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Micrographia by Robert Hooke

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Now the cause why all these included Fluids, newly mentioned, or as many
others as are wholly included within a heterogeneous fluid, are not
_exactly_ of a _Spherical Figure_ (seeing that if caused by these
Principles only, it could be of no other) must proceed from some other kind
of _pressure_ against the two opposite flatted sides. This _adventitious_
or _accidental pressure_ may proceed from _divers causes_, and accordingly
must _diversifie_ the Figure of the included heterogeneous fluid: For
seeing that a body may be included either with a fluid only, or only with a
solid, or partly with a fluid, and partly with a solid, or partly with one
fluid, and partly with another; there will be found a very great variety of
the terminating _surfaces_, much differing from a _Spherical_, according to
the various resistance or pressure that belongs to each of these
encompassing bodies.

Which Properties may in general be deduced from two heads, _viz._ _Motion_,
and _Rest_. For, either this Globular Figure is altered by a _natural
Motion_, such as is _Gravity_, or a _violent_, such as is any _accidental
motion_ of the fluids, as we see in the _wind_ ruffling up the water, and
the _purlings_ of _Streams_, and _foaming_ of _Catarracts_, and the like.
Or thirdly, By the _Rest_, _Firmness_ and _Stability_ of the ambient
_Solid_. For if the including _Solid_ be of an _angular_ or any other
_irregular_ Form, the included _fluid_ will be near of the _like_, as a
Pint-_Pot_ full of _water_, or a _Bladder_ full of _Air_. And next, if the
including or included fluid have a greater _gravity_ one than another, then
will the _globular_ Form be deprest into an _Elliptico-spherical_: As if,
for example, we suppose the Circle _ABCD_, in the _fourth Figure_, to
represent a _drop of water_, _Quick-silver_, or the like, included with the
_Air_ or the like, which supposing there were no _gravity_ at all in either
of the _fluids_, or that the _contained_ and _containing_ were of the _same
weight,_ would be _equally comprest_ into an exactly _spherical_ body (the
ambient fluid _forcing equally_ against every side of it.) But supposing
either a greater _gravity_ in the included, by reason whereof the parts of
it being _prest_ from _A_ towards _B_, and thereby the whole put into
_motion_, and that _motion_ being _hindred_ by the _resistance_ of the
_subjacent_ parts of the ambient, the _globular_ Figure _ADBC_ will be
_deprest_ into the _Elliptico-spherical_, _EGFH_. For the side _A_ is
_detruded_ to _E_ by the _Gravity_, and _B_ to _F_ by the _resistance_ of
the subjacent medium: and therefore _C_ must necessarily be thrust to _G_;
and _D_ to _H_. Or else, supposing a greater _gravity_ in the _ambient_, by
whose more then ordinary _pressure_ against the under side of the included
globule; _B_ will be forced to _F_, and by its _resistance_ of the motion
_upwards_, the side _A_ will be _deprest_ to _E_, and therefore _C_ being
thrust to _G_ and _D_ to _H_; the _globular_ Figure by this means also will
be made an _Elliptico-spherical_. Next if a fluid be included _partly_ with
one, and _partly_ with another fluid, it will be found to be shaped
_diversly_, according to the proportion of the _gravity_ and _incongruity_
of the 3 _fluids_ one to another: As in the _second Figure_, let the upper
_MMM_ be _Air_, the middle _LMNO_ be common _Oyl_, the lower _OOO_ be
_Water_, the _Oyl_ will be form'd, not into a _spherical_ Figure, such as
is represented by the _pricked Line_, but into such a Figure as LMNO, whose
side LMN will be of a flatter _Elliptical_ Figure, by reason of the great
disproportion between the _Gravity_ of _Oyl_ and _Air_, and the side LOM of
a rounder, because of the smaller difference between the weight of _Oyl_
and _Water_. Lastly, The _globular_ Figure will be changed, if the
_ambient_ be partly _fluid_ and partly _solid_. And here the termination of
the incompassed _fluid_ towards the incompassing is shap'd according to the
proportion of the congruity or incongruity of the _fluids_ to the _solids_,
and of the gravity and incongruity of the _fluids_ one to another. As
suppose the subjacent _medium_ that hinders an included fluids descent, be
a _solid_, as let KI, in the _fourth Figure_, represent the smooth
superficies of a _Table_; EGFH, a parcel of _running Mercury_; the side GFH
will be more flatted, according to the proportion of the incongruity of the
_Mercury_ and _Air_ to the _Wood_, and of the _gravity_ of _Mercury_ and
_Air_ one to another; The side GEH will likewise be a little more deprest
by reason the subjacent parts are now at rest, which were before in motion.

