Francis Bacon (1561-1626)
(1620) excerpt on the nature of heat Second Book [from Great Books of the Western
11. The investigation of forms proceeds thus: a nature being given, we must first
present to the understanding all the known instances which agree in the same nature,
although the subject matter be considerably diversified. And this collection must be made
as a mere history, and without any premature reflection, or too great degree of
refinement. For instance; take the investigation of the form of heat.
Table I. Instances agreeing in the Form of Heat.
- The rays of the sun, particularly in summer, and at noon.
- The same reflected and condensed, as between mountains, or along walls, and particularly
in burning mirrors.
- Ignited meteors.
- Burning lightning
- Eruptions of flames from the cavities of mountains, etc.
- Flame of every kind.
- Ignited solids.
- Natural warm water baths.
- Warm or heated liquids.
- Warm vapors and smoke; and the air itself, which admits a most powerful and violent heat
if confined, as in reverberating furnaces.
- Damp hot weather, arising from the constitution of the air, without any reference to the
time of the year.
- Confined and subterraneous air in some caverns, particularly in winter.
- All shaggy substances, as wool, the skins of animals, and the plumage of birds, contain
- All bodies, both solid and liquid, dense and rare (as the air itself), placed near fire
for any time.
- Sparks arising from the violent percussion of flint and steel.
- All bodies rubbed violently, as stone, wood, cloth, etc., so that rudders, and axles of
wheels, sometimes catch fire, and the West Indians obtain fire by attrition.
- Green and moist vegetable matter confined and rubbed together, as roses, peas in
baskets; so hay, if it be damp when stacked, often catches fire.
- Quick lime sprinkled with water.
- Iron, when first dissolved by acids in a glass, and without any application to fire; the
same of tin, but not so intensely.
- Animals, particularly internally; although the heat is not perceivable by the touch in
insects, on account of their small size.
- Horse dung, and the like excrement from other animals, when fresh.
- Strong oil of sulphur and of vitriol exhibit the operation of heat in burning linen.
- As does the oil of marjoram, and like substances, in burning the bony substance of the
- Strong and well rectified spirits of wine exhibit the same effects; so that white of
eggs when thrown into it grows hard and white, almost in the same manner as when boiled,
and bread becomes burnt and brown as if toasted.
- Aromatic substances and warm plants, as the dracunculus [arum], old nasturtium, etc.,
which, though they be not warm to the touch (whether whole or pulverized), yet are
discovered by the tongue and palate to be warm and almost burning when slightly
- Strong vinegar and all acids, on any part of the body not clothed with the epidermis, as
the eye, tongue, or any wounded part, or where the skin is removed, excite a pain
differing but little from that produced by heat.
- Even a severe and intense cold produces a sensation of burning.
Nec Boreae penetrabile frigus adurit. --Virgil.
- Other instances.
We are wont to call this a Table of existence and presence.
12. We must next present to the understanding instances which do not admit of the given
nature, for form (as we have observed) ought no less to be absent where the given nature
is absent, than to be present where it is present. If, however, we were to examine every
instance, our labor would be infinite.
Negatives, therefore, must be classed under the affirmatives, and the want of the given
nature must be inquired into more particularly in objects which have a very close
connection with those others in which it is present and manifest. And this we are wont to
term a table of deviation or of absence in proximity.
Table II. Proximate Instances wanting the Nature of Heat.
- The rays of the moon, stars, and comets, are not found to be warm to the touch, nay the
severest cold has been observed to take place at the full of the moon. Yet the larger
fixed stars are supposed to increase and render more intense the heat of the sun, as he
approaches them, when the sun is in the sign of the Lion for instance, and in the
- The rays of the sun in what is called the middle region of the air give no heat, to
account for which the commonly assigned reason is satisfactory; namely, that that region
is neither sufficiently near to the body of the sun whence the rays emanate, nor to the
earth whence they are reflected. And the fact is manifested by snow being perpetual on the
tops of mountains, unless extremely lofty. But it is observed on the other hand by some,
that at Peak of Teneriffe, and also among the Andes of Peru, the tops of the mountains are
free from snow, which only lies in the lower part as you ascend. Besides, the air on the
summit of these mountains is found to be by no means cold, but only thin and sharp; so
much so, that in the Andes it pricks and hurts the eyes from its extreme sharpness, and
even excites the orifice of the stomach and produces vomiting. The ancients also observed,
that the rarity of the air on the summit of Olympus was such, that those who ascended it
were obliged to carry sponges moistened with vinegar and water, and to apply them now and
then to their nostrils, as the air was not dense enough for their respiration; on the
summit of which mountain it is also related, there reigned so great a serenity and calm,
free from rain, snow, or wind, that the letters traced upon the ashes of the sacrifices on
the altar of Jupiter, by the fingers of those who had offered them, would remain
undisturbed till the next year. Those even, who at this day go to the top of the Peak of
Teneriffe, walk by night and not in the day-time, and are advised and pressed by their
guides, as soon as the sun rises, to make haste in their descent, on account of the danger
(apparently arising from the rarity of the atmosphere) lest their breathing should be
relaxed and suffocated.
