whose limit is heat appears to be motion. This is chiefly exhibited in
flame, which is in constant motion, and in warm or boiling liquids,
which are likewise in constant motion. It is also shown in the
excitement or increase of heat by motion, as by bellows and draughts:
for which see Inst. 29, Tab. 3, and by other species of motion, as in
Inst. 28 and 31, Tab. 3. It is also shown by the extinction of fire
and heat upon any strong pressure, which restrains and puts a stop to
motion; for which see Inst. 30 and 32, Tab. 3. It is further shown
by this circumstance, namely, that every substance is destroyed, or
at least materially changed, by strong and powerful fire and heat:
whence it is clear that tumult and confusion are occasioned by heat,
together with a violent motion in the internal parts of bodies; and
this gradually tends to their dissolution.
What we have said with regard to motion must be thus understood, when
taken as the genus of heat: it must not be thought that heat generates
motion, or motion heat (though in some respects this be true), but
that the very essence of heat, or the substantial self[104] of heat,
is motion and nothing else, limited, however, by certain differences
which we will presently add, after giving some cautions for avoiding
ambiguity.
Sensible heat is relative, and regards man, not universe; and is
rightly held to be merely the effect of heat on animal spirit. It is
even variable in itself, since the same body (in different states of
sensation) excites the feeling of heat and of cold; this is shown by
Inst. 41, Tab. 3.
Nor should we confound the communication of heat or its transitive
nature, by which a body grows warm at the approach of a heated body,
with the form of heat; for heat is one thing and heating another. Heat
can be excited by friction without any previous heating body, and,
therefore, heating is excluded from the form of heat. Even when heat is
excited by the approach of a hot body, this depends not on the form of
heat, but on another more profound and common nature; namely, that of
assimilation and multiplication, about which a separate inquiry must be
made.
The notion of fire is vulgar, and of no assistance; it is merely
compounded of the conjunction of heat and light in any body, as in
ordinary flame and red-hot substances.
Laying aside all ambiguity, therefore, we must lastly consider the true
differences which limit motion and render it the form of heat.
I. The first difference is, that heat is an expansive motion, by which
the body strives to dilate itself, and to occupy a greater space than
before. This difference is principally seen in flame, where the smoke
or thick vapor is clearly dilated and bursts into flame.
It is also shown in all boiling liquids, which swell, rise, and boil
up to the sight, and the process of expansion is urged forward till
they are converted into a much more extended and dilated body than the
liquid itself, such as steam, smoke, or air.
It is also shown in wood and combustibles where exudation sometimes
takes place, and evaporation always.
It is also shown in the melting of metals, which, being very compact,
do not easily swell and dilate, but yet their spirit, when dilated
and desirous of further expansion, forces and urges its thicker parts
into dissolution, and if the heat be pushed still further, reduces a
considerable part of them into a volatile state.
It is also shown in iron or stones, which though not melted or
dissolved, are however softened. The same circumstance takes place in
sticks of wood, which become flexible when a little heated in warm
ashes.
It is most readily observed in air, which instantly and manifestly
expands with a small degree of heat, as in Inst. 38, Tab. 3.
It is also shown in the contrary nature of cold; for cold contracts
and narrows every substance;[105] so that in intense frosts nails fall
out of the wall and brass cracks, and heated glass exposed suddenly to
the cold cracks and breaks. So the air, by a slight degree of cold,
contracts itself, as in Inst. 38, Tab. 3. More will be said of this in
the inquiry into cold.
Nor is it to be wondered at if cold and heat exhibit many common
effects (for which see Inst. 32, Tab. 2), since two differences, of
which we shall presently speak, belong to each nature: although in
the present difference the effects be diametrically opposed to each
other. For heat occasions an expansive and dilating motion, but cold a
contracting and condensing motion.