Or further in the _third figure_, let AILD represent an including _solid_
medium of a cylindrical shape (as suppose a small _Glass Jar_) Let FGEMM
represent a contain'd _fluid_, as water; this towards the bottom and sides,
is figured according to the concavity of the _Glass_: But its upper
_Surface_, (which by reason of its gravity, (not considering at all the Air
above it, and so neither the congruity or incongruity of either of them to
the Glass) should be terminated by part of a _Sphere_ whose diameter should
be the same with that of the earth, which to our sense would appear a
straight _Line_, as FGE, Or which by reason of its having a greater
congruity to Glass than Air has, (not considering its Gravity) would be
thrust into a _concave Sphere_, as CHB, whose diameter would be the same
with that of the concavity of the Vessel:) Its upper Surface, I say, by
reason of its having a greater gravity then the Air, and having likewise a
greater congruity to Glass then the Air has, is terminated, by a _concave
Elliptico-spherical Figure_, as CKB. For by its congruity it easily
conforms it self, and adheres to the Glass, and constitutes as it were one
containing body with it, and therefore should thrust the contained Air on
that side it touches it, into a _spherical_ Figure, as BHC, but the motion
of Gravity depressing a little the Corners B and C, reduces it into the
aforesaid Figure CKB. Now that it is the greater congruity of one of the
two _contiguous fluids_, then of the other, to the containing _solid_, that
causes the separating surfaces to be thus or thus figured: And that it is
not because this or that figurated surface is more proper, natural, or
peculiar to one of these fluid bodies, then to the other, will appear from
this; that the same _fluids_ will by being put into differing _solids_,
change their _surfaces_. For the same water, which in a Glass or wooden
Vessel will have a concave surface upwards, and will rise higher in a
smaller then a greater Pipe, the same water, I say, in the same Pipes
greased over or oyled, will produce quite contrary effects; for it will
have a _protuberant_ and _convex_ surface upwards, and will not rise so
high in small, as in bigger Pipes: Nay, in the very same solid Vessel, you
may make the very same two contiguous _Liquids_ to alter their Surfaces;
for taking a small Wine-glass, or such like Vessel, and pouring water
gently into it, you shall perceive the _surface_ of the water all the way
_concave_, till it rise even with the top, when you shall find it (if you
gently and carefully pour in more) to grow very _protuberant_ and _convex_;
the reason of which is plain, for that the _solid_ sides of the containing
body are no longer extended, to which the water does more readily adhere
then the air; but it is henceforth to be included with air, which would
reduce it into a _hemisphere_, but by reason of its _gravity_, it is
flatted into an _Oval_. _Quicksilver_ also which to _Glass_ is more
incongruous then _Air_ (and thereby being put into a _Glass-pipe_, will not
adhere to it, but by the more _congruous air_ will be forced to have a very
_protuberant_ surface, and to rise higher in a greater then a lesser Pipe)
this _Quicksilver_ to clean _Metal_, especially to _Gold_, _Silver_, _Tin_,
_Lead_, &c. _Iron_ excepted, is more _congruous_ then _Air_, and will not
only stick to it, but have a _concave_ Surface like _water_, and rise
higher in a less, then in a greater Pipe.

