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Micrographia by Robert Hooke

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That all kind of solid bodies consist of pretty massie particles in respect
of the particles of this fluid _medium_, which in many places do so touch
each other, that none of this fluid _medium_ interposes much after the same
mannner (to use a gross similitude) as a heap of great stones compose one
great _congeries_ or mass in the midst of the water.

That all fluid bodies which we may call _tangible_, are nothing but some
more subtile parts of those particles, that serve to constitute all
_tangible_ bodies.

That the water, and such other fluid bodies, are nothing but a _congeries_
of particles agitated or made fluid by it in the same manner as the
particles of _Salt_ are agitated or made fluid by a parcel of water, in
which they are dissolv'd, and subsiding to the bottom of it, constitute a
fluid body, much more massie and dense, and less fluid then the pure water
it self.

That the air on the other side is a certain company of particles of quite
another kind, that is, such as are very much smaller, and more easiely
moveable by the motion of this fluid _medium_; much like those very subtile
parts of _Cochenel_, other very deep tinging bodies, where by a very small
parcel of matter is able to tinge and diffuse it self over a very great
quantity of the fluid dissolvent; or somewhat after that manner, as smoak,
and such like minute bodies, or steams, are observ'd to tinge a very great
quantity of air; onely this last similitude is deficient in one propriety,
and that is a perpetuity or continuance in that state of commixture with
the air, but the former does more neerly approach to the nature and manner
of the air's being dissolv'd by this fluid or _AEther_. And this Similitude
will further hold in these proprieties; that as those tinctures may be
increased by certain bodies, so may they be precipitated by others, as I
shall afterwards shew it to be very probable, that the like accidents
happen even to the Air it self.

Further, as these solutions and tinctures do alter the nature of these
fluid bodies, as to their aptness to propagate a motion or impulse through
them, even so does the particles of the Air, Water, and other fluid bodies,
and of Glass, Crystal, &c. which are commixt with this bulk of the _AEther_
alter the motion of the propagated pulse of light; that is, where these
more bulkie particles are more plentifull, and consequently a lesser
quantity of the _AEther_ between them to be mov'd, there the motion must
necessarily be the swifter, though not so robust, which will produce those
effects, which I have (I hope) with some probability, ascribed to it in the
digression about Colours, at the end of the _Observations_ on
_Muscovy-glass._

Now, that other Stones, and those which have the closest and hardest
textures, and seem (as far as we are able to discover with our eyes, though
help'd with the best _Microscopes_) freest from pores, are yet
notwithstanding replenish'd with them, an Instance or two will, I suppose,
make more probable.

A very solid and unflaw'd piece of cleer white _Marble_, if it be well
polish'd and glaz'd, has so curiously smooth a surface, that the best and
most polish'd surface of any wrought-glass, seems not to the naked eye, nor
through a _Microscope_, to be more smooth, and less porous. And yet, that
this hard close body is replenish'd with abundance of pores, I think these
following Experiments will sufficiently prove.

The first is, That if you take such a piece, and for a pretty while boyl it
in Turpentine and Oyl of Turpentine, you shall find that the stone will be
all imbu'd with it; and whereas before it look'd more white, but more
opacous, now it will look more greasie, but be much more transparent, and
if you let it lie but a little while, and then break off a part of it, you
shall find the unctuous body to have penetrated it to such a determinate
depth every way within the surface. This may be yet easier try'd with a
piece of the same _Marble_, a little warm'd in the fire, and then a little
Pitch or Tarr melted on the top of it; for these black bodies, by their
insinuating themselves into the invisible pores of the stone, ting it with
so black a hue, that there can be no further doubt of the truth of this
assertion, that it abounds with small imperceptible pores.

Now, that other bodies will also sink into the pores of _Marble_, besides
unctuous, I have try'd, and found, that a very Blue tincture made in
_spirit of Urine_ would very readily and easily sink into it, as would also
several tinctures drawn with _spirit of Wine_.

Nor is _Marble_ the only seemingly close stone, which by other kinds of
Experiments may be found porous; for I have by this kind of Experiment on
divers other stones found much the same effect, and in some, indeed much
more notable. Other stones I have found so porous, that with the
_Microscope_ I could perceive several small winding holes, much like
Worm-holes, as I have noted in some kind of _Purbeck-stone_, by looking on
the surface of a piece newly flaw'd off, for if otherwise, the surface has
been long expos'd to the Air, or has been scraped with any tool, those
small caverns are fill'd with dust, and disappear.

