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Micrographia by Robert Hooke

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The ends of others of these drops I nipt off whilst all the bodies and ends
of them lay buried under the water, which, like the former, flew all to
pieces with as brisk a noise, and as strong a motion.

Others of these I tried to break, by grinding away the blunt end, and
though I took a seemingly good one, and had ground away neer two thirds of
the Ball, yet would it not fly to pieces, but now and then some small rings
of it would snap and fly off, not without a brisk noise and quick motion,
leaving the Surface of the drop whence it flew very prettily branched or
creased, which was easily discoverable by the _Microscope_. This drop,
after I had thus ground it, without at all impairing the remnant that was
not ground away, I caused to fly immediately all into sand upon the nipping
off the very tip of its slender end.

Another of these drops I began to grind away at the smaller end, but had
not worn away on the stone above a quarter of an inch before the whole drop
flew with a brisk crack into sand or small dust; nor would it have held so
long, had there not been a little flaw in the piece that I ground away, as
I afterwards found.

Several others of these drops I covered over with a thin but very tuff skin
of _Icthyocolla_, which being very tough and very transparent, was the most
convenient substance for these tryals that I could imagine, having dipt, I
say, several of these drops in this transparent Glue whilst hot, and
suffering them to hang by a string tied about the end of them till they
were cold, and the skin pretty tough; then wrapping all the body of the
drop (leaving out only the very tip) in fine supple Kids-leather very
closely, I nipped off the small top, and found, as I expected, that
notwithstanding this skin of Glue, and the close wrapping up in Leather,
upon the breaking of the top, the drop gave a crack like the rest, and gave
my hand a pretty brisk impulse: but yet the skin and leather was so strong
as to keep the parts from flying out of their former posture; and, the skin
being transparent, I found that the drop retained exactly its former figure
and polish, but was grown perfectly opacous and all over flaw'd, all those
flaws lying in the manner of rings, from the bottom or blunt end, to the
very top or small point. And by several examinations with a _Microscope_,
of several thus broken, I found the flaws, both within the body of the
drop, and on the outward surface, to lye much in this order.

Let AB in the Figure X of the fourth Scheme represent the drop cased over
with _Icthyocolla_ or _Isinglass_, (by being ordered as is before
prescribed) crazed or flawed into pieces, but by the skin or case kept in
its former figure, and each of its flawed parts preserved exactly in its
due posture; the outward appearance of it somewhat plainly to the naked
eye, but much more conspicuous if viewed with a small lens appeared much
after this shape. That is, the blunt end B for a pretty breadth, namely, as
far as the Ring CCC seemed irregularly flawed with divers clefts, which all
seemed to tend towards the Center of it, being, as I afterwards found, and
shall anon shew in the description of the figure Y, the Basis, as it were,
of a Cone, which was terminated a little above the middle of the drop, all
the rest of the Surface from CCC to A was flawed with an infinite number of
small and parallel Rings, which as they were for the most part very round,
so were they very thick and close together, but were not so exactly flaw'd
as to make a perfect Ring, but each circular part was by irregular cracks
flawed likewise into multitudes of irregular flakes or tiles; and this
order was observed likewise the whole length of the neck.

Now though I could not so exactly cut this _conical Body_ through the
_Axis_, as is represented by the figure Y; yet by _anatomizing_, as it
were, of several, and taking notice of divers particular circumstances, I
was informed, that could I have artificially divided a flaw'd drop through
the _Axis_ or _Center_, I should with a _Microscope_ have found it to
appear much of this form, where A signifies the _Apex_, and B the blunt
end, CC the Cone of the Basis, which is terminated at T the top or end of
it, which seems to be the very middle of the blunt end in which, not only
the conical body of the Basis CC is terminated, but as many of the parts of
the drop as reach as high as DD.

And it seemed to be the head or beginning of a Pith, as it were, or a part
of the body which seemed more spungy then the rest, and much more
irregularly flawed, which from T ascended by EE, though less visible, into
the small neck towards A. The Grain, as it were, of all the flaws, that
proceeds from all the outward Surface ADCCDA, was much the same, as is
represented by the black strokes that meet in the middle DT, DT, DE, DE,
&c.