II. The second difference is a modification of the preceding, namely,
that heat is an expansive motion, tending toward the exterior, but at
the same time bearing the body upward. For there is no doubt that there
be many compound motions, as an arrow or dart, for instance, has both a
rotatory and progressive motion. In the same way the motion of heat is
both expansive and tending upward.
This difference is shown by putting the tongs or poker into the fire.
If placed perpendicularly with the hand above, they soon burn it, but
much less speedily if the hand hold them sloping or from below.
It is also conspicuous in distillations _per descensum_, which men are
wont to employ with delicate flowers, whose scent easily evaporates.
Their industry has devised placing the fire above instead of below,
that it may scorch less; for not only flame but all heat has an upward
tendency.
Let an experiment be made on the contrary nature of cold, whether its
contraction be downward, as the expansion of heat is upward. Take,
therefore, two iron rods or two glass tubes, alike in other respects,
and warm them a little, and place a sponge, dipped in cold water, or
some snow, below the one and above the other. We are of opinion that
the extremities will grow cold in that rod first where it is placed
beneath, as the contrary takes place with regard to heat.
III. The third difference is this; that heat is not a uniform expansive
motion of the whole, but of the small particles of the body; and this
motion being at the same time restrained, repulsed, and reflected,
becomes alternating, perpetually hurrying, striving, struggling, and
irritated by the repercussion, which is the source of the violence of
flame and heat.
But this difference is chiefly shown in flame and boiling liquids,
which always hurry, swell, and subside again in detached parts.
It is also shown in bodies of such hard texture as not to swell or
dilate in bulk, such as red-hot iron, in which the heat is most violent.
It is also shown by the fires burning most briskly in the coldest
weather.
It is also shown by this, that when the air is dilated in the
thermometer uniformly and equably, without any impediment or repulsion,
the heat is not perceptible. In confined draughts also, although they
break out very violently, no remarkable heat is perceived, because
the motion affects the whole, without any alternating motion in the
particles; for which reason try whether flame do not burn more at the
sides than in its centre.
It is also shown in this, that all burning proceeds by the minute pores
of bodies--undermining, penetrating, piercing, and pricking them as if
with an infinite number of needle-points. Hence all strong acids (if
adapted to the body on which they act) exhibit the effects of fire,
from their corroding and pungent nature.
The difference of which we now speak is common also to the nature of
cold, in which the contracting motion is restrained by the resistance
of expansion, as in heat the expansive motion is restrained by the
resistance of contraction.
Whether, therefore, the particles of matter penetrate inward or
outward, the reasoning is the same, though the power be very different,
because we have nothing on earth which is intensely cold.
IV. The fourth difference is a modification of the preceding, namely,
that this stimulating or penetrating motion should be rapid and never
sluggish, and should take place not in the very minutest particles, but
rather in those of some tolerable dimensions.
It is shown by comparing the effects of fire with those of time. Time
dries, consumes, undermines, and reduces to ashes as well as fire, and
perhaps to a much finer degree; but as its motion is very slow, and
attacks very minute particles, no heat is perceived.
It is also shown in a comparison of the dissolution of iron and gold;
for gold is dissolved without the excitement of any heat, but iron with
a vehement excitement of it, although most in the same time, because in
the former the penetration of the separating acid is mild, and gently
insinuates itself, and the particles of gold yield easily, but the
penetration of iron is violent, and attended with some struggle, and
its particles are more obstinate.
It is partially shown, also, in some gangrenes and mortifications of
flesh, which do not excite great heat or pain, from the gentle nature
of the putrefaction.
Let this suffice for a first vintage, or the commencement of the
interpretation of the form of heat by the liberty of the understanding.
From this first vintage the form or true definition of heat (considered
relatively to the universe and not to the sense) is briefly thus--Heat
is an expansive motion restrained, and striving to exert itself in the
smaller particles.[106] The expansion is modified by its tendency to
rise, though expanding toward the exterior; and the effort is modified
by its not being sluggish, but active and somewhat violent.