- The reflection of the solar rays in the polar regions is found to be weak and
inefficient in producing heat, so that the Dutch, who wintered in Nova Zembla, and
expected that their vessel would be freed about the beginning of July from the obstruction
of the mass of ice which had blocked it up, were disappointed and obliged to embark in
their boat. Hence the direct rays of the sun appear to have but little power even on the
plain, and when reflected, unless they are multiplied and condensed, which takes place
when the sun tends more to the perpendicular; for, then, the incidence of the rays occurs
at more acute angles, so that the reflected rays are nearer to each other, whilst, on the
contrary, when the sun is in a very oblique position, the angles of incidence are very
obtuse, and the reflected rays at a greater distance. In the mean time it must be
observed, that there may be many operations of the solar rays, relating, too, to the
nature of heat, which are not proportioned to our touch, so that, with regard to us, they
do not tend to produce warmth, but, with regard to some other bodies, have their due
effect in producing it.
- Let the following experiment be made. Take a lens the reverse of a burning glass, and
place it between the hand and the solar rays, and observe whether it diminish the heat of
the sun as a burning glass increases it. For it is clear, with regard to the visual rays,
that in proportion as the lens is made of unequal thickness in the middle and at its
sides, the images appear either more diffused or contracted. It should be seen, therefore,
if the same be true with regard to heat.
- Let the experiment be well tried, whether the lunar rays can be received and collected
by the strongest and best burning-glasses, so as to produce even the least degree of heat.
But if that degree be, perhaps, so subtile and weak, as not to be perceived and
ascertained by the touch, we must have recorse [sic] to those glasses which indicate the
warm or cold state of the atmosphere, and let the lunar rays fall through the
burning-glass on the top of this thermometer, and then notice if the water be depressed by
- Let the burning-glass be tried on warm objects which emit no luminous rays, as heated
but not ignited iron or stone, or hot water, or the like; and observe whether the heat
become increased and condensed, as happens with the solar rays.
- Let it be tried on common flame.
- The effect of comets (if we can reckon them amongst meteors) in augmenting the heat of
the season is not found to be constant or clear, although droughts have generally been
observed to follow them. However, luminous lines, and pillars, and openings, and the like,
appear more often in winter than in summer, and especially with the most intense cold but
joined with drought. Lightning, and coruscations, and thunder, however, rarely happen in
winter, and generally at the time of the greatest heats. The appearances we term falling
stars are generally supposed to consist of some shining and inflamed viscous substance,
rather than of violently hot matter; but let this be further investigated.
- Some coruscations emit light without burning, but are never accompanied by thunder.
- Eructations and eruptions of flame are to be found in cold climates as well as in hot,
as in Iceland and Greenland; just as the trees of cold countries are sometimes inflammable
and more pitchy and resinous than in warm, as the fir, pine, and the like. But the
position and nature of the soil, where such eruptions are wont to happen, are not yet
sufficiently investigated to enable us to subjoin a negative instance to the affirmative.
- All flame is constantly more or less warm, and this instance is not altogether negative;
yet it is said that the ignis fatuus (as it is called), and which sometimes is driven
against walls, has but little heat; perhaps it resembles that of spirits of wine, which is
mild and gentle. That flame, however, appears yet milder, which in some well authenticated
and serious histories is said to have appeared round the head and hair of boys and
virgins, and instead of burning their hair, merely to have played about it. And it is most
certain that a sort of flash, without any evident heat, has sometimes been seen about a
horse when sweating at night, or in damp weather. It is also a well-known fact, and it was
almost considered as a miracle, that a few years since a girl's apron sparkled when a
little shaken or rubbed, which as, perhaps, occasioned by the alum or salts after having
been struck together and incrusted rather strongly were broken by the friction. It is well
known that all sugar, whether candied or plain, if it be hard, will sparkle when broken or
scraped in the dark. In like manner sea and salt water is sometimes found to shine at
night when struck violently by the oar. The foam of the sea when agitated by tempests also
sparkles at night, and the Spaniards call this appearance the sea's lungs. It has not been
sufficiently ascertained what degree of heat attends the flame which the ancient sailors
called Castor and Pollux, and the moderns call St. Ermu's [sic] fire.
- Every ignited body that is red-hot is always warm, although without flame, nor is any
negative instance subjoined to this affirmative. Rotten wood, however, approaches nearly
to it, for it shines at night, and yet is not found to be warm; and the putrefying scales
of fish which shine in the same manner are not warm to the touch, nor the body of the
glowworm, or of the fly called Luciola.
- The situation and nature of the soil of natural warm baths have not been sufficiently
investigated, and therefore a negative instance is not subjoined.
- To the instances of warm liquids we may subjoin the negative one of the peculiar nature
of liquids in general; for no tangible liquid is known that is at once warm in its nature
and constantly continues warm; but their heat is only superinduced as an adventitious
nature for a limited time, so that those which are extremely warm in their power and
effect, as spirits of wine, chemical aromatic oils, the oils of vitriol and sulphur, and
the like, and which speedily burn, are yet cold at first to the touch, and the water of
natural baths, poured into any vessel and separated from its source, cools down like water
heated by the fire. It is, however, true that oily substances are rather less cold to the
touch than those that are aqueous, oil for instance than water, silk than linen; but this
belongs to the table of degrees of cold.
