be
all cast out.
_A._ What is that you call fire? Is it a hard or fluid body?
_B._ It is not any other body but that of the shining coal; which coal,
though extinguished with water, is still the same body. So also in a
very hot furnace, the hollow spaces between the shining coals, though
they burn that you put into them, are no other body than air moved.
_A._ Is it not flame?
_B._ No. For flame is nothing but a multitude of sparks, and sparks are
but the atoms of the fuel dissipated by the incredible swift motion of
the movent, which makes every spark to seem a hundred times greater than
it is, as appears by this; that, when a man swings in the air a small
stick fired at one end, though the motion cannot be very swift, yet the
fire will appear to the eye to be a long, straight, or crooked line.
Therefore a great many sparks together flying upward, must needs appear
unto the sight as one continued flame. Nor are the sparks stricken out
of a flint any thing else but small particles of the stone, which by
their swift motion are made to shine. But that fire is not a substance
of itself, is evident enough by this, that the sun-beams passing through
a globe of water will burn as other fire does. Which beams, if they were
indeed fire, would be quenched in the passage.
_A._ This is so evident, that I wonder so wise men as Aristotle and his
followers, for so long a time could hold it for an element, and one of
the primary parts of the universe. But the natural heat of a man or
other living creature, whence proceedeth it? Is there anything within
their bodies that hath this compounded motion?
_B._ At the breaking up of a deer I have seen it plainly in his bowels
as long as they were warm. And it is called the peristaltic motion, and
in the heart of a beast newly taken out of his body; and this motion is
called systole and diastole. But they are both of them this compounded
motion, whereof the former causeth the food to wind up and down through
the guts, and the latter makes the circulation of the blood.
_A._ What kind of motion is the cause of cold? Methinks it should be
contrary to that which causeth heat.
_B._ So it is in some respect. For seeing the motion that begets heat,
tendeth to the separation of the parts of the body whereon it acteth, it
stands with reason, that the motion which maketh cold, should be such as
sets them closer together. But contrary motions are, to speak properly,
when upon two ends of a line two bodies move towards each other, the
effect whereof is to make them meet. But each of them, as to this
question, is the same.
_A._ Do you think (as many philosophers have held and now hold), that
cold is nothing but a privation of heat?
_B._ No. Have you never heard the fable of the satyr that dwelling with
a husbandman, and seeing him blow his fingers to warm them, and his
pottage to cool it, was so scandalized, that he ran from him, saying he
would no longer dwell with one that could blow both hot and cold with
one breath? Yet the cause is evident enough. For the air which had
gotten a calefactive power from his vital parts, was from his mouth and
throat gently diffused on his fingers, and retained still that power.
But to cool his pottage he straightened the passage at his lips, which
extinguished the calefactive motion.
_A._ Do you think wind the general cause of cold? If that were true, in
the greatest winds we should have the greatest frosts.
_B._ I mean not any of those uncertain winds which, I said, were made by
the clouds, but such as a body moved in the air makes to and against
itself; (for it is all one motion of the air whether it be carried
against the body, or the body against it); such a wind as is constant,
if no other be stirring, from east to west; and made by the earth
turning daily upon its own centre; which is so swift, as, except it be
kept off by some hill, to kill a man, as by experience hath been found
by those who have passed over great mountains, and specially over the
Andes which are opposed to the east. And such is the wind which the
earth maketh in the air by her annual motion, which is so swift, as
that, by the calculation of astronomers, to go sixty miles in a minute
of an hour. And therefore this must be the motion which makes it so cold
about the poles of the ecliptic.
_A._ Does not the earth make the wind as great in one part of the
ecliptic as in another?
_B._ Yes. But when the sun is in Cancer, it tempers the cold, and still
less and less, but least of all in the winter-solstice, where his beams
are most oblique to the superficies of the earth.
_A._ I thought the greatest cold had been about the poles of the
equator.
_B._ And so did I once. But the reason commonly given for it is so
improbable, that I do not think so now. For the cause they render of it
is only, that the motion of the earth is swiftest in the equinoctial,
and slowest about the poles; and consequently, since motion is the cause
of heat, and cold is but, as it was thought, a want of the same, they
inferred that the greatest cold must be about the poles of the
equinoctial. Wherein they miscounted. For not every motion causeth heat,
but this agitation only, which we call compounded motion; though some
have alleged experience for that opinion; as that a bullet out of a gun
will with its own swiftness melt. Which I never shall believe.