In all these Examples it is evident, that there is an _extraordinary_ and
_adventitious force_, by which the _globular_ Figure of the contained
_heterogeneous_ fluid is altered; neither can it be imagined, how it should
otherwise be of any other Figure then _Globular_: For being by the
_heterogeneous_ fluid equally _protruded_ every way, whatsoever part is
_protuberant_, will be thereby _deprest_. From this cause it is, that in
its effects it does very much resemble a _round Spring_ (such as a _Hoop_.)
For as in a _round Spring_ there is required an additional _pressure_
against two opposite sides, to reduce it into an _Oval_ Form, or to force
it in between the sides of a _Hole,_ whose _Diameter_ is less then that of
the _Spring_, there must be a considerable force or _protusion_ against
_the concave_ or inner side of the _Spring_; So to alter this _spherical_
constitution of an included fluid body, there is required more pressure
against opposite sides to reduce it into an _Oval_; and, to press it into
an _Hole_ less in _Diameter_ then it self, it requires a greater
_protrusion_ against all the other sides, What degrees of force are
requisite to reduce them into longer and longer _Ovals_, or to press them
into less and less _holes_, I have not yet experimentally calculated; but
thus much by experiment I find in general, that there is alwayes required a
greater pressure to close them into longer _Ovals_, or protude them into
smaller _holes_. The necessity and reason of this, were it requisite, I
could easily explain: but being not so necessary, and requiring more room
and time then I have for it at present, I shall here omit it; and proceed
to shew, that this may be presently found true, if Experiment be made with
a _round Spring_ (the way of making which trials is _obvious_ enough.) And
with the fluid bodies of _Mercury_, _Air_, _&c_, the way of trying which,
will be somewhat more difficult; and therefore I shall in brief describe
it. He therefore that would try with _Air_, must first be provided of a
_Glass-pipe_, made of the shape of that in the _fifth Figure_, whereof the
side AB, represents a straight _Tube_ of about three foot long, C,
represents another part of it, which consists of a _round Bubble_; so
ordered, that there is left a _passage_ or _hole_ at the top, into which
may be fastened with _cement_ several _small Pipes_ of determinate
_cylindrical_ cavities: as let _hollow_ of

F. 1/4
G. 1/6
H. 1/8
I. be 1/12 of an inch.
K. 1/16
L. 1/24
M. 1/32
&c----
There may be added as many more, as the Experimenter shall think fit, with
holes continually decreasing by known quantities, so far as his senses are
able to help him; I say, so far, because there may be made _Pipes_ so small
that it will be impossible to perceive the _perforation_ with ones naked
eye, though by the help of a _Microscope_, it may easily enough be
perceived: Nay, I have made a _Pipe_ perforated from end to end, so small,
that with my naked eye I could very hardly see the body of it, insomuch
that I have been able to knit it up into a knot without breaking: And more
accurately examining one with my _Microscope_, I found it not so big as a
sixteenth part of one of the smaller hairs of my head which was of the
smaller and finer sort of hair, so that sixteen of these _Pipes_ bound
faggot-wise together, would but have equalized one single hair; how small
therefore must its _perforation_ be? It appearing to me through the
_Microscope_ to be a proportionably _thick-sided Pipe_.

To proceed then, for the trial of the Experiment, the Experimenter must
place the _Tube_ AB, perpendicular, and fill the _Pipe_ F (cemented into
the hole E) with water, but leave the _bubble_ C full of _Air_, and then
gently pouring in water into the Pipe AB, he must observe diligently how
high the water will rise in it before it protrude the _bubble_ of Air C,
through the narrow passage of F, and denote exactly the height of the
_Cylinder_ of water, then cementing in a second Pipe as G, and filling it
with water; he may proceed as with the former, denoting likewise the height
of the _Cylinder_ of water, able to protrude the _bubble_ C through the
passage of G, the like may he do with the next _Pipe_, and the next, &c. as
far as he is able: then comparing the several heights of the _Cylinders_,
with the several _holes_ through which each _Cylinder_ did force the _air_
(having due regard to the _Cylinders_ of water in the small _Tubes_) it
will be very easie to determine, what force is requisite to press the _Air_
into such and such _a hole_, or (to apply it to our present experiment) how
much of the pressure of the _Air_ is taken off by its ingress into smaller
and smaller _holes_. From the application of which to the entring of the
_Air_ into the bigger _hole_ of the _Vessel_, and into the smaller _hole_
of the _Pipe_, we shall clearly find, that there is a greater pressure of
the air upon the water in the _Vessel_ or greater _pipe_, then there is
upon that in the lesser _pipe_: For since the pressure of the _air_ every
way is found to be equal, that is, as much as is able to press up and
sustain a _Cylinder_ of _Quicksilver_ of two foot and a half high, or
thereabouts; And since of this pressure so many more degrees are required
to force the _Air_ into a smaller then into a greater _hole_ that is full
of a more congruous fluid. And lastly, since those degrees that are
requisite to press it in, are thereby taken off from the _Air_ within, and
the _Air_ within left with so many degrees of pressure less then the _Air_
without; it will follow, that the _Air_ in the less _Tube_ or _pipe_, will
have less pressure against the superficies of the _water_ therein, then the
_Air_ in the bigger: which was the minor Proposition to be proved.