- In like manner we may subjoin a negative instance to that of warm vapor, derived from
the nature of the vapor itself, as far as we are acquainted with it. For exhalations from
oily substances, though easily inflammable, are yet never warm unless recently exhaled
from some warm substance.
- The same may be said of the instance of air; for we never perceive that air is warm
unless confined or pressed, or manifestly heated by the sun, by fire, or some other warm
- A negative instance in exhibited in weather by its coldness with an east or north wind,
beyond what the season would lead us to expect, just as the contrary takes place with the
south or west winds. An inclination to rain (especially in winter) attends warm weather,
and to frost cold weather.
- A negative instance as to air confined in caverns may be observed in summer. Indeed, we
should make a more diligent inquiry into the nature of confined air. For in the first
place the qualities of air in its own nature with regard to heat and cold may reasonably
be the subject of doubt; for air evidently derives its heat from the effects of celestial
bodies, and possibly its cold from the exhalation of the earth, and in the mid region of
air (as it is termed) from cold vapors and snow, so that no judgment can be formed of the
nature of air by that which is out of doors and exposed, but a more correct one might be
derived from confined air. It is necessary, however, that the air should be enclosed in a
vessel of such materials as would not imbue it with heat or cold of itself, nor easily
admit the influence of the external atmosphere. The experiment should be made therefore
with an earthen jar, covered with folds of leather to protect it from the external air,
and the air should be kept three or four days in this vessel well closed. On opening the
jar, the degree of heat may be ascertained either by the hand or a graduated glass tube.
- There is a similar doubt as to whether the warmth of wool, skins, feathers, and the
like, is derived from a slight inherent heat, since they are animal excretions, or from
their being of a certain fat and oily nature that accords with heat, or merely from the
confinement and separation of air which we spoke of in the preceding paragraph; for all
air appears to possess a certain degree of warmth when separated from the external
atmosphere. Let an experiment be made, therefore, with fibrous substances of linen, and
not of wool, feathers, or silk, which are animal excretions. For it is to be observed that
all powders (where air is manifestly enclosed) are less cold than the substances when
whole, just as we imagine froth (which contains air) to be less cold than the liquid
- We have here no exactly negative instance, for we are not acquainted with any body
tangible or spiritous which does not admit of heat when exposed to the fire. There is,
however, this difference, that some admit it more rapidly, as air, oil, and water; others
more slowly, as stone and metals. This, however, belongs to the table of degrees.
- No negative is here subjoined, except the remark that sparks are not kindled by flint
and steel, or any other hard substance, unless some small particles of the stone or metal
are struck off, and that the air never forms them by friction, as is commonly supposed;
besides, the sparks from the weight of the ignited substance have a tendency to descend
rather than to rise, and when extinguished become a sort of dark ash.
- We are of opinion that here again there is no negative; for we are not acquainted with
any tangible body which does not become decidedly warm by friction, so that the ancients
feigned that the gods had no other means or power of creating heat than the friction of
air, by rapid and violent rotation. On this point, however, further inquiry must be made,
whether bodies projected by machines (as balls from cannon) do not derive some degree of
heat from meeting the air, which renders them somewhat warm when they fall. The air in
motion rather cools than heats, as in the winds, the bellows, or breath when the mouth is
contracted. The motion, however, in such instances is not sufficiently rapid to excite
heat, and is applied to a body of air, and not to its component parts, so that it is not
surprising that heat should not be generated.
- We must make a more diligent inquiry into this instance; for herbs and green and moist
vegetables appear to possess a latent heat so small, however, as not to be perceived by
the touch in single specimens, but when they are united and confined, so that their spirit
cannot exhale into the air, and they rather warm each other, their heat is at once
manifested, and even flame occasionally in suitable substances.
- Here, too, we must make a more diligent inquiry; for quick lime, when sprinkled with
water, appears to conceive heat, either from its being collected into one point (as we
observed of herbs when confined), or from the irritation and exasperation of the fiery
spirit by water which occasions a conflict and struggle. The true reason will more readily
be shown if oil be used instead of water, for oil will equally tend to collect the
confined spirit, but not to irritate. The experiment may be made more general both by
using the ashes and calcined products of different bodies and by pouring different liquids
- A negative instance may be subjoined of other metals which are more soft and soluble;
for leaf gold dissolved by aqua regia, or lead by aqua fortis, are not warm to the touch
whilst dissolving, no more is quicksilver (as far as I remember), but silver excites a
slight heat, and so does copper, and tin yet more plainly, and most of all iron and steel,
which excite not only a powerful heat, but a violent bubbling. The heat, therefore,
appears to be occasioned by the struggle which takes place when these strong dissolvents
penetrate, dig into, and tear asunder the parts of those substances, whilst the substances
themselves resist. When, however, the substances yield more easily, scarcely any heat is
- There is no negative instance with regard to the heat of animals, except in insects (as
has been observed), owing to their small size; for in fishes, as compared with land
animals, a lower degree rather than a deprivation of heat is observable. In plants and
vegetables, both as to their exudations, and pith when freshly exposed, there is no
sensible degree of heat. But in animals there is a great difference in the degree, both in
particular parts (for the heat varies near the heart, the brain, and the extremities) and
in the circumstances in which they are placed, such as violent exercise and fevers.
