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Micrographia by Robert Hooke

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This Experiment I tried with several other Liquors, and particularly with
fresh Water and Salt (which I made by dissolving Salt in warm Water) which
two though they are nothing heterogeneous, yet before they would perfectly
mix one with another, I made trial of the Experiment: Nay, letting the Tube
wherein I tried the Experiment remain for many dayes, I observed them not
to mix; but the superficies of the fresh was rather more then less elevated
above that of the Salt. Now the proportion of the gravity of Sea-water, to
that of River-water, according to _Stevinus_ and _Varenius_, and as I have
since found pretty true by making trial my self, is as 46. to 45. that is,
46. Ounces of the salt Water will take up no more room then 45. of the
fresh. Or reciprocally 45 pints of salt-water weigh as much as 46 of fresh.

But I found the proportion of Brine to fresh Water to be near 13 to 12:
Supposing therefore GHM to represent the Sea, and FI the height of the
Mountain above the Superficies of the Sea, FM a Cavern in the Earth,
beginning at the bottom of the Sea, and terminated at the top of the
Mountain, LM the Sand at the bottom, through which the Water is as it were
strained, so as that the fresher parts are only permitted to transude, and
the saline kept back; if therefore the proportion of G M to FM be as 45 to
46, then may the Cylinder of Salt-water GM make the Cylinder of Fresh-water
to rise as high as E, and to run over at N. I cannot here stand to examine
or confute their Opinion, who make the depth of the Sea, below its
Superficies, to be no more perpendicularly measured then the height of the
Mountains above it: 'Tis enough for me to say, there is no one of those
that have asserted it, have experimentally known the perpendicular of
either; nor shall I here determine, whether there may not be many other
causes of the separation of the fresh water from the salt, as perhaps some
parts of the Earth through which it is to pass, may contain a Salt, that
mixing and uniting with the Sea-salt, may precipitate it; much after the
same manner as the _Alkalizate_ and _Acid Salts_ mix and precipitate each
other in the preparation of _Tartarum Vitriolatum._ I know not also whether
the exceeding cold (that must necessarily be) at the bottom of the Water,
may not help towards this separation, for we find, that warm Water is able
to dissolve and contain more Salt, then the same cold; insomuch that Brines
strongly impregnated by heat, if let cool, do suffer much of their Salt to
subside and crystallize about the bottom and sides. I know not also whether
the exceeding pressure of the parts of the Water one against another, may
not keep the Salt from descending to the very bottom, as finding little or
no room to insert it self between those parts, protruded so violently
together, or else squeeze it upwads into the superiour parts of the Sea,
where it may more easily obtain room for it self, amongst the parts of the
Water, by reason that there is more heat and less pressure. To this Opinion
I was somewhat the more induced by the relations I have met with in
_Geographical Writers_, of drawing fresh Water from the bottom of the Sea,
which is salt above. I cannot now stand to examine, whether this natural
perpetual motion may not artificially be imitated: Nor can I stand to
answer the Objections which may be made against this my Supposition: As,
First, How it comes to pass, that there are sometimes salt Springs much
higher then the Superficies of the Water? And, Secondly, Why Springs do not
run faster and slower, according to the varying height made of the Cylinder
of Sea-water, by the ebbing and flowing of the Sea?

As to the First, In short, I say, the fresh Water may receive again a
saline Tincture near the Superficies of the Earth, by passing through some
salt _Mines_, or else many of the saline parts of the Sea may be kept back,
though not all.