The Conclusion therefore will necessarily follow, _viz._ That _this unequal
pressure of the Air caused by its ingress into unequal holes, is a cause
sufficient to produce this effect, without the help of any other
concurrent_; therefore is probably the principal (if not the only) cause of
these _Phaenomena_.

This therefore being thus explained, there will be divers _Phaenomena_
explicable thereby, as, the rising of _Liquors_ in a _Filtre_, the rising
of _Spirit of Wine_, _Oyl_, _melted Tallow_, &c. in the _Week_ of a _Lamp_,
(though made of small _Wire_, _Threeds_ of _Asbestus_, _Strings_ of
_Glass_, or the like) the rising of _Liquors_ in a _Spunge_, piece of
_Bread_, _Sand_, &c. perhaps also the ascending of the _Sap_ in _Trees_ and
_Plants_, through their small, and some of them _imperceptible pores_, (of
which I have said more, on another occasion) at least the passing of it out
of the earth into their roots. And indeed upon the consideration of this
Principle, multitudes of other uses of it occurr'd to me, which I have not
yet so well examined and digested as to propound for _Axioms_, but only as
_Queries_ and _Conjectures_ which may serve as _hints_ toward some further
_discoveries_.

As first, Upon the consideration of the _congruity_ and _incongruity_ of
Bodies, as to _touch_, I found also the like _congruity_ and _incongruity_
(if I may so speak) as to the _Transmitting_ of the _Rates_ of Light: For
as in this regard, _water_ (not now to mention other Liquors) seems nearer
of affinity to _Glass_ then _Air_, and _Air_ then _Quicksilver_: whence an
_oblique Ray_ out of _Glass_, will pass into _water_ with very little
_refraction_ from the _perpendicular_, but none out of _Glass_ into _Air_,
excepting a _direct_, will pass without a very great refraction from the
perpendicular, nay any oblique Ray under thirty degrees, will not be
admitted into the Air at all. And _Quicksilver_ will neither admit oblique
or direct, but reflects all; seeming, as to the transmitting of the Raies
of Light, to be of a quite differing constitution, from that of _Air_,
_Water_, _Glass_, &c. and to resemble most those opacous and strong
reflecting bodies of Metals: So also as to the property of cohesion or
congruity, Water seems to keep the same order, being more congruous to
Glass then Air, and Air then Quicksilver.

A Second thing (which was hinted to me, by the consideration of the
included fluids globular form, caused by the protrusion of the ambient
heterogeneous fluid) was, whether the _Phaenomena_ of gravity might not by
this means be explained, by supposing the _Globe_ of Earth, Water, and Air
to be included with a _fluid_, heterogeneous to all and each of them, so
subtil, as not only to be every where _interspersed_ through the _Air_, (or
rather the _air_ through it) but to _pervade_ the bodies of _Glass_, and
even the _closest Metals_, by which means it may endeavour to _detrude_ all
earthly bodies as far from it as it can; and partly thereby, and partly by
other of its properties may move them towards the Center of the Earth. Now
that there is some such fluid, I could produce many Experiments and
Reasons, that do seem to prove it: But because it would ask some time and
room to set them down and explain them, and to consider and answer all the
Objections (many whereof I foresee) that may be alledged against it; I
shall at present proceed to other _Queries_, contenting my self to have
here only given a hint of what I may say more elswhere.