- Here, again, there is scarcely a negative instance. I might add that the excrements of
animals, even when they are no longer fresh, possess evidently some effective heat, as is
shown by their enriching the soil.
- Such liquids (whether oily or watery) as are intensely acrid exhibit the effects of
heat, by the separation and burning of bodies after some little action upon them, yet they
are not at first warm to the touch, but they act according to their affinity and the pores
of the substances to which they are applied; for aqua regia dissolves gold but not
silver--on the contrary, aqua fortis dissolves silver but not gold; neither of them
dissolves glass, and so of the rest.
- Let spirits of wine be tried on wood, or butter, wax, or pitch, to see if this will melt
them at all by their heat; for the twenty-fourth instance shows that they possess
properties resembling those of heat in causing incrustation. Let an experiment also be
made with a graduated glass or colander, concave at the top, by pouring well-rectified
spirits of wine into the cavity, and covering it up in order that they may the better
retain their heat, then observe whether their heat make the water descend.
- Spices and acrid herbs are sensibly warm to the palate, and still more so when taken
internally; one should see, therefore, on what other substances they exhibit the effects
of heat. Now, sailors tell us that when large quantities of spices are suddenly opened,
after having been shut up for some time, there is some danger of fever and inflammation to
those who stir them or take them out. An experiment might, therefore, be made whether such
spices and herbs, when produced, will, like smoke, dry fish and meat hung up over them.
- There is an acrid effect and a degree of penetration in cold liquids, such as vinegar
and oil of vitriol, as well as in warm, such as oil of marjoram and the like; they have,
therefore, an equal effect in causing animated substances to smart, and separating and
consuming inanimate parts. There is not any negative instance as to this, nor does there
exist any animal pain unaccompanied by the sensation of heat.
- There are many effects common to cold and heat, however, different in their process; for
snowballs appear to burn boys' hands after a little time, and cold no less than fire
preserves bodies from putrefaction--besides both heat and cold contract bodies. But it is
better to refer these instances and the like to the investigation of cold.
13. In the third place we must exhibit to the understanding the instances in which that
nature, which is the object of our inquiries, is present in a greater or less degree,
either by comparing its increase and decrease in the same object, or its degree in
different objects; for since the form of a thing is its very essence, and the thing only
differs from its form as the apparent from the actual object, or the exterior from the
interior, or that which is considered with relation to man from that which is considered
with relation to the universe; it necessarily follows that no nature can be considered a
real form which does not uniformly diminish and increase with the given nature. We are
wont to call this our Table of degrees or comparative instances.
Table III. Degrees or Comparative Instances of Heat.
We will first speak of those bodies which exhibit no degree of heat sensible to the
touch, but appear rather to possess a potential heat, or disposition and preparation for
it. We will then go on to others, which are actually warm to the touch, and observe the
strength and degree of it.
- There is no known solid or tangible body which is by its own nature originally warm; for
neither stone, metal, sulphur, fossils, wood, water, nor dead animal carcasses are found
warm. The warm springs in baths appear to be heated accidentally, by flame, subterraneous
fire (such as is thrown up by Etna and many other mountains), or by the contact of certain
bodies, as heat is exhibited in the dissolution of iron and tin. The degree of heat,
therefore, in inanimate objects is not sensible to our touch; but they differ in their
degrees of cold, for wood and metal are not equally cold. This, however, belongs to the
table of degrees of cold.
- But with regard to potential heat and predisposition to flame, we find many inanimate
substances wonderfully adapted to it, as sulphur, naphtha, and saltpetre.
- Bodies which have previously acquired heat, as horse-dung from the animal, or lime, and
perhaps ashes and soot from fire, retain some latent portion of it. Hence distillations
and separations of substances are effected by burying them in horse-dung, and heat is
excited in lime by sprinkling it with water (as has been before observed).
- In the vegetable world we know of no plant, nor part of any plant (as the exudations or
pith) that is warm to man's touch. Yet (as we have before observed) green weeks grow warm
when confined, and some vegetables are warm and others cold to our internal touch, i.e.,
the palate and stomach, or even after a while to our external skin (as is shown in
plasters and ointments).
- We know of nothing in the various parts of animals, when dead or detached from the rest,
that is warm to the touch; for horse-dung itself does not retain its heat, unless it be
confined and buried. All dung, however, appears to possess a potential heat, as in
manuring fields; so also dead bodies are endued with this latent and potential heat to
such a degree, that in cemeteries where people are interred daily the earth acquires a
secret heat, which consumes any recently-deposited body much sooner than pure earth; and
they tell you that the people of the East are acquainted with a fine soft cloth, made of
the down of birds, which can melt butter wrapped gently up in it by its own warmth.