_A._ It is a common thing with many philosophers to maintain their
fancies with any rash report, and sometimes with a lie. But how is it
possible that so soft a substance as water should be turned into so hard
a substance as ice?
_B._ When the air shaves the globe of the earth with such swiftness, as
that of sixty miles in a minute of an hour, it cannot, where it meets
with still water, but beat it up into small and undistinguishable
bubbles, and involve itself in them as in so many bladders or skins of
water. And ice is nothing else but the smallest imaginable parts of air
and water mixed; which is made hard by this compounded motion, that
keeps the parts so close together, as not to be separated in one place
without disordering the motion of them all. For when a body will not
easily yield to the impression of an external movent in one place
without yielding in all, we call it hard; and when it does, we say it is
soft.
_A._ Why is not ice as well made in a moved as in a still water? Are
there not great seas of ice in the northern parts of the earth?
_B._ Yes, and perhaps also in the southern parts. But I cannot imagine
how ice can be made in such agitation as is always in the open sea, made
by the tides and by the winds. But how it may be made at the shore, it
is not hard to imagine. For in a river or current, though swift, the
water that adhereth to the banks is quiet, and easily by the motion of
the air driven into small insensible bubbles; and so may the water that
adhereth to those bubbles, and so forwards till it come into a stream
that breaks it, and then it is no wonder though the fragments be driven
into the open sea, and freeze together into greater lumps. But when in
the open sea, or at the shore, the tide or a great wave shall arise,
this young and tender ice will presently be washed away. And therefore I
think it evident, that as in the Thames the ice is first made at the
banks where the tide is weak or none, and, broken by the stream, comes
down to London, and part goes to the sea floating till it dissolve, and
part, being too great to pass the bridge, stoppeth there and sustains
that which follows, till the river be quite frozen over; so also the ice
in the northern seas begins first at the banks of the continent and
islands which are situated in that climate, and then broken off, are
carried up and down, and one against another, till they become great
bodies.
_A._ But what if there be islands, and narrow inlets of the sea, or
rivers also about the pole of the equinoctial?
_B._ If there be, it is very likely the sea may also there be covered
all over with ice. But for the truth of this, we must stay for some
farther discovery.
_A._ When the ice is once made and hard, what dissolves it?
_B._ The principal cause of it, is the weight of the water itself; but
not without some abatement in the stream of the air that hardeneth it;
as when the sunbeams are less oblique to the earth, or some contrary
wind resisteth the stream of the air. For when the impediment is
removed, then the nature of the water only worketh, and, being a heavy
body, downward.
_A._ I forgot to ask you, why two pieces of wood rubbed swiftly one
against another, will at length set on fire.
_B._ Not only at length, but quickly, if the wood be dry. And the cause
is evident, viz. the compounded motion which dissipates the external
small parts of the wood. And then the inner parts must of necessity, to
preserve the plentitude of the universe, come after; first the most
fluid, and then those also of greater consistence, which are first
erected, and the motion continued, made to fly swiftly out; whereby the
air driven to the eye of the beholder, maketh that fancy which is called
light.
_A._ Yes; I remember you told me before, that upon any strong pressure
of the eye, the resistance from within would appear a light. But to
return to the enquiry of heat and cold, there be two things that beyond
all other put me into admiration. One is the swiftness of kindling in
gunpowder. The other is the freezing of water in a vessel, though not
far from the fire, set about with other water with ice and snow in it.
When paper or flax is flaming, the flame creeps gently on; and if a
house full of paper were to be burnt with putting a candle to it, it
will be long in burning; whereas a spark of fire would set on flame a
mountain of gunpowder in almost an instant.
_B._ Know you not gunpowder is made of the powder of charcoal,
brimstone, and saltpetre? Whereof the first will kindle with a spark,
the second flame as soon as touched with fire; and the third blows it,
as being composed of many orbs of salt filled with air, and as it
dissolveth in the flame, furiously blowing increaseth it. And as for
making ice by the fireside; it is manifest that whilst the snow is
dissolving in the external vessel, the air must in the like manner break
forth, and shave the superficies of the inner vessel, and work through
the water till it be frozen.