A Third _Query_ then was, Whether the _heterogeneity_ of the _ambient
fluid_ may not be accounted a _secondary cause_ of the _roundness_ or
_globular form_ of the _greater bodies_ of the world, such as are those of
the _Sun_, _Stars_, and _Planets_, the _substance_ of each of which seems
altogether _heterogeneous_ to the _circum-ambient fluid aether_? And of
this I shall say more in the Observation of the Moon.

A Fourth was, Whether the _globular form_ of the _smaller parcels_ of
matter here upon the _Earth_, as that of _Fruits_, _Pebbles_, or _Flints_,
&c. (which seem to have been a _Liquor_ at first) may not be caused by the
_heterogeneous ambient fluid_. For thus we see that melted _Glass_ will be
naturally formed into a _round Figure_; so likewise any small Parcel of any
_fusible body_, if it be perfectly enclosed by the _Air_, will be driven
into a _globular_ Form; and, when cold, will be found a _solid Ball_. This
is plainly enough manifested to us by their way of making _shot_ with the
_drops of Lead_; which being a very pretty curiosity, and known but to a
very few, and having the liberty of publishing it granted me, by that
_Eminent Virtuoso_ Sir _Robert Moray_, who brought in this Account of it to
the _Royal Society_, I have here transcribed and inserted.

To make small shot of different sizes; Communicated by his Highness _P.R._

_Take Lead out of the Pig what quantity you please, melt it down, stir and
clear it with an iron Ladle, gathering together the blackish parts that
swim at top like scum, and when you see the colour of the clear Lead to be
greenish, but no sooner, strew upon it _Auripigmentum_ powdered according
to the quantity of Lead, about as much as will lye upon a half Crown piece
will serve for eighteen or twenty pound weight of some sorts of Lead;
others will require more, or less. After the _Auripigmentum_ is put in,
stir the Lead well, and the _Auripigmentum_ will flame: when the flame is
over, take out some of the Lead in a Ladle having a lip or notch in the
brim for convenient pouring out of the Lead, and being well warmed amongst
the melted Lead, and with a stick make some single drops of Lead trickle
out of the Ladle into water in a Glass, which if they fall to be round and
without tails, there is _Auripigmentum_ enough put in, and the temper of
the heat is right, otherwise put in more. Then lay two bars of Iron (or
some more proper Iron-tool made on purpose) upon a Pail of water, and place
upon them a round Plate of Copper, of the size and figure of an ordinary
large Pewter or Silver Trencher, the hollow whereof is to be about three
inches over, the bottom lower then the brims about half an inch, pierced
with thirty, forty, or more small holes; the smaller the holes are, the
smaller the shot will be; and the brim is to be thicker then the bottom, to
conserve the heat the better._

_The bottom of the Trencher being some four inches distant from the water
in the Pail, lay upon it some burning Coles, to keep the Lead melted upon
it. Then with the hot Ladle take Lead off the Pot where it stands melted,
and pour it softly upon the burning Coles over the bottom of the Trencher,
and it will immediately run through the holes into the water in small round
drops. Thus pour on new Lead still as fast as it runs through the Trencher
till all be done; blowing now and then the Coles with hand-Bellows, when
the Lead in the Trencher cools so as to stop from running._

_While one pours on the Lead, another must, with another Ladle, thrusted
four or five inches under water in the Pail, catch from time to time some
of the shot, as it drops down, to see the size of it, and whether there be
any faults in it. The greatest care is to keep the Lead upon the Trencher
in the right degree of heat; if it be too cool, it will not run through the
Trencher, though it stand melted upon it; and this is to be helped by
blowing the Coals a little, or pouring on new Lead that is hotter: but the
cooler the Lead, the larger the Shot; and the hotter, the smaller; when it
it too hot, the drops will crack and fly; then you must stop pouring on new
Lead, and let it cool; and so long as you observe the right temper of the
heat, the Lead will constantly drop into very round Shot, without so much
as one with a tail in many pounds._