- Manures, such as every kind of dung, chalk, sea-sand, salt, and the like, have some
disposition towards heat.
- All putrefaction exhibits some slight degree of heat, though not enough to be
perceptible by the touch; for neither the substances which by putrefaction are converted
into animalculae, as flesh and cheese, nor rotten wood which shines in the dark, are warm
to the touch. The heat, however, of putrid substances displays itself occasionally in a
disgusting and strong scent.
- The first degree of heat, therefore, in substances which are warm to the human touch
appears to be that of animals, and this admits of a great variety of degrees, for the
lowest (as in insects) is scarcely perceptible, the highest scarcely equals that of the
sun's rays in warm climates and weather, and is not so acute as to be insufferable to the
hand. It is said, however, of Constantius, and some others of a very dry constitution and
habit of body, that when attacked with violent fevers, they became so warm as to appear
almost to burn the hand applied to them.
- Animals become more warm by motion and exercise, wine and feasting, venery, burning
fevers, and grief.
- In the paroxysm of intermittent fevers the patients are at first seized with cold and
shivering, but soon afterwards become more heated than at first--in burning and
pestilential fevers they are hot from the beginning.
- Let further inquiry be made into the comparative heat of different animals, as fishes,
quadrupeds, serpents, birds, and also of the different species, as the lion, the kite, or
man; for, according to the vulgar opinion, fishes are the least warm internally, and birds
the most, particularly doves, hawks, and ostriches.
- Let further inquiry be made as to the comparative heat in different parts and limbs of
the same animal; for milk, blood, seed, and eggs are moderately warm, and less hot than
the outward flesh of the animal when in motion or agitated. The degree of heat of the
brain, stomach, heart, and the rest, has not yet been equally well investigated.
- All animals are externally cold in winter and cold weather, but are thought to be
- The heat of the heavenly bodies, even in the warmest climates and seasons, never reaches
such a pitch as to light or burn the driest wood or straw, or even tinder without the aid
of burning-glasses. It can, however, raise vapor from moist substances.
- Astronomers tell us that some stars are hotter than others. Mars is considered the
warmest after the Sun, then Jupiter, then Venus. The Moon and, above all, Saturn, are
considered to be cold. Among the fixed stars Sirius is thought the warmest, then Cor
Leonis or Regulus, then the lesser Dog-star.
- The sun gives out more heat as it approaches towards the perpendicular or zenith, which
may be supposed to be the case with the other planets, according to their degree of heat;
for instance, that Jupiter gives out more heat when situated beneath Cancer or Leo than
when he is beneath Capricorn and Aquarius.
- It is to be supposed that the sun and other planets give more heat in perigee, from
their approximation to the earth, than when in apogee. But if in any country the sun
should be both in its perigee and nearer to the perpendicular at the same time, it must
necessarily give out more heat than in a country where it is also in perigee, but situated
more obliquely; so that the comparative altitude of the planets should be observed, and
their approach to or declination from the perpendicular in different countries.
- The sun and other planets are thought also to give out more heat in proportion as they
are nearer to the larger fixed stars, as when the sun is in Leo he his nearer Cor Leonis,
Cauda Leonis, Spica Virginis, Sirius, and the lesser Dog-star, than when he is in Cancer,
where, however, he approaches nearer the perpendicular. It is probable, also, that the
quarters of the heavens produce a greater heat (though not perceptibly), in proportion as
they are adorned with a greater number of stars, particularly those of the first
- On the whole, the heat of the heavenly bodies is augmented in three ways. 1. The
approach to the perpendicular; 2. Proximity or their perigee; 3. The conjunction or union
- There is a very considerable difference between the degree of heat in animals, and even
in the rays of the heavenly bodies (as they reach us), and the heat of the most gentle
flame, and even of all ignited substances, nay liquids, or the air itself when unusually
heated by fire. For the flame of spirit of wine, though diffused and uncollected, is yet
able to set straw, linen, or paper on fire, which animal heat, or that of the sun, will
never accomplish without a burning-glass.
- There are, however, many degrees of strength and weakness in flame and ignited bodies;
but no diligent inquiry has been made in this respect, and we must, therefore, pass it
hastily over. Of all flames, that of spirits of wine appears to be the most gentle,
except, perhaps, the Ignis Fatuus, or the flashes from the perspiration of animals.
After this we should be inclined to place the flame of light and porous vegetables, such
as straw, reeds, and dried leaves; from which the flame of hair or feathers differs but
little. Then, perhaps, comes the flame of wood, particularly that which contain but little
rosin or pitch; that of small wood, however, (such as is usually tied up in fagots), is
milder than that of the trunks or roots of trees. This can be easily tried in iron
furnaces, where a fire of fagots or branches of trees is of little service. Next follows
the flame of oil, tallow, wax, and the like oily and fat substances, which are not very
violent. But a most powerful heat is found in pitch and rosin, and a still greater in
sulphur, camphor, naphtha, saltpetre, and salts (after they have discharged their crude
matter), and in their compounds; as in gunpowder, Greekfire (vulgarly called wildfire),
and its varieties, which possess such a stubborn heat as scarcely to be extinguished by
- We consider that the flame which results from some imperfect metals is very strong and
active; but on all these points further inquiry should be made.