Nor is this kind of Grain, as I may call it, peculiar to Glass drops thus
quenched; for (not to mention _Coperas-stones_, and divers other
_Marchasites_ and _Minerals_, which I have often taken notice of to be in
the very same manner flaked or grained, with a kind of Pith in the middle)
I have observed the same in all manner of cast Iron, especially the coarser
sort, such as Stoves, and Furnaces, and Backs, and Pots are made of: For
upon the breaking of any of those Substances it is obvious to observe, how
from the out-sides towards the middle, there is a kind of Radiation or
Grain much resembling this of the Glass-drop; but this Grain is most
conspicuous in Iron-bullets, if they be broken: the same _Phaenomena_ may
be produced by casting _regulus_ of _Antimony_ into a Bullet-mold, as also
with _Glass of Antimony_, or with almost any such kind of _Vitrified
substance_, either cast into a cold Mold or poured into Water.

Others of these Drops I heat red hot in the fire, and then suffered them to
cool by degrees. And these I found to have quite lost all their
_fulminating_ or flying quality, as also their hard, brittle and springy
texture; and to emerge of a much softer temper, and much easier to be
broken or snapt with ones finger; but its strong and brittle quality was
quite destroyed, and it seemed much of the same consistence with other
green Glass well nealed in the Oven.

The Figure and bigness of these for the most part was the same with that of
the Figure Z; that is, all the surface of them was very smooth and polisht,
and for the most part round, but very rugged or knobbed about D, and all
the length of the stem was here and there pitted or flatted. About D, which
is at the upper part of the drop under that side of the stem which is
concave, there usually was made some one or more little Hillocks or
Prominences. The drop it self, before it be broken, appears very
transparent, and towards the middle of it, to be very full of small
Bubbles, of some kind of aerial substance, which by the refraction of the
outward surface appear much bigger then really they are, and this may be in
good part removed, by putting the drop under the surface of clear Water,
for by that means most part of the refraction of the convex Surface of the
drop is destroyed, and the bubbles will appear much smaller. And this, by
the by, minds me of the appearing magnitude of the _aperture_ of the
_iris_, or _pupil_ of the eye, which though it appear, and be therefore
judged very large, is yet not above a quarter of the bigness it appears of,
by the _lenticular_ refraction of the _Cornea_.

The cause of all which _Phaenomena_ I imagine to be no other then this,
That the Parts of the Glass being by the excessive heat of the fire kept
off and separated one from another, and thereby put into a kind of sluggish
fluid consistence, are suffered to drop off with that heat or agitation
remaining in them, into cold Water; by which means the outsides of the drop
are presently cool'd and _crusted_, and are thereby made of a loose
texture, because the parts of it have not time to settle themselves
leisurely together, and so to lie very close together: And the innermost
parts of the drop, retaining still much of their former heat and
agitations, remain of a loose texture also, and, according as the cold
strikes inwards from the bottom and sides, are quenched, as it were, and
made rigid in that very posture wherein the cold finds them. For the parts
of the _crust_ being already hardened, will not suffer the parts to shrink
any more from the outward Surface inward; and though it shrink a little by
reason of the small parcels of some Aerial substances dispersed through
the matter of the Glass, yet that is not neer so much as it appears (as I
just now hinted;) nor if it were, would it be sufficient for to consolidate
and condense the body of Glass into a _tuff_ and close _texture_, after it
had been so excessively rarified by the heat of the glass-Furnace.

But that there may be such an expansion of the aerial substance contained
in those little _blebbs_ or bubbles in the body of the drop, this following
Experiment will make more evident.

* * * * *

Take a small Glass-Cane about a foot long, seal up one end of it
_hermetically_, then put in a very small bubble of Glass, almost of the
shape of an Essence-viol with the open mouth towards the sealed end, then
draw out the other end of the Pipe very small, and fill the whole Cylinder
with water, then set this Tube by the Fire till the Water begin to boyl,
and the Air in the bubble be in good part rarified and driven out, then by
sucking at the smalling Pipe, more of the Air or vapours in the bubble may
be suck'd out, so that it may sink to the bottom; when it is sunk to the
bottom, in the flame of a Candle, or Lamp, nip up the slender Pipe and let
it cool: whereupon it is obvious to observe, first, that the Water by
degrees will subside and shrink into much less room: Next, that the Air or
vapours in the Glass will expand themselves so, as to buoy up the little
Glass: Thirdly, that all about the inside of the Glass-pipe there will
appear an infinite number of small bubbles, which as the Water grows colder
and colder will swell bigger and bigger, and many of them buoy themselves
up and break at the top.