_When all is done, take your Shot out of the Pail of water, and put it in a
Frying-pan over the fire to dry them, which must be done warily, still
shaking them that they melt not; and when they are dry you may separate the
small from the great, in Pearl Sives made of Copper or Lattin let into one
another, into as many sizes at you please. But if you would have your Shot
larger then the Trencher makes them, you may do it with a Stick, making
them trickle out of the Ladle, as hath been said._

_If the Trencher be but toucht a very little when the Lead stops from going
through it, and be not too cool, it will drop again, but it it better not
to touch it at all. At the melting of the Lead take care that there be no
kind of Oyl, Grease, or the like, upon the Pots, or Ladles, or Trencher._

_The Chief cause of this Globular Figure of the Shot, seems to be the
_Auripigmentum_; for, as soon as it is put in among the melted Lead, it
loses its shining brightness, contracting instantly a grayish film or skin
upon it, when you scum it to make it clean with the Ladle. So that when the
Air comes at the falling drop of the melted Lead, that skin constricts them
every where equally: but upon what account, and whether this be the true
cause, is left to further disquisition._

Much after this same manner, when the Air is exceeding cold through which
it passes; do we find the drops of Rain, falling from the Clouds, congealed
into round Hail-stones by the freezing Ambient.

To which may be added this other known Experiment, That if you gently let
fall a drop of _water_ upon small _sand_ or _dust_, you shall find, as it
were, an artificial _round stone_ quickly generated. I cannot upon this
occasion omit the mentioning of the strange kind of _Grain_, which I have
observed in a _stone_ brought from _Kettering_ in _Northamptonshire_, and
therefore called by Masons _Kettering-Stone_, of which see the Description.
Which brings into my mind what I long since observed in the fiery Sparks
that are struck out of a Steel. For having a great desire to see what was
left behind, after the Spark was gone out, I purposely struck fire over a
very white piece of Paper, and observing diligently where some conspicuous
sparks went out, I found a very little black spot no bigger then the point
of a Pin, which through a _Microscope_ appeared to be a perfectly round
Ball, looking much like a polisht ball of Steel, insomuch that I was able
to see the Image of the window reflected from it. I cannot here stay
(having done it more fully in another place) to examine the particular
Reasons of it, but shall only hint, that I imagine it to be some small
parcel of the Steel, which by the violence of the motion of the stroke
(most of which seems to be imprest upon those small parcels) is made so
glowing hot, that it is melted into a _Vitrum_, which by the ambient Air is
thrust into the form of a Ball.

A Fifth thing which I thought worth Examination was, Whether the motion of
all kind of Springs, might not be reduced to the Principle whereby the
included _heterogeneous fluid_ seems to be moved; or to that whereby two
Solids, as Marbles, or the like, are thrust and kept together by the
_ambient fluid_.

A Sixth thing was, Whether the Rising and Ebullition of the Water out of
Springs and Fountains (which lie much higher from the Center of the Earth
then the Superficies of the Sea, from whence it seems to be derived) may
not be explicated by the rising of Water in a smaller Pipe: For the
Sea-water being strained through the Pores or Crannies of the Earth, is, as
it were, included in little Pipes, where the pressure of the Air has not so
great a power to resist its rising: But examining this way, and finding in
it several difficulties almost irremovable, I thought upon a way that would
much more naturally and conceivably explain it, which was by this following
Experiment: I took a Glass-Tube, of the form of that described in the sixth
Figure, and chusing two _heterogeneous fluids_, such as Water and Oyl, I
poured in as much Water as filled up the Pipes as high as AB, then putting
in some Oyl into the Tube AC, I deprest the superficies A of the Water to
F, and B I raised to G, which was not so high perpendicularly as the
superficies of the Oyl F, by the space FI, wherefore the proportion of the
gravity of these two Liquors was as GH to FE.

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