And to confirm this _Conjecture_, yet further, I shall here insert an
excellent account, given into the _Royal Society_ by that Eminently Learned
Physician, Doctor _Goddard_, of an Experiment, not less instructive then
curious and accurate, made by himself on a very hard and seemingly close
stone call'd _Oculus Mundi_, as I find it preserv'd in the Records of that
Honourable Society.

A small stone of the kind, call'd by some Authours, _Oculus Mundi_,
being dry and cloudy, weigh'd 5-209/256 _Grains_.

The same put under water for a night, and somewhat more, became
transparent, and the superficies being wiped dry, weighed 6-3/256
_Grains_.

The difference between these two weights, 0-50/256 of a _Grain_.

The same Stone kept out of water one Day and becoming cloudy again
weighed, 5-225/256 _Graines_.

Which was more then the first weight, 0-16/256 of a _Grain_.

The same being kept two Days longer weighed, 5-202/256 _Graines_.

Which was less then at first, 0-7/256 a _Grain_.

Being kept dry something longer it did not grow sensibly lighter.

Being put under water for a night and becoming again transparent and
wiped dry, the weight was, _6-3/256 Grains_, the same with the first
after putting in water, and more then the last weight after keeping of
it dry, 0-57/256 of a _Grain_.

Another Stone of the same kind being variegated with milky _white_ and
_gray_ like some sorts of _Agates_, while it lay under water, was
alwaies invironed with little Bubbles, such as appear in water a little
before boyling, next the sides of the Vessel.

There were also some the like Bubbles on the Surface of the water just
over it, as if either some exhalations came out of it, or that it did
excite some fermentation in the parts of the water contiguous to it.

There was little sensible difference in the transparency of this Stone,
before the putting under water, and after: To be sure the milky-_white_
parts continued as before, but more difference in weight then in the
former. For whereas before the putting into the water the weight was
18-97/128 _Graines_. After it had lyen in about four and twenty hours
the weight was 20-27/128 _Graines_, so the difference was, 1-58/128
_Graines_.

The same Stone was infused in the water scalding hot, and so continued
for a while after it was cold, but got no more weight then upon
infusing in the cold, neither was there any sensible Difference in the
weight both times.

In which Experiment, there are three Observables that seem very manifestly
to prove the porousness of these seemingly close bodies: the first is their
acquiring a transparency, and losing their whiteness after steeping in
water, which will seem the more strongly to argue it, if what I have
already said about the making transparent, or clarifying of some bodies, as
the white powder of beaten Glass, and the froth of some glutinous
transparent liquor be well consider'd; for thereby it will seem rational to
think that this transparency arises from the insinuation of the water
(which has much the same refraction with such stony particles, as may be
discover'd by Sand view'd with a _Microscope_) into those pores which were
formerly repleat with air (that has a very differing refraction, and
consequently is very reflective) which seems to be confirm'd by the second
Observable, namely, the increase of weight after keeping, and decrease upon
drying. And thirdly, seem'd yet more sensibly confirm'd by the multitude of
bubbles in the last Experiment.

We find also most Acid Salts very readily to dissolve and separate the
parts of this body one from another; which is yet a further Argument to
confirm the porousness of bodies, and will serve as such, to shew that even
Glass also has an abundance of pores in it, since there are several
liquors, that with long staying in a Glass, will so _Corrode_ and eat into
it, as at last, to make it pervious to the liquor it contain'd, of which I
have seen very many Instances.

Since therefore we find by other proofs, that many of those bodies which we
think the most solid ones, and appear so to our sight, have notwithstanding
abundance of those grosser kind of pores, which will admit several kinds of
liquors into them, why should we not believe that Glass, and all other
transparent bodies abound with them, since we have many other arguments,
besides the propagation of light, which seem to argue for it?

And whereas it may be objected, that the propagation of light is no
argument that there are those atomical pores in glass, since there are
_Hypotheses_ plausible enough to solve those _Phaenomena_, by supposing the
pulse onely to be communicated through the transparent body.