And as to the Second, The same _Spring_ may be fed and supplyed by divers
_Caverns_, coming from very far distant parts of the _Sea_, so as that it
may in one place be _high_, in another _low water_; and so by that means
the _Spring_ may be equally supply'd at all times. Or else the _Cavern_ may
be so straight and narrow, that the water not having so ready and free
passage through it, cannot upon so short and quick mutations of pressure,
be able to produce any sensible effect at such a distance. Besides that, to
confirm this _hypothesis_, there are many _Examples_ found in _Natural
Historians_, of _Springs_ that do ebb and flow like the Sea: As
particularly, those recorded by the Learned _Camden_, and after him by
_Speed,_ to be found in this _Island_: One of which, they relate to be on
the Top of a Mountain, by the small Village _Kilken_ in _Flintshire_,
_Maris aemulus qui statis temporibus suos evomit & resorbet Aquas_; Which
at certain times riseth and falleth after the manner of the Sea. A Second
in _Caermardenshire,_ near _Caermarden_, at a place called _Cantred
Bichan_; _Qui (ut scribit Giraldus) naturali die bis undis deficiens, &
toties exuberans, marinas imitatur instabilitates_; That twice in four and
twenty hours ebbing and flowing; resembleth the unstable motions of the
Sea. The _Phaenomena_ of which two may be easily made out, by supposing the
_Cavern_, by which they are fed, to arise from the bottom of the next Sea.
A Third, is a Well upon the River _Ogmore_ in _Glamorganshire_, and near
unto _Newton_, of which _Camden_ relates himself to be certified, by a
Letter from a Learned Friend of his that observed it, _Fons abest hinc,
&c._ The Letter is a little too long to be inserted, but the substance is
this; That this Well ebbs and flows quite contrary to the flowing and
ebbing of the Sea in those parts: for 'tis almost empty at Full Sea, but
full at Low water. This may happen from the Channel by which it is
supplied, which may come from the bottom of a Sea very remote from those
parts, and where the Tides are much differing from those of the approximate
shores. A Fourth, lies in _Westmorland_, near the River _Leder_; _Qui
instar Euripi saepius in die reciprocantibus undis fluit & refluit_, which
ebbs and flows many times a day. This may proceed from its being supplyed
from many Channels, coming from several parts of the Sea, lying
sufficiently distant asunder to have the times of High-water differing
enough one from the other; so as that whensoever it shall be High water
over any of those places, where these Channels begin, it shall likewise be
so in the Well; but this is but a supposition.

A Seventh _Query_ was, Whether the _dissolution_ or mixing of several
bodies, whether fluid or solid, with saline or other Liquors, might not
partly be attributed to this Principle of the congruity of those bodies and
their dissolvents? As of Salt in Water, Metals in several _Menstruums_,
Unctuous Gums in Oyls, the mixing of Wine and Water, &c. And whether
_precipitation_ be not partly made from the same Principle of Incongruity?
I say _partly_, because there are in some Dissolutions, some other Causes
concurrent.

I shall lastly make a much more seemingly strange and unlikely _Query_; and
that is, Whether this Principle, well examined and explained, may not be
found a _co-efficient_ in the most considerable Operations of Nature? As in
those of _Heat_, and _Light_ consequently of _Rarefaction_ and
_Condensation_, _Hardness_, and _Fluidness_, _Perspicuity_ and
_Opacousness_, _Refractions_ and _Colours. &c._ Nay, I know not whether
there may be many things done in Nature, in which this may not (be said to)
have a Finger? This I have in some other passages of this Treatise further
enquired into and shewn, that as well _Light_ as _Heat_ may be caused by
_corrosion_, which is applicable to _congruity_, and consequently all the
rest will be but _subsequents_: In the mean time I would not willingly be
guilty of that _Error_, which the thrice Noble and Learned _Verulam_ justly
takes notice of, as such, and calls _Philosophiae Genus Empiricum, quod in
paucorum Experimentorum Angustiis & Obscuritate fundatum est_. For I
neither conclude from one single Experiment, nor are the Experiments I make
use of all made upon one Subject: Nor wrest I any Experiment to make it
_quadrare_ with any preconceiv'd Notion. But on the contrary, I endeavour
to be conversant in divers kinds of Experiments, and all and every one of
those Trials, I make the Standards or Touchstones, by which I try all my
former Notions, whether they hold out in weight, and measure, and touch,
&c. For as that Body is no other then a Counterfeit Gold, which wants any
one of the Proprieties of Gold, (such as are the Malleableness, Weight,
Colour, Fixtness in the Fire, Indissolubleness in _Aqua fortis_, and the
like) though it has all the other; so will all those Notions be found to be
false and deceitful, that will not undergo all the Trials and Tests made of
them by Experiments. And therefore such as will not come up to the desired
_Apex_ of Perfection, I rather wholly reject and take new, then by piecing
and patching, endeavour to retain the old, as knowing such things at best
to be but lame and imperfect. And this course I learned from Nature; whom
we find neglectful of the old Body, and suffering its Decaies and
Infirmities to remain without repair, and altogether sollicitous and
careful of perpetuating the _Species_ by new _Individuals_. And it is
certainly the most likely way to erect a glorious Structure and Temple to
_Nature_, such as she will be found (by any _zealous Votary_) to reside in;
to begin to build a new upon a sure Foundation of Experiments.