With regard to the operative definition, the matter is the same. If you
are able to excite a dilating or expansive motion in any natural body,
and so to repress that motion and force it on itself as not to allow
the expansion to proceed equally, but only to be partially exerted and
partially repressed, you will beyond all doubt produce heat, without
any consideration as to whether the body be of earth (or elementary, as
they term it), or imbued with celestial influence, luminous or opaque,
rare or dense, locally expanded or contained within the bounds of its
first dimensions, verging to dissolution or remaining fixed, animal,
vegetable, or mineral, water, or oil, or air, or any other substance
whatever susceptible of such motion. Sensible heat is the same, but
considered relatively to the senses. Let us now proceed to further
helps.
XXI. After our tables of first review, our rejection or exclusive
table, and the first vintage derived from them, we must advance to the
remaining helps of the understanding with regard to the interpretation
of nature, and a true and perfect induction, in offering which we
will take the examples of cold and heat where tables are necessary,
but where fewer instances are required we will go through a variety
of others, so as neither to confound investigation nor to narrow our
doctrine.
In the first place, therefore, we will treat of prerogative
instances;[107] 2. Of the supports of induction; 3. Of the correction
of induction; 4. Of varying the investigation according to the
nature of the subject; 5. Of the prerogative natures with respect to
investigation, or of what should be the first or last objects of our
research; 6. Of the limits of investigation, or a synopsis of all
natures that exist in the universe; 7. Of the application to practical
purposes, or of what relates to man; 8. Of the preparations for
investigation; 9. And lastly, of the ascending and descending scale of
axioms.[108]
XXII. Among the prerogative instances we will first mention solitary
instances. Solitary instances are those which exhibit the required
nature in subjects that have nothing in common with any other subject
than the nature in question, or which do not exhibit the required
nature in subjects resembling others in every respect except that
of the nature in question; for these instances manifestly remove
prolixity, and accelerate and confirm exclusion, so that a few of them
are of as much avail as many.
For instance, let the inquiry be the nature of color. Prisms,
crystalline gems, which yield colors not only internally but on the
wall, dews, etc., are solitary instances; for they have nothing in
common with the fixed colors in flowers and colored gems, metals,
woods, etc., except the color itself. Hence we easily deduce that
color is nothing but a modification of the image of the incident and
absorbed light, occasioned in the former case by the different degrees
of incidence, in the latter by the various textures and forms of
bodies.[109] These are solitary instances as regards similitude.
Again, in the same inquiry the distinct veins of white and black in
marble, and the variegated colors of flowers of the same species, are
solitary instances; for the black and white of marble, and the spots of
white and purple in the flowers of the stock, agree in every respect
but that of color. Thence we easily deduce that color has not much to
do with the intrinsic natures of any body, but depends only on the
coarser and as it were mechanical arrangement of the parts. These are
solitary instances as regards difference. We call them both solitary or
wild, to borrow a word from the astronomers.
XXIII. In the second rank of prerogative instances we will consider
migrating instances. In these the required nature passes toward
generation, having no previous existence, or toward corruption, having
first existed. In each of these divisions, therefore, the instances
are always twofold, or rather it is one instance, first in motion or
on its passage, and then brought to the opposite conclusion. These
instances not only hasten and confirm exclusion, but also reduce
affirmation, or the form itself, to a narrow compass; for the form
must be something conferred by this migration, or, on the contrary,
removed and destroyed by it; and although all exclusion advances
affirmation, yet this takes place more directly in the same than in
different subjects; but if the form (as it is quite clear from what
has been advanced) exhibit itself in one subject, it leads to all. The
more simple the migration is, the more valuable is the instance. These
migrating instances are, moreover, very useful in practice, for since
they manifest the form, coupled with that which causes or destroys it,
they point out the right practice in some subjects, and thence there
is an easy transition to those with which they are most allied. There
is, however, a degree of danger which demands caution, namely, lest
they should refer the form too much to its efficient cause, and imbue,
or at least tinge, the understanding with a false notion of the form
from the appearance of such cause, which is never more than a vehicle
or conveyance of the form. This may easily be remedied by a proper
application of exclusion.