To this I answer, that that _Hypothesis_ which the industrious _Mersennus_
has publish'd about the slower motion of the end of a Ray in a denser
_medium_, then in a more rare and thin, seems altogether unsufficient to
solve abundance of _Phaenomena_, of which this is not the least
considerable, that it is impossible from that supposition, that any colours
should be generated from the refraction of the Rays; for since by that
_Hypothesis_ the _undulating pulse_ is always carried perpendicular, or at
right angles with the Ray or Line of direction, it follows, that the stroke
of the pulse of light, after it has been once or twice refracted (through a
Prisme, for example) must affect the eye with the same kind of stroke as if
it had not been refracted at all. Nor will it be enough for a Defendant of
that _Hypothesis_, to say, that perhaps it is because the refractions have
made the Rays more weak, for if so, then two refractions in the two
parallel sides of a _Quadrangular Prisme_ would produce colours, but we
have no such _Phaenomena_ produc'd.

There are several Arguments that I could bring to evince that there are in
all transparent bodies such atomical pores. And that there is such a fluid
body as I am arguing for, which is the _medium_, or Instrument, by which
the pulse of Light is convey'd from the _lucid body_ to the enlightn'd. But
that it being a digression from the Observations I was recording, about the
Pores of _Kettering Stone_, it would be too much such, if I should protract
it too long; and therefore I shall proceed to the next _Observation_.

* * * * *


Observ. XVI. _Of _Charcoal_, or burnt _Vegetables_._

Charcoal, or a Vegetable burnt black, affords an object no less pleasant
than instructive, for if you take a small round Charcoal, and break it
short with your fingers, you may perceive it to break with a very smooth
and sleek surface, almost like the surface of black sealing Wax; this
surface, if it be look'd on with an ordinary _Microscope_, does manifest
abundance of those pores which are also visible to the eye in many kinds of
_Wood_, rang'd round the pith, both a in kind of circular order, and a
radiant one. Of these there are a multitude in the substance of the Coal,
every where almost perforating and drilling it from end to end; by means of
which, be the Coal never so long, you may easily blow through it; and this
you may presently find, by wetting one end of it with Spittle, and blowing
at the other.

But this is not all, for besides those many great and conspicuous irregular
spots or pores, if a better _Microscope_ be made use of, there will appear
an infinite company of exceedingly small, and very regular pores, so thick
and so orderly set, and so close to one another, that they leave very
little room or space between them to be fill'd with a solid body, for the
apparent _interstitia_ or separating sides of these pores seem so thin in
some places, that the texture of a Honey-comb cannot be more porous. Though
this be not every where so, the intercurrent partitions in some places
being very much thicker in proportion to the holes.

Most of these small pores seem'd to be pretty round, and were rang'd in
rows that radiated from the pith to the bark; they all of them seem'd to be
continued open pores, running the whole length of the Stick; and that they
were all perforated, I try'd by breaking off a very thin sliver of the Coal
cross-ways, and then with my _Microscope_, diligently surveying them
against the light, for by that means I was able to see quite through them.

These pores were so exceeding small and thick, that in a line of them, 1/18
part of an Inch long, I found by numbring them no less then 150. small
pores; and therefore in a line of them an Inch long, must be no less then
2700. pores, and in a circular _area_ of an Inch diameter, must be about
5725350. of the like pores; so that a Stick of an Inch Diameter, may
containe no less then seven hundred and twenty five thousand, besides 5
Millions of pores, which would, I doubt not, seem even incredible, were not
every one left to believe his own eyes. Nay, having since examin'd _Cocus,
black and green Ebony, Lignum Vitae_, &c. I found, that all these Woods
have their pores, abundantly smaller then those of soft light Wood; in so
much, that those of _Guajacum_ seem'd not above an eighth part of the
bigness of the pores of Beech, but then the _Interstitia_ were thicker; so
prodigiously curious are the contrivances, pipes, or sluces by which the
_Succus nutritius_, or Juyce of a Vegetable is convey'd from place to
place.

This _Observation_ seems to afford us the true reason of several
_Phaenomena_ of Coals; as

First, why they look black; and for this we need go no further then the
_Scheme_, for certainly, a body that has so many pores in it as this is
discover'd to have, from each of which no light is reflected, must
necessarily look black, especially, when the pores are somewhat bigger in
proportion to the intervals then they are cut in the _Scheme_, black being
nothing else but a privation of Light, or a want of reflection; and
wheresover this reflecting quality is deficient, there does that part look
black, whether it be from a porousness of the body, as in this Instance, or
in a deadning and dulling quality, such as I have observ'd in the _Scoria_
of Lead, Tin, Silver, Copper, &c.