But to digress no further from the consideration of the _Phaenomena,_ more
immediately explicable by this Experiment, we shall proceed to shew, That,
as to the rising of Water in a _Filtre_, the reason of it will be manifest
to him, that does take notice, that a _Filtre_ is constituted of a great
number of small long solid bodies, which lie so close together, that the
Air in its getting in between them, doth lose of its pressure that it has
against the _Fluid_ without them, by which means the Water or Liquor not
finding so strong a resistance between them as is able to counter-ballance
the pressure on its superficies without, is raised upward, till it meet
with a pressure of the Air which is able to hinder it. And as to the Rising
of Oyl, melted Tallow, Spirit of Wine, &c. in the Week of a Candle or Lamp,
it is evident, that it differs in nothing from the former, save only in
this, that in a _Filtre_ the Liquor descends and runs away by another part;
and in the Week the Liquor is dispersed and carried away by the Flame;
something there is ascribable to the Heat, for that it may rarifie the more
volatil and spirituous parts of those combustible Liquors, and so being
made lighter then the Air, it maybe protruded upwards by that more
ponderous fluid body in the Form of Vapours; but this can be ascribed to
the ascension of but a very little, and most likely of that only which
ascends without the Week. As for the Rising of it in a Spunge, Bread,
Cotton, &c. above the superficies of the subjacent Liquor, what has been
said about the _Filtre_ (if considered) will easily suggest a reason,
considering that all these bodies abound with small holes or pores.

From this same Principle also (_viz. the unequal pressure of the Air
against the unequal superficies of the water_) proceeds the cause of the
accession or incursion of any floating body against the sides of the
containing Vessel; or the _appropinquation_ of two floating bodies, as
_Bubbles_, _Corks_, _Sticks_, _Straws_, &c. one towards another. As for
instance, Take a Glass-jar, such as AB in the seventh _Figure_, and filling
it pretty near the top with water, throw into it a small round piece of
Cork, as C, and plunge it all over in water, that it be wet, so as that the
water may rise up by the sides of it, then placing it any where upon the
superficies, about an inch, or one inch and a quarter from any side, and
you shall perceive it by degrees to make _perpendicularly_ toward the
nearest part of the side, and the nearer it approaches, the faster to be
moved, the reason of which _Phaenomenon_ will be found no other then this,
that the Air has a greater pressure against the middle of the
_superficies_, then it has against those parts that approach nearer, and
are _contiguous_ to the sides. Now that the pressure is greater, may (as I
shewed before in the explication of the third _Figure_) be evinced from the
flatting of the water in the middle, which arises from the gravity of the
under _fluid_: for since, as I shewed before, if there were no gravity in
the under _fluid_, or that it were equal to that of the upper, the
terminating Surface would be _Spherical_, and since it is the additional
pressure of the gravity of water that makes it so flat, it follows, that
the pressure upon the middle must be greater then towards the sides. Hence
the Ball having a stronger pressure against that side of it which respects
the middle of the _superficies_, then against that which respects the
_approximate_ side, must necessarily move towards that part, from whence it
finds least resistance, and so be _accelerated_, as the resistance
decrease. Hence the more the water is raised under that part of its way it
is passing above the middle, the faster it is moved: And therefore you will
find it to move faster in E then in D, and in D then in C. Neither could I
find the floating substance to be moved at all, until it were placed upon
some part of the _Superficies_ that was sensibly elevated above the height
of the middle part. Now that this may be the true cause, you may try with a
blown Bladder, and an exactly round Ball upon a very smooth side of some
pliable body, as _Horn_ or _Quicksilver._ For if the Ball be placed under a
part of the Bladder which is upon one side of the middle of its pressure,
and you press strongly against the Bladder, you shall find the Ball moved
from the middle towards the sides.