_A._ I could easily assent to this, if I could conceive how the air that
shaves, as you say, the outside of the vessel, could work through it. I
conceive well enough a pail of water with ice or snow dissolving in it,
and how it causeth wind. But how that wind should communicate itself
through the vessel of wood or metal, so as to make it shave the
superficies of the water which is within it, I do not so well
understand.
_B._ I do not say the inner superficies of the vessel shaves the water
within it. But it is manifest that the wind made in the pail of water by
the melting snow or ice presseth the sides of the vessel that standeth
in it; and that the pressure worketh clean through, how hard soever the
vessel be; and that again worketh on the water within, by restitution of
its parts, and so hardeneth the water by degrees.
_A._ I understand you now. The ice in the pail by its dissolution
transfers its hardness to the water within.
_B._ You are merry. But supposing, as I do, that the ice in the pail is
more than the water in the vessel, you will find no absurdity in the
argument. Besides, the experiment, you know, is common.
_A._ I confess it is probable. The Greeks have the word φρίκη (whence
the Latins have their word _frigus_) to signify the curling of the water
by the wind; and use the same also for horror, which is the passion of
one that cometh suddenly into a cold air, or is put into a sudden
affright, whereby he shrinks, and his hair stands upright. Which
manifestly shows that the motion which causeth cold, is that which
pressing the superficies of a body, sets the parts of it closer
together. But to proceed in my queries. Monsieur Des Cartes, whom you
know, hath written somewhere, that the noise we hear in thunder,
proceeds from breaking of the ice in the clouds; what think you of it?
Can a cloud be turned into ice?
_B._ Why not? A cloud is but water in the air?
_A._ But how? For he has not told us that.
_B._ You know that it is only in summer, and in hot weather, that it
thunders; or if in winter, it is taken for a prodigy. You know also,
that of clouds, some are higher, some lower, and many in number, as you
cannot but have oftentimes observed, with spaces between them.
Therefore, as in all currents of water, the water is there swiftest
where it is straitened with islands, so must the current of air made by
the annual motion be swiftest there, where it is checked with many
clouds, through which it must, as it were, be strained, and leave behind
it many small particles of earth always in it, and in hot weather more
than ordinary.
_A._ This I understand, and that it may cause ice. But when the ice is
made, how is it broken? And why falls it not down in shivers?
_B._ The particles are enclosed in small caverns of the ice; and their
natural motion being the same which we have ascribed to the globe of the
earth, requires a sufficient space to move in. But when it is imprisoned
in a less room than that, then a great part of the ice breaks: and this
is the thunder-clap. The murmur following is from the settling of the
air. The lightning is the fancy made by the recoiling of the air against
the eye. The fall is in rain, not in shivers; because the prisons which
they break are extreme narrow, and the shivers being small, are
dissolved by the heat. But in less heat they would fall in drops of
hail, that is to say, half frozen by the shaving of the air as they
fall, and be in a very little time, much less than snow or ice,
dissolved.
_A._ Will not that lightning burn?
_B._ No. But it hath often killed men with cold. But this extraordinary
swiftness of lightning consisteth not in the expansion of the air, but
in a straight and direct stream from where it breaks forth; which is in
many places successively, according to the motion of the cloud.
_A._ Experience tells us that. I have now done with my problems
concerning the great bodies of the world, the stars, and element of air
in which they are moved, and am therein satisfied, and the rather,
because you have answered me by the supposition of one only motion, and
commonly known, and the same with that of Copernicus, whose opinion is
received by all the learned; and because you have not used any of these
empty terms, sympathy, antipathy, antiperistasis, etc., for a natural
cause, as the old philosophers have done to save their credit. For
though they were many of them wise men, as Plato, Aristotle, Seneca, and
others, and have written excellently of morals and politics, yet there
is very little natural philosophy to be gathered out of their writings.
_B._ Their ethics and politics are pleasant reading, but I find not any
argument in their discourses of justice or virtue drawn from the supreme
authority, on whose laws all justice, virtue, and good politics depend.
_A._ Concerning this cover, or, as some have called it, the scurf or
scab of the terrestrial star, I will begin with you tomorrow. For it is
a large subject, containing animals, vegetables, metals, stones, and
many other kinds of bodies, the knowledge whereof is desired by most
men, and of the greatest and most general profit.
_B._ And this is it, in which I shall give you the least satisfaction;
so great is the variety of motion, and so concealed from human senses.
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