Next, we may also as plainly see the reason of its shining quality, and
that is from the even breaking off of the stick, the solid _interstitia_
having a regular termination or surface, and having a pretty strong
reflecting quality, the many small reflections become united to the naked
eye, and make a very pretty shining surface.

Thirdly, the reason of its hardness and brittleness seems evident, for
since all the watery or liquid substance that moistn'd and toughn'd those
_Interstitia_ of the more solid parts, are evaporated and remov'd, that
which is left behind becomes of the nature almost of a stone, which will
not at all, or very little, bend without a _divulsion_ or _solution_ of its
_continuity_.

It is not my design at present, to examine the use and _Mechanisme_ of
these parts of Wood, that being more proper to another Enquiry; but rather
to hint, that from this Experiment we may learn,

First, what is the cause of the blackness of many burnt bodies, which we
may find to be nothing else but this; that the heat of the fire agitating
and rarifying the waterish, transparent, and volatile water that is
contain'd in them, by the continuation of that action, does so totally
expel and drive away all that which before fill'd the pores, and was
dispers'd also through the solid mass of it, and thereby caus'd an
universal kind of transparency, that it not onely leaves all the pores
empty, but all the _Interstitia_ also so dry and _opacous_, and perhaps
also yet further perforated, that that light onely is reflected back which
falls upon the very outward edges of the pores, all they that enter into
the pores of the body, never returning, but being lost in it.

Now, that the Charring or coaling of a body is nothing else, may be easily
believ'd by one that shall consider the means of its production, which may
be done after this, or any such manner. The body to be charr'd or coal'd,
may be put into a _Crucible_, Pot, or any other Vessel that will endure to
be made red-hot in the fire without breaking, and then cover'd over with
Sand, so as no part of it be suffer'd to be open to the Air, then set into
a good fire, and there kept till the Sand has continu'd red hot for a
quarter, half, an hour or two, or more, according to the nature and bigness
of the body to be coal'd or charr'd, then taking it out of the fire, and
letting it stand till it be quite cold, the body may be taken out of the
Sand well charr'd and cleans'd of its waterish parts; but in the taking of
it out, care must be had that the Sand be very neer cold, for else, when it
comes into the free air, it will take fire, and readily burn away.

This maybe done also in any close Vessel of Glass, as a _Retort_, or the
like, and the several fluid substances that come over may be receiv'd in a
fit _Recipient_, which will yet further countenance this _Hypothesis_: And
their manner of charring Wood in great quantity comes much to the same
thing, namely, an application of a great heat to the body, and preserving
it from the free access of the devouring air; this may be easily learn'd
from the History of Charring of Coal, most excellently describ'd and
publish'd by that most accomplish'd Gentleman, Mr. _John Evelin_, in the
100, 101, 103, pages of his _Sylva_, to which I shall therefore refer the
curious Reader that desires a full information of it.

Next, we may learn what part of the Wood it is that is the _combustible_
matter, for since we shall find that none, or very little of those fluid
substances that are driven over into the Receiver are _combustible_, and
that most of that which is left behind is so, it follows, that the solid
_interstitia_ of the Wood are the _combustible_ matter. Further, the reason
why uncharr'd Wood burns with a greater flame then that which is charr'd,
is as evident, because those waterish or volatil parts issuing out of the
fired Wood, every way, not onely shatter and open the body, the better for
the fire to enter, but issuing out in vapours or wind, they become like so
many little _aeolipiles_, or Bellows, whereby they blow and agitate the
fir'd part, and conduce to the more speedy and violent consumption or
dissolution of the body.

Thirdly, from the Experiment of charring of Coals (whereby we see that
notwithstanding the great heat, and the duration of it, the solid parts of
the Wood remain, whilest they are preserv'd from the free access of the air
undissipated) we may learn, that which has not, that I know of, been
publish'd or hinted, nay, not so much as thought of, by any; and that in
short is this.

First, _that the Air_ in which we live, move, and breath, and which
encompasses very many, and cherishes most bodies it encompasses, that this
Air is the _menstruum_, or universal dissolvent of all _Sulphureous_
bodies.

Secondly, _that this action_ it performs not, till the body be first
sufficiently heated, as we find requisite also to the dissolution of many
other bodies by several other _menstruums_.

Thirdly, _that this action_ of dissolution, produces or generates a very
great heat, and that which we call Fire; and this is common also to many
dissolutions of other bodies, made by _menstruums_, of which I could give
multitudes of Instances.