- The flame of vivid lightning appears to exceed all the above, so as sometimes to have
melted even wrought iron into drops, which the other flames cannot accomplish.
- In ignited bodies there are different degrees of heat, concerning which, also, a
diligent inquiry has not been made. We consider the faintest heat to be that of tinder,
touchwood, and dry match-rope, such as is used for discharging cannon. Next follows that
of ignited charcoal or cinders, and even bricks, and the like; but the most violent is
that of ignited metals, as iron, copper, and the like. Further inquiry, however, must be
made into this also.
- Some ignited bodies are found to be much warmer than some flames; for instance, red hot
iron is much warmer, and burns more than the flame of spirits of wine.
- Some bodies even not ignited, but only heated by the fire, as boiling water, and the air
confined in reverberatories, surpass in heat many flames and ignited substances.
- Motion increases heat, as is shown in the bellows and the blow-pipe; for the harder
metals are not dissolved or melted by steady quiet fire, without the aid of the blow-pipe.
- Let an experiment be made with burning-glasses; in which respect I have observed, that
if a glass be placed at the distance of ten inches, for instance, from the combustible
object, it does not kindle or burn it so readily, as if the glass be placed at the
distance of five inches (for instance), and be then gradually and slowly withdrawn to the
distance of ten inches. The cone and focus of the rays, however, are the same, but the
mere mention increases the effect of the heat.
- Conflagrations, which take place with a high wind, are thought to make greater way
against than with the wind, because when the wind slackens, the lame recoils more rapidly
than it advances when the wind is favorable.
- Flame does not burst out or arise unless it have some hollow space to move and exert
itself in, except in the exploding flame of gunpowder, and the like, where the compression
and confinement of the flame increase its fury.
- The anvil becomes so hot by the hammer, that if it were a thin plate it might probably
grow red, like ignited iron by repeated strokes. Let the experiment be tried.
- But in ignited bodies that are porous, so as to leave room for the fire to move itself,
if its motion be prevented by strong compression, the fire is immediately extinguished;
thus it is with tinder, or the burning snuff of a candle or lamp, or even hot charcoal or
cinders; for when they are squeezed by snuffers, or the foot, and the like, the effect of
the fire instantly ceases.
- The approach towards a hot body increases heat in proportion to the approximation; a
similar effect to that of light, for the nearer any object is placed towards the light,
for more visible it becomes.
- The union of different heats increases heat, unless the substances be mixed; for a large
and small fire in the same spot tend mutually to increase each other's heat, but lukewarm
water poured into boiling water cools it.
- The continued neighborhood of a warm body increases heat. For the heat, which
perpetually passes and emanates from it, being mixed with that which preceded it,
multiplies the whole. A fire, for instance, does not warm a room in half an hour as much
as the same fire would in an hour. This does not apply to light, for a lamp or candle
placed in a spot gives no more light by remaining there, than it did at first.
- The irritation of surrounding cold increases heat, as may be seen in fires during a
sharp frost. We think that this is owing not merely to the confinement and compression of
the heat (which forms a sort of union), but also by the exasperation of it, as when the
air or a stick is violently compressed or bent, it recoils, not only to the point it first
occupied, but still farther back. Let an accurate experiment, therefore, be made with a
stick, or something of the kind, put into the flame, in order to see whether it be not
sooner burnt at the sides than in the middle of it.
- There are many degrees in the susceptibility of heat. And, first, it must be observed
how much a low gentle heat changes and partially warms even the bodies least susceptible
of it. For even the heat of the hand imparts a little warmth to a ball of lead or other
metal held a short time in it; so easily is heat transmitted and excited, without any
apparent change in the body.
- Of all bodies that we are acquainted with, air admits and loses heat the most readily,
which is admirably seen in weather-glasses, whose construction is as follows. Take a glass
with a hollow belly, and a thin and long neck; turn it upside down, and place it with its
mouth downwards into another glass vessel containing water; the end of the tube touching
the bottom of the vessel, and the tube itself leaning a little on the edge, so as to be
fixed upright. In order to do this more readily, let a little wax be applied to the edge,
not however so as to block up the orifice, lest by preventing the air from escaping, the
motion, which we shall presently speak of, and which is very gentle and delicate, should
be impeded. Before the first glass be inserted in the other, its upper part (the belly)
should be warmed at the fire. Then upon placing it as we have described, the air (which
was dilated by the heat), after a sufficient time has been allowed for it to lose the
additional temperature, will restore and contract itself to the same dimensions as that of
the external or common atmosphere at the moment of immersion, and the water will be
attracted upwards in the tube to a proportionate extent. A long narrow slip of paper
should be attached to the tube, divided into as many degrees as you please. You will then
perceive, as the weather grows warmer or colder, that the air contracts itself into a
narrower space in cold weather and dilates in the warm, which will be exhibited by the
rising of the water as the air contracts itself, and its depression as the air dilates.