Having therefore shewn the reason of the motion of any float towards the
sides, the reason of the incursion of any two floating bodies will easily
appear: For the rising of the water against the sides of either of them, is
an Argument sufficient, to shew the pressure of the Air to be there less,
then it is further from it, where it is not so much elevated; and therefore
the reason of the motion of the other toward it, will be the same as
towards the side of the Glass, only here from the same reason, they are
mutually moved toward each other, whereas the side of the Glass in the
former remains fixt. If also you gently fill the Jar so full with water,
that the water is _protuberant_ above the sides, the same piece of Cork
that before did hasten towards the sides, does now fly from it as fast
towards the middle of the Superficies; the reason of which will be found no
other then this, that the pressure of the Air is stronger against the sides
of the Superficies G and H, then against the middle I; for since, as I
shewed before, the Principle of congruity would make the terminating
Surface Spherical, and that the flatting of the Surface in the middle is
from the abatement of the waters pressure outwards, by the contrary
indeavour of its gravity; it follows that the pressure in the middle must
be less then on the sides; and therefore the consecution will be the same
as in the former. It is very odd to one that considers not the reason of
it, to see two floating bodies of wood to approach each other, as though
they were indued with some magnetical vigour; which brings into my mind
what I formerly tried with a piece of Cork or such like body, which I so
ordered, that by putting a little stick into the same water, one part of
the said Cork would approach and make toward the stick, whereas another
would discede and fly away, nay it would have a kind of verticity, so as
that if the _AEquator_ (as I may so speak) of the Cork were placed towards
the stick, if let alone, it would instantly turn its appropriate Pole
toward it, and then run a-tilt at it: and this was done only by taking a
dry Cork, and wetting one side of it with one small stroak; for by this
means gently putting it upon the water, it would depress the superficies on
every side of it that was dry, and therefore the greatest pressure of the
Air, being near those sides, caused it either to chase away, or else to fly
off from any other floating body, whereas that side only, against which the
water ascended, was thereby able to attract.

It remains only, that I should determine how high the Water or other Liquor
may by this means be raised in a smaller Pipe above the Superficies of that
without it, and at what height it may be sustained: But to determine this,
will be exceeding difficult, unless I could certainly know how much of the
Airs pressure is taken off by the smalness of such and such a Pipe, and
whether it may be wholly taken off, that is, whether there can be a hole or
pore so small, into which Air could not at all enter, though water might
with its whole force, for were there such, 'tis manifest, that the water
might rise in it to some five or six and thirty English Foot high. I know
not whether the capillary Pipes in the bodies of small Trees, which we call
their _Microscopical pores_, may not be such; and whether the congruity of
the sides of the Pore may not yet draw the juyce even higher then the Air
was able by its bare pressure to raise it: For, Congruity is a principle
that not only unites and holds a body joyned to it, but, which is more,
attracts and draws a body that is very near it, and holds it above its
usual height.

And this is obvious even in a drop of water suspended under any Similar or
Congruous body: For, besides the ambient pressure that helps to keep it
sustein'd, there is the Congruity of the bodies that are contiguous. This
is yet more evident in Tenacious and Glutinous bodies; such as Gummous
Liquors, Syrups, Pitch, and Rosin melted, &c. Tar, Turpentine, Balsom,
Bird-lime, &c. for there it is evident, that the Parts of the tenacious
body, as I may so call it, do stick and adhere so closely together, that
though drawn out into long and very slender Cylinders, yet they will not
easily relinquish one another; and this, though the bodies be _aliquatenus_
fluid, and in motion by one another, which, to such as consider a fluid
body only as its parts are in a confused irregular motion, without taking
in also the congruity of the parts one among another, and incongruity to
some other bodies, does appear not a little strange. So that besides the
incongruity of the ambient fluid to it, we are to consider also the
congruity of the parts of the contein'd fluid one with another.