Fourthly, _that this action_ is perform'd with so great a violence, and
does so minutely act, and rapidly agitate the smallest parts of the
_combustible_ matter, that it produces in the _diaphanous medium_ of the
Air, the action or pulse of light, which what it is, I have else-where
already shewn.

Fifthly, _that the dissolution_ of sulphureous bodies is made by a
substance inherent, and mixt with the Air, that is like, if not the very
same, with that which is fixt in _Salt-peter_, which by multitudes of
Experiments that may be made with _Saltpeter_, will, I think, most
evidently be demonstrated.

Sixthly, _that in this dissolution_ of bodies by the Air, a certain part is
united and mixt, or dissolv'd and turn'd into the Air, and made to fly up
and down with it in the same manner as a _metalline_ or other body
dissolved into any _menstruums_, does follow the motions and progresses of
that _menstruum_ till it be precipitated.

Seventhly, That as there is one part that is dissoluble by the Air, so are
there other parts with which the parts of the Air mixing and uniting, do
make a _Coagulum_, or _precipitation_, as one may call it, which causes it
to be separated from the Air, but this _precipitate_ is so light, and in so
small and rarify'd or porous clusters, that it is very volatil, and is
easily carry'd up by the motion of the Air, though afterwards, when the
heat and agitation that kept it rarify'd ceases, it easily condenses, and
commixt with other indissoluble parts, it sticks and adheres to the next
bodies it meets withall; and this is a certain _Salt_ that may be extracted
out of _Soot_.

Eighthly, that many indissoluble parts being very apt and prompt to be
rarify'd, and so, whilest they continue in that heat and agitation, are
lighter then the Ambient Air, are thereby thrust and carry'd upwards with
great violence, and by that means carry along with them, not onely that
_Saline concrete_ I mention'd before, but many terrestrial, or indissoluble
and irrarefiable parts, nay, many parts also which are dissoluble, but are
not suffer'd to stay long enough in a sufficient heat to make them prompt
and apt for that action. And therefore we find in _Soot_, not onely a part,
that being continued longer in a competent heat, will be dissolv'd by the
Air, or take fire and burn; but a part also which is fixt, terrestrial, and
irrarefiable.

Ninthly, that as there are these several parts that will rarifie and fly,
or be driven up by the heat, so are there many others, that as they are
indissoluble by the _aerial menstruum_, so are they of such sluggish and
gross parts, that they are not easily rarify'd by heat, and therefore
cannot be rais'd by it; the volatility or fixtness of a body seeming to
consist only in this, that the one is of a texture, or has component parts
that will be easily rarify'd into the form of Air, and the other, that it
has such as will not, without much ado, be brought to such a constitution;
and this is that part which remains behind in a white body call'd Ashes,
which contains a substance, or _Salt_, which Chymists call _Alkali_: what
the particular natures of each of these bodies are, I shall not here
examine, intending it in another place, but shall rather add that this
_Hypothesis_ does so exactly agree with all _Phaenomena_, of Fire, and so
genuinely explicate each particular circumstance that I have hitherto
observ'd, that it is more then probable, that this cause which I have
assign'd is the true adequate, real, and onely cause of those _Phaenomena_;
And therefore I shall proceed a little further, to shew the nature and use
of the Air.

Tenthly, therefore the dissolving parts of the Air are but few, that is, it
seems of the nature of those _Saline menstruums_, or spirits, that have
very much flegme mixt with the spirits, and therefore a small parcel of it
is quickly glutted, and will dissolve no more; and therefore unless some
fresh part of this _menstruum_ be apply'd to the body to be dissolv'd, the
action ceases, and the body leaves to be dissolv'd and to shine, which is
the Indication of it, though plac'd or kept in the greatest heat; whereas
_Salt-peter_ is a _menstruum_, when melted and red-hot, that abounds more
with those Dissolvent particles, and therefore as a small quantity of it
will dissolve a great sulphureous body, so will the dissolution be very
quick and violent.

Therefore in the _Eleventh_ place, it is observable, that, as in other
solutions, if a copious and quick supply of fresh _menstruum_, though but
weak, be poured on, or applied to the dissoluble body, it quickly consumes
it: So this _menstruum_ of the Air, if by Bellows, or any other such
contrivance, it be copiously apply'd to the shining body, is found to
dissolve it as soon, and as violently as the more strong _menstruum_ of
melted _Nitre_.

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