The sensibility of the air with regard to heat or cold is so delicate and exquisite, that
it far exceeds the human touch, so that a ray of sunshine, the heat of the breath, and
much more, that of the hand laced on the top of the tube, immediately causes an evident
depression of the water. We think, however, that the spirit of animals possesses a much
more delicate susceptibility of heat and cold, only that it is impeded and blunted by the
grossness of their bodies.
- After air, we consider those bodies to be most sensible of heat, which have been
recently changed and contracted by cold, as snow and ice; for they begin to be dissolved
and melt with the first mild weather. Next, perhaps, follows quicksilver; then greasy
substances, as oil, butter, and the like; then wood; then water; lastly, stones and
metals, which do not easily grow hot, particularly towards their centre. When heated,
however, they retain their temperature for a very long time; so that a brick or stone, or
hot iron, plunged in a basin of cold water, and kept there for a quarter of an hour or
thereabouts, retains such a heat as not to admit of being touched.
- The less massive the body is, the more readily it grows warm at the approach of a heated
body, which shows that heat with us is somewhat averse to a tangible mass.
- Heat with regard to the human senses and touch is various and relative, so that lukewarm
water appears hot if the hand be cold, and cold if the hand be hot.
14. Anyone may readily see how poor we are in history, since in the above tables,
besides occasionally inserting traditions and report instead of approved history and
authentic instances (always, however, adding some note if their credit or authority be
doubtful), we are often forced to subjoin, "Let the experiment be tried. Let further
inquiry be made."
15. We are wont to term the office and use of these three tables the presenting a
review of instances to the understanding; and when this has been done, induction itself is
to be brought into action. For on an individual review of all the instances a nature is to
be found, such as always to be present and absent with the given nature, to increase and
decrease with it, and, as we have said, to form a more common limit of the nature. If the
mind attempt this affirmatively from the first (which it always will when left to itself),
there will spring up phantoms, mere theories and ill-defined notions, with axioms
requiring daily correction. These will, doubtless, be better or worse, according to the
power and strength of the understanding which creates them. But it is only for God (the
bestower and creator of forms), and perhaps for angels and intelligences, at once to
recognize forms affirmatively at the first glance of contemplating: man, at least, is
unable to do so, and is only allowed to proceed first by negatives, and then to conclude
with affirmatives, after every species of exclusion.
16. We must, therefore, effect a complete solution and separation of nature; not by
fire, but by the mind, that divine fire. The first work of legitimate induction, in the
discovery of forms, is rejection, or the exclusive instances of individual natures, which
are not found in some one instance where the given nature is present, or are found in any
one instance where it is absent, or are found to increase in any one instance where the
given nature decreases, or the reverse. After an exclusion correctly affected, an
affirmative form will remain as the residuum, solid, true, and well defined, whilst all
volatile opinions go off in smoke. This is readily said; but we must arrive at it by a
circuitous route. We shall perhaps, however, omit nothing that can facilitate our
17. The first and almost perpetual precaution and warning which we consider necessary
is this; that none should suppose from the great part assigned by us to forms, that we
mean such forms as the meditations and thoughts of men have hitherto been accustomed to.
In the first place, we do not at present mean the concrete forms, which (as we have
observed) are in the common course of things compounded of simple natures, as those of a
lion, an eagle, a rose, gold, or the like. The moment for discussing these will arrive
when we come to treat of the latent process and latent conformation, and the discovery of
them as they exist in what are called substances, or concrete natures.
Nor again, would we be thought to mean (even when treating of simple natures) any
abstract forms or ideas either undefined or badly defined in matter. For when we speak of
forms, we mean nothing else than those laws and regulations of simple action which arrange
and constitute any simple nature, such as heat, light, weight, in every species of matter,
and in a susceptible subject. The form of heat or form of light, therefore, means no more
than the law of heat or the law of light. Nor do we ever abstract or withdraw ourselves
from things, and the operative branch of philosophy. When therefore, we say (for instance)
in our investigation of the form of heat, "Reject rarity," or, "Rarity is
not of the form of heat," it is the same as if we were to say, "Man can abstract
or ward off heat from a rare body." But if our forms appear to any one to be somewhat
abstracted, from their mingling and uniting heterogeneous objects (the heat, for instance,
of the heavenly bodies appears to be very different from that of fire; the fixed red of
the rose and the like, from that which is apparent in the rainbow, or the radiation of
opal or the diamond; death by drowning, from that by burning, the sword, apoplexy, or
consumption; and yet they all agree in the common natures of heat, redness, and death),
let him be assured that his understanding is enthralled by habit, by general appearances
and hypotheses. For it is most certain that, however heterogeneous and distinct, they
agree in the form or law which regulates heat, redness, or death; and that human power
cannot be emancipated and freed from the common course of nature, and expanded and exalted
to new efficients and new modes of operation, except by this revelation and invention of
forms of this nature. But after this union of nature, which is the principal point, we
will afterwards, in its proper place, treat of the divisions and ramifications of nature,
whether ordinary or internal and more real.