And this Congruity (that I may here a little further explain it) is both a
Tenaceous and an Attractive power; for the Congruity, in the Vibrative
motions, may be the cause of all kind of attraction, not only Electrical,
but Magnetical also, and therefore it may be also of Tenacity and
Glutinousness. For, from a perfect congruity of the motions of two distant
bodies, the intermediate fluid particles are separated and droven away from
between them, and thereby those congruous bodies are, by the incompassing
mediums, compell'd and forced neerer together; wherefore that
attractiveness must needs be stronger, when, by an immediate contact, they
are forc'd to be exactly the same: As I shew more at large in my _Theory_
of the _Magnet_. And this hints to me the reason of the suspension of the
_Mercury_ many inches, nay many feet, above the usual station of 30 inches.
For the parts of _Quick-Silver,_ being so very similar and congruous to
each other, if once united, will not easily suffer a divulsion: And the
parts of water, that were any wayes _heterogeneous_, being by _exantlation_
or rarefaction exhausted, the remaining parts being also very similar, will
not easily part neither. And the parts of the Glass being solid, are more
difficultly disjoyn'd; and the water, being somewhat similar to both, is,
as it were, a medium to unite both the _Glass_ and the _Mercury_ together.
So that all three being united, and not very dissimilar, by means of this
contact, if care be taken that the Tube in erecting be not shogged, the
_Quicksilver_ will remain suspended, notwithstanding its contrary indeavour
of Gravity, a great height above its ordinary Station; but if this
immediate Contact be removed, either by a meer separation of them one from
another by the force of a shog, whereby the other becomes imbodied between
them, and licks up from the surface some agil parts, and so hurling them
makes them air, or else by some small heterogeneous agil part of the Water,
or Air, or Quicksilver, which appears like a bubble, and by its jumbling to
and fro there is made way for the _heterogeneous AEther_ to obtrude it self
between the Glass and either of the other Fluids, the Gravity of _Mercury
precipitates_ it downward with very great violence; and if the Vessel that
holds the restagnating _Mercury_ be convenient, the _Mercury_ will for a
time _vibrate_ to and fro with very large _reciprocations_, and at last
will remain kept up by the pressure of the external Air at the height of
neer thirty inches. And whereas it may be objected, that it cannot be, that
the meer imbodying of the _AEther_ between these bodies can be the cause,
since the _AEther_ having a free passage alwayes, both through the Pores of
the Glass, and through those of the Fluids, there is no reason why it
should not make a separation at all times whilst it remains suspended, as
when it is violently dis-joyned by a shog. To this I answer, That though
the _AEther_ passes between the Particles, that is, through the Pores of
bodies, so as that any chasme or separation being made, it has infinite
passages to admit its entry into it, yet such is the tenacity or attractive
virtue of Congruity, that till it be overcome by the meer strength of
Gravity, or by a shog assisting that Conatus of Gravity, or by an agil
Particle, that is like a leaver agitated by the _AEther_; and thereby the
parts of the congruous substances are separated so far asunder, that the
strength of congruity is so far weakened, as not to be able to reunite
them, the parts to be taken hold of being removed out of the attractive
Sphere, as I may so speak, of the congruity; such, I say, is the tenacity
of congruity, that it retains and holds the almost contiguous Particles of
the Fluid, and suffers them not to be separated, till by meer force that
attractive or retentive faculty be overcome: But the separation being once
made beyond the Sphere of the attractive activity of congruity, that virtue
becomes of no effect at all, but the _Mercury_ freely falls downwards till
it meet with a resistance from the pressure of the _ambient_ Air, able to
resist its gravity, and keep it forced up in the Pipe to the height of
about thirty inches.

Thus have I gently raised a Steel _pendulum_ by a Loadstone to a great
Angle, till by the shaking of my hand I have chanced to make a separation
between them, which is no sooner made, but as if the Loadstone had retained
no attractive virtue, the _Pendulum_ moves freely from it towards the other
side. So vast a difference is there between the attractive virtue of the
_Magnet_ when it acts upon a contiguous and upon a disjoyned body: and much
more must there be between the attractive virtues of congruity upon a
contiguous and disjoyned body; and in truth the attractive virtue is so
little upon a body disjoyned, that though I have with a _Microscope_
observed very diligently, whether there were any extraordinary
_protuberance_ on the side of a drop of water that was exceeding neer to
the end of a green stick, but did not touch it, I could not perceive the
least; though I found, that as soon as ever it toucht it the whole drop
would presently unite it self with it; so that it seems an absolute contact
is requisite to the exercising of the tenacious faculty of congruity.

* * * * *


Observ. VII. _Of some _Phaenomena_ of Glass drops._

These _Glass Drops_ are small parcels of coarse green Glass taken out of
the Pots that contain the _Metal_ (as they call it) in fusion, upon the end
of an Iron Pipe; and being exceeding hot, and thereby of a kind of sluggish
fluid Confidence, are suffered to drop from thence into a Bucket of cold
Water, and in it to lye till they be grown sensibly cold.

Some of these I broke in the open air, by snapping off a little of the
small stem with my fingers, others by crushing it with a small pair of
Plyers; which I had no sooner done, then the whole bulk of the drop flew
violently, with a very brisk noise, into multitudes of small pieces, some
of which were as small as dust, though in some there were remaining pieces
pretty large, without any flaw at all, and others very much flaw'd, which
by rubbing between ones fingers was easily reduced to dust; these dispersed
every way so violently, that some of them pierced my skin. I could not find
either with my naked Eye, or a _Microscope_, that any of the broken pieces
were of a regular figure, nor any one like another, but for the most part
those that flaw'd off in large pieces were prettily branched.

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