From this _Disceding_ of the heat in Glass drops, that is, by the quenching
or cooling Irradiations propagated from the Surface upwards and inwards, by
the lines CT, CT, DT, DE, &c. the bubbles in the drop have room to expand
themselves a little, and the parts of the Glass contract themselves; but
this operation being too quick for the sluggish parts of the Glass, the
contraction is performed very unequally and irregularly, and thereby the
Particles of the Glass are bent, some one way, and some another, yet so as
that most of them draw towards the Pith or middle TEEE, or rather from that
outward: so that they cannot _extricate_ or unbend themselves, till some
part of TEEE be broken and loosened, for all the parts about that are
placed in the manner of an Arch, and so till their hold at TEEE be loosened
they cannot fly asunder, but uphold, and shelter, and fix each other much
like the stones in a Vault, where each stone does concurre to the stability
of the whole Fabrick, and no one stone can be taken away but the whole Arch
falls. And wheresoever any of those radiating wedges DTD, &c. are removed,
which are the component parts of this Arch, the whole Fabrick presently
falls to pieces; for all the Springs of the several parts are set at
liberty, which immediately extricate themselves and fly asunder every way;
each part by its spring contributing to the darting of it self and some
other contiguous part. But if this drop be heat so hot as that the parts by
degrees can unbend themselves, and be settled and annealed in that posture,
and be then suffered gently to subside and cool; The parts by this nealing
losing their springiness, constitute a drop of a more soft but less brittle
texture, and the parts being not at all under a flexure, though any part of
the middle or Pith TEEE be broken, yet will not the drop at all fly to
pieces as before.

This Conjecture of mine I shall indeavour to make out by explaining each
particular Assertion with _analogous_ Experiments: The Assertions are
there.

First, That the parts of the Glass, whilst in a fluid Consistence and
hot, are more rarified, or take up more room, then when hard and cold.

Secondly, That the parts of the drop do suffer a twofold contraction.

Thirdly, That the dropping or quenching the glowing metal in the Water
makes it of a hard, springing, and rarified texture.

Fourthly, That there is a flexion or force remaining upon the parts of
the Glass thus quenched, from which they indeavour to extricate
themselves.

Fifthly, That the Fabrick of the drop, that is able to hinder the parts
from extricating themselves, is _analogus_ to that of an Arch.

Sixthly, That the sudden flying asunder of the parts proceeds from
their springiness.

Seventhly, That a gradual heating and cooling does anneal or reduce the
parts of Glass to a texture that is more loose, and easilier to be
broken, but not so brittle.

That the first of these is true may be gathered from this, That _Heat is a
property of a body arising from the motion or agitation of its parts_; and
therefore whatever body is thereby toucht must necessarily receive some
part of that motion, whereby its parts will be shaken and agitated, and so
by degrees free and extricate themselves from one another, and each part so
moved does by that motion _exert_ a _conatus_ of _protruding_ and
displacing all the adjacent Particles. Thus Air included in a vessel, by
being heated will burst it to pieces. Thus have I broke a Bladder held over
the fire in my hand, with such a violence and noise, that it almost made me
deaf for the present, and much surpassed the noise of a Musket: The like
have I done by throwing into the fire small glass Bubbles hermetically
sealed, with a little drop of Water included in them. Thus Water also, or
any other Liquor, included in a convenient vessel, by being warmed,
manifestly expands it self with a very great violence, so as to break the
strongest vessel, if when heated it be narrowly imprisoned in it. This is
very manifest by the _Sealed Thermometers_, which I have, by several
tryals, at last brought to a great certainty and tenderness: for I have
made some with stems above four foot long, in which the expanding Liquor
would so far vary, as to be very neer the very top in the heat of Summer,
and prety neer the bottom at the coldest time of the Winter. The Stems I
use for them are very thick, straight, and even Pipes of Glass, with a very
small _perforation_, and both the head and body I have made on purpose at
the Glass-house, of the same metal whereof the Pipes are drawn: these I can
easily in the flame of a Lamp, urged with the blast of a pair of Bellows,
seal and close together, so as to remain very firm, close and even; by this
means I joyn on the body first, and then fill both it and a part of the
stem, proportionate to the length of the stem and the warmth of the season
I fill it in with the best rectified _Spirit of Wine_ highly _ting'd_ with
the lovely colour of _Cocheneel_, which I deepen the more by pouring some
drops of common _Spirit of Urine_, which must not be too well rectified,
because it will be apt to make the Liquor to curdle and stick in the small
perforation of the stem. This Liquor I have upon tryal found the most
tender of any spirituous Liquor, and those are much more sensibly affected
with the variations of heat and cold then other more flegmatick and
ponderous Liquors, and as capable of receiving a deep tincture, and keeping
it, as any Liquor whatsoever; and (which makes it yet more acceptable) is
not subject to be frozen by any cold yet known. When I have thus filled it,
I can very easily in the forementioned flame of a Lamp seal and joyn on the
head of it.