18. We must now offer an example of the exclusion or rejection of natures found by the
tables of review, not to be of the form of heat; first premising that not only each table
is sufficient for the rejection of any nature, but even each single instance contained in
them. For it is clear from what has been said that every contradictory instance destroys
an hypothesis as to the form. Still, however, for the sake of clearness, and in order show
more plainly the use of the tables, we redouble or repeat the exclusive.
Table IV. An Example of the Exclusive Table, or of the Rejection of Natures from the
Form of Heat
- On account of the sun's rays, reject elementary (or terrestrial) nature.
- On account of common fire, and particularly subterranean fires (which are the most
remote and secluded from the rays of the heavenly bodies), reject celestial nature.
- On account of the heat acquired by every description of substances (as minerals,
vegetables, the external parts of animals, water, oil, air, etc.) by mere approximation to
the fire or any warm body, reject all variety and delicate texture of bodies.
- On account of iron and ignited metals, which warm other bodies, and yet neither lose
their weight nor substance, reject the imparting or mixing of the substance of the heating
- On account of boiling water and air, and also those metals and other solid bodies which
are heated, but not to ignition, or red heat, reject flame or light.
- On account of the rays of the moon and other heavenly bodies (except the sun), again
reject flame or light.
- On account of the comparison between red-hot iron and the flame of spirits of wine (for
the iron is more hot and less bright, whilst the flame of spirits of wine is more bright
and less hot), again reject flame and light.
- On account of gold and other ignited metals, which are of the greatest specific density,
- On account of air, which is generally found to be cold and yet continues rare, reject
- On account of ignited iron, which does not swell in bulk, but retains the same apparent
dimension, reject the absolute expansive motion of the whole.
- On account of the expansion of the air in thermometers and the like, which is absolutely
moved and expanded to the eye, and yet acquires no manifest increase of heat, again reject
absolute or expansive motion of the whole.
- On account of the ready application of heat to all substances without any destruction or
remarkable alteration of them, reject destructive nature or the violent communication of
any new nature.
- On account of the agreement and conformity of the effects produced by cold and heat,
reject both expansive and contracting motion as regards the whole.
- On account of the heat excited by friction, reject principal nature, by which we mean
that which exists positively, and is not caused by a preceding nature.
There are other natures to be rejected; but we are merely offering examples, and not
perfect tables. None of the above natures is of the form of heat; and man is freed from
them all in his operation upon heat.
19. In the exclusive table are laid the foundations of true induction, which is not,
however, completed until the affirmative be attained. Nor is the exclusive table perfect,
nor can it be so at first. For it is clearly a rejection of simple natures; but if we have
not as yet good and just notions of simple natures, how can the exclusive table be made
correct? Some of the above, as the notion of elementary and celestial nature, and rarity,
are vague and ill-defined. We, therefore, who are neither ignorant nor forgetful of the
great work which we attempt, in rendering the human understanding adequate to things and
nature, by no means rest satisfied with what we have hitherto enforced, but push the
matter further, and contrive and prepare more powerful aid for the use of the
understanding, which we will next subjoin. And, indeed, in the interpretation of nature
the mind is to be so prepared and formed, as to rest itself on proper degrees of
certainty, and yet to remember (especially at first) that what is present depends much
upon what remains behind.
20. Since, however, truth emerges more readily from error than confusion, we consider
it useful to leave the understanding at liberty to exert itself and attempt the
interpretation of nature in the affirmative, after having constructed and weighed the
three tables of preparation, such as we have laid them down, both from the instances there
collected, and others occurring elsewhere. Which attempt we are wont to call the liberty
of the understanding, or the commencement of interpretation, or the first vintage.
The First Vintage of the Form of Heat
It must be observed that the form of anything is inherent (as appears clearly from our
premises) in each individual instance in which the thing itself is inherent or it would
not be a form. No contradictory instance, therefore, can be alleged. The form, however, is
found to be much more conspicuous and evident in some instances than in others; in those
(for example) where its nature is less restrained and embarrassed, and reduced to rule by
other natures. Such instances we are wont to term coruscations, or conspicuous instances.
We must proceed, then, to the first vintage of the form of heat.
From the instances taken collectively, as well as singly, the nature 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 drafts: for which
see Inst. 29, Tab. III and by other species of motion, as in Inst. 28 and 31, Tab. III. 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. III. 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
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. III.
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.
(1) 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
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
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 stated in Inst. 38, Tab. III.
It is also shown in the contrary nature of cold; for cold contracts and narrows every
substance; 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. III. 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. II), 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 occasion an expansive and dilating motion, but cold a contracting and
(2) 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
upwards. 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 upwards.
This difference is shown by putting 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
downwards, as the expansion of heat is upwards. 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 the 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.
(3) 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
drafts 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 at 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 effect 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 inwards or outwards, the
reasoning is the same, though the power be very different, because we have nothing on
earth which is intensely cold.
(4) 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
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
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. The expansion is
modified by its tendency to rise, though expanding towards 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 or such motion. Sensible heat is the same, but considered relatively
to the senses. Let us now proceed to further helps.