Let us then give an example of a migrating instance. Let whiteness be
the required nature. An instance which passes toward generation is
glass in its entire and in its powdered state, or water in its natural
state, and when agitated to froth; for glass when entire, and water in
its natural state, are transparent and not white, but powdered glass
and the froth of water are white and not transparent. We must inquire,
therefore, what has happened to the glass or water in the course of
this migration; for it is manifest that the form of whiteness is
conveyed and introduced by the bruising of the glass and the agitation
of the water; but nothing is found to have been introduced but a
diminishing of the parts of the glass and water and the insertion
of air. Yet this is no slight progress toward discovering the form
of whiteness, namely, that two bodies, in themselves more or less
transparent (as air and water, or air and glass), when brought into
contact in minute portions, exhibit whiteness from the unequal
refraction of the rays of light.
But here we must also give an example of the danger and caution of
which we spoke; for instance, it will readily occur to an understanding
perverted by efficients, that air is always necessary for producing the
form of whiteness, or that whiteness is only generated by transparent
bodies, which suppositions are both false, and proved to be so by
many exclusions; nay, it will rather appear (without any particular
regard to air or the like), that all bodies which are even in such of
their parts as affect the sight exhibit transparency, those which are
uneven and of simple texture whiteness, those which are uneven and of
compound but regular texture all the other colors except black, but
those which are uneven and of a compound irregular and confused texture
exhibit blackness. An example has been given, therefore, of an instance
migrating toward generation in the required nature of whiteness. An
instance migrating toward corruption in the same nature is that of
dissolving froth or snow, for they lose their whiteness and assume the
transparency of water in its pure state without air.
Nor should we by any means omit to state, that under migrating
instances we must comprehend not only those which pass toward
generation and destruction, but also those which pass toward increase
or decrease, for they, too, assist in the discovery of the form, as is
clear from our definition of a form and the Table of Degrees. Hence
paper, which is white when dry, is less white when moistened (from
the exclusion of air and admission of water), and tends more to
transparency. The reason is the same as in the above instances.[110]
XXIV. In the third rank of prerogative instances we will class
conspicuous instances, of which we spoke in our first vintage of the
form of heat, and which we are also wont to call coruscations, or free
and predominant instances. They are such as show the required nature
in its bare substantial shape, and at its height or greatest degree
of power, emancipated and free from all impediments, or at least
overcoming, suppressing, and restraining them by the strength of its
qualities; for since every body is susceptible of many united forms of
natures in the concrete, the consequence is that they mutually deaden,
depress, break, and confine each other, and the individual forms are
obscured. But there are some subjects in which the required nature
exists in its full vigor rather than in others, either from the absence
of any impediment, or the predominance of its quality. Such instances
are eminently conspicuous. But even in these care must be taken, and
the hastiness of the understanding checked, for whatever makes a show
of the form, and forces it forward, is to be suspected, and recourse
must be had to severe and diligent exclusion.
For example, let heat be the required nature. The thermometer is a
conspicuous instance of the expansive motion, which (as has been
observed) constitutes the chief part of the form of heat; for although
flame clearly exhibits expansion, yet from its being extinguished every
moment, it does not exhibit the progress of expansion. Boiling water
again, from its rapid conversion into vapor, does not so well exhibit
the expansion of water in its own shape, while red-hot iron and the
like are so far from showing this progress, that, on the contrary, the
expansion itself is scarcely evident to the senses, on account of its
spirit being repressed and weakened by the compact and coarse particles
which subdue and restrain it. But the thermometer strikingly exhibits
the expansion of the air as being evident and progressive, durable and
not transitory.[111]
Take another example. Let the required nature be weight. Quicksilver
is a conspicuous instance of weight; for it is far heavier than any
other substance except gold, which is not much heavier, and it is a
better instance than gold for the purpose of indicating the form of
weight; for gold is solid and consistent, which qualities must be
referred to density, but quicksilver is liquid and teeming with spirit,
yet much heavier than the diamond and other substances considered to
be most solid; whence it is shown that the form of gravity or weight
predominates only in the quantity of matter, and not in the close
fitting of it.[112]
XXV. In the fourth rank of prerogative instances we will class
clandestine instances, which we are also wont to call twilight
instances; they are as it were opposed to the conspicuous instances,
for they show the required nature in its lowest state of efficacy, and
as it were its cradle and first rudiments, making an effort and a sort
of first attempt, but concealed and subdued by a contrary nature. Such
instances are, however, of great importance in discovering forms, for
as the conspicuous tend easily to differences, so do the clandestine
best lead to genera, that is, to those common natures of which the
required natures are only the limits.