Then, for graduating the stem, I fix that for the beginning of my division
where the surface of the liquor in the stem remains when the ball is placed
in common distilled water, that is so cold that it just begins to freeze
and shoot into flakes; and that mark I fix at a convenient place of the
stem, to make it capable of exhibiting very many degrees of cold, below
that which is requisite to freeze water: the rest of my divisions, both
above and below this (which I mark with a [0] or nought) I place according
to the Degrees of _Expansion_, or _Contraction_ of the Liquor in proportion
to the bulk it had when it indur'd the newly mention'd freezing cold. And
this may be very easily and accurately enough done by this following way;
Prepare a Cylindrical vessel of very thin plate Brass or Silver, ABCD of
the figure Z; the Diameter AB of whose cavity let be about two inches, and
the depth BC the same; let each end be cover'd with a flat and smooth plate
of the same substance, closely soder'd on, and in the midst of the upper
cover make a pretty large hole EF, about the bigness of a fifth part of the
Diameter of the other; into this fasten very well with cement a straight
and even Cylindrical pipe of Glass, EFGH, the Diameter of whose cavity let
be exactly one tenth of the Diameter of the greater Cylinder. Let this pipe
be mark'd at GH with a Diamant, so that G from E may be distant just two
inches, or the same height with that of the cavity of the greater Cylinder,
then divide the length EG exactly into 10 parts, so the capacity of the
hollow of each of these divisions will be 1/1000 part of the capacity of
the greater Cylinder. This vessel being thus prepared, the way of marking
and graduating the _Thermometers_ may be very easily thus performed:

Fill this Cylindrical vessel with the same liquor wherewith the
_Thermometers_ are fill'd, then place both it and the _Thermometer_ you are
to _graduate_, in water that is ready to be frozen, and bring the surface
of the liquor in the _Thermometer_ to the first marke or [0]; then so
proportion the liquor in the Cylindrical vessel, that the surface of it may
just be at the lower end of the small glass-Cylinder; then very gently and
gradually warm the water in which both the _Thermometer_ and this
Cylindrical vessel stand, and as you perceive the ting'd liquor to rise in
both stems, with the point of a Diamond give several marks on the stem of
the _Thermometer_ at those places, which by comparing the expansion in both
Stems, are found to correspond to the divisions of the cylindrical vessel,
and having by this means marked some few of these divisions on the Stem, it
will be very easie by these to mark all the rest of the Stem, and
accordingly to assign to every division a proper character.

A _Thermometer_, thus marked and prepared, will be the fittest Instrument
to make a Standard of heat and cold that can be imagined. For being sealed
up, it is not at all subject to variation or wasting, nor is it liable to
be changed by the varying pressure of the Air, which all other kind of
_Thermometers_ that are open to the Air are liable to. But to proceed.

This property of Expansion with Heat, and Contraction with Cold, is not
peculiar to Liquors only, but to all kind of solid Bodies also, especially
Metals, which will more manifestly appear by this Experiment.

Take the Barrel of a Stopcock of Brass, and let the Key, which is well
fitted to it, be riveted into it, so that it may slip, and be easily turned
round, then heat this Cock in the fire, and you will find the Key so
swollen, that you will not be able to turn it round in the Barrel; but if
it be suffered to cool again, as soon as it is cold it will be as movable,
and as easie to be turned as before.

This Quality is also very observable in _Lead_, _Tin_, _Silver_,
_Antimony_, _Pitch_, _Rosin_, _Bees-wax_, _Butter_, and the like; all
which, if after they be melted you suffer gently to cool, you shall find
the parts of the upper Surface to subside and fall inwards, losing that
plumpness and smoothness it had whilst in fusion. The like I have also
observed in the cooling of _Glass of Antimony_, which does very neer
approach the nature of Glass,

But because these are all Examples taken from other materials then Glass,
and argue only, that possibly there may be the like property also in Glass,
not that really there is; we shall by three or four Experiments indeavour
to manifest that also.

