Micrographia / Some Physiological Descriptions of Minute Bodies Made by Magnifying Glasses with Observations and Inquiries Thereupon

by Robert Hooke

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.

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 Alcalizate 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 upwards 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 æmulus qui statis temporibus suas 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 Phænomena 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 sæpius 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.