As an example, let consistency, or that which confines itself, be
the required nature, the opposite of which is a liquid or flowing
state. The clandestine instances are such as exhibit some weak and low
degree of consistency in fluids, as a water bubble, which is a sort
of consistent and bounded pellicle formed out of the substance of the
water. So eaves’ droppings, if there be enough water to follow them,
draw themselves out into a thin thread, not to break the continuity
of the water, but if there be not enough to follow, the water forms
itself into a round drop, which is the best form to prevent a breach
of continuity; and at the moment the thread ceases, and the water
begins to fall in drops, the thread of water recoils upward to avoid
such a breach. Nay, in metals, which when melted are liquid but more
tenacious, the melted drops often recoil and are suspended. There is
something similar in the instance of the child’s looking-glass, which
little boys will sometimes form of spittle between rushes, and where
the same pellicle of water is observable; and still more in that other
amusement of children, when they take some water rendered a little
more tenacious by soap, and inflate it with a pipe, forming the water
into a sort of castle of bubbles, which assumes such consistency, by
the interposition of the air, as to admit of being thrown some little
distance without bursting. The best example is that of froth and snow,
which assume such consistency as almost to admit of being cut, although
composed of air and water, both liquids. All these circumstances
clearly show that the terms liquid and consistent are merely vulgar
notions adapted to the sense, and that in reality all bodies have a
tendency to avoid a breach of continuity, faint and weak in bodies
composed of homogeneous parts (as is the case with liquids), but more
vivid and powerful in those composed of heterogeneous parts, because
the approach of heterogeneous matter binds bodies together, while the
insinuation of homogeneous matter loosens and relaxes them.
Again, to take another example, let the required nature be attraction
or the cohesion of bodies. The most remarkable conspicuous instance
with regard to its form is the magnet. The contrary nature to
attraction is non-attraction, though in a similar substance. Thus
iron does not attract iron, lead lead, wood wood, nor water water.
But the clandestine instance is that of the magnet armed with iron,
or rather that of iron in the magnet so armed. For its nature is such
that the magnet when armed does not attract iron more powerfully at
any given distance than when unarmed; but if the iron be brought in
contact with the armed magnet, the latter will sustain a much greater
weight than the simple magnet, from the resemblance of substance in the
two portions of iron, a quality altogether clandestine and hidden in
the iron until the magnet was introduced. It is manifest, therefore,
that the form of cohesion is something which is vivid and robust in
the magnet, and hidden and weak in the iron. It is to be observed,
also, that small wooden arrows without an iron point, when discharged
from large mortars, penetrate further into wooden substances (such
as the ribs of ships or the like), than the same arrows pointed with
iron,[113] owing to the similarity of substance, though this quality
was previously latent in the wood. Again, although in the mass air does
not appear to attract air, nor water water, yet when one bubble is
brought near another, they are both more readily dissolved, from the
tendency to contact of the water with the water, and the air with the
air.[114] These clandestine instances (which are, as has been observed,
of the most important service) are principally to be observed in small
portions of bodies, for the larger masses observe more universal and
general forms, as will be mentioned in its proper place.[115]