And the First is an Observation that is very obvious even in these very
drops, to wit, that they are all of them terminated with an unequal or
irregular Surface, especially about the smaller part of the drop, and the
whole length of the stem; as about D, and from thence to A, the whole
Surface, which would have been round if the drop had cool'd leisurely, is,
by being quenched hastily, very irregularly flatted and pitted; which I
suppose proceeds partly from the Waters unequally cooling and pressing the
parts of the drop, and partly from the self-contracting or subsiding
quality of the substance of the Glass: For the vehemency of the heat of the
drop causes such hidden motions and bubbles in the cold Water, that some
parts of the Water bear more forcibly against one part then against
another, and consequently do more suddenly cool those parts to which they
are contiguous.

A Second Argument may be drawn from the Experiment of cutting Glasses with
a hot Iron. For in that Experiment the top of the Iron heats, and thereby
rarifies the parts of the Glass that lie just before the crack, whence each
of those agitated parts indeavouring to expand its self and get elbow-room,
thrusts off all the rest of the contiguous parts, and consequently promotes
the crack that was before begun.

A Third Argument may be drawn from the way of producing a crack in a sound
piece or plate of Glass, which is done two wayes, either First, by suddenly
heating a piece of Glass in one place more then in another. And by this
means _chymists_ usually cut off the necks of Glass-bodies, by two kinds of
Instruments, either by a glowing hot round Iron-Ring, which just
incompasses the place that is to be cut, or else by a _Sulphur'd_ Threed,
which is often wound about the place where the separation is to be made,
and then fired. Or Secondly, A Glass may be cracked by cooling it suddenly
in any place with Water, or the like, after it has been all leisurely and
gradually heated very hot. Both which _Phaenomena_ seem manifestly to
proceed from the _expansion_ and contraction of the parts of the Glass,
which is also made more probable by this circumstance which I have
observed, that a piece of common window-glass being heated in the middle
very suddenly with a live Coal or hot Iron, does usually at the first crack
fall into pieces, whereas if the Plate has been gradually heated very hot,
and a drop of cold Water and the like be put on the middle of it, it only
flaws it, but does not break it asunder immediately.

A Fourth Argument may be drawn from this Experiment; Take a Glass-pipe, and
fit into a solid stick of Glass, so as it will but just be moved in it.
Then by degrees heat them whilst they are one within another, and they will
grow stiffer, but when they are again cold, they will be as easie to be
turned as before. This Expansion of Glass is more manifest in this
Experiment.

Take a stick of Glass of a considerable length, and fit it so between the
two ends or screws of a Lath, that it may but just easily turn, and that
the very ends of it may be just toucht and susteined thereby; then applying
the flame of the Candle to the middle of it, and heating it hot, you will
presently find the Glass to stick very fast on those points, and not
without much difficulty to be convertible on them, before that by removing
the flame for a while from it, it be suffered to cool, and when you will
find it as easie to be turned round as at the first.

From all which Experiments it is very evident, that all those Bodies, and
particularly Glass, suffers an Expansion by Heat, and that a very
considerable one, whilst they are in a state of Fusion. For _Fluidity_, as
I elsewhere mention, _being nothing but an effect of very strong and quick
shaking motion, whereby the parts are, as it were, loosened from each
other, and consequently leave an interjacent space or vacuity_; it follows,
that all those shaken Particles must necessarily take up much more room
then when they were at rest, and lay quietly upon each other. And this is
further confirmed by a Pot of _boyling Alabaster_, which will manifestly
rise a sixth or eighth part higher in the Pot, whilst it is boyling, then
it will remain at, both before and after it be boyled. The reason of which
odd _Phaenomenon_ (to hint it here only by the way) is this, that there is
in the curious powder of Alabaster, and other calcining Stones, a certain
watery substance, which is so fixt and included with the solid Particles,
that till the heat be very considerable they will not fly away; but after
the heat is increased to such a degree, they break out every way in
vapours, and thereby so shake and loosen the small corpusles of the Powder
from each other, that they become perfectly of the nature of a fluid body,
and one may move a stick to and fro through it, and stir it as easily as
water, and the vapours burst and break out in bubbles just as in boyling
water, and the like; whereas, both before those watery parts are flying
away, and after they are quite gone; that is, before and after it have done
boyling, all those effects cease, and a stick is as difficultly moved to
and fro in it as in sand, or the like. Which Explication I could easily
prove, had I time; but this is not a fit place for it.

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