And the second is, That in this experiment there is such an unequal pressure.
That the first is true, the following Experiment will evince. For if you take any Vessel so contrived, as that you can at pleasure either increase or diminish the pressure of the Air upon this or that part of the Superficies of the water, the equality of the height of those parts will presently be lost; and that part of the Superficies that sustains the greater pressure, will be inferior to that which undergoes the less. A fit Vessel for this purpose, will be an inverted Glass Syphon, such an one as is described in the Sixth Figure. For if into it you put Water enough to fill it as high as AB, and gently blow in at D, you shall depress the Superficies B, and thereby raise the opposite Superficies A to a considerable height, and by gently sucking you may produce clean contrary effects.
Next, That there is such an unequal pressure, I shall prove from this, That there is a much greater incongruity of Air to Glass, and some other Bodies, then there is of Water to the same.
By Congruity, I mean a property of a fluid Body, whereby any part of it is readily united with any other part, either of itself, or of any other Similar, fluid, or solid body: And by Incongruity a property of a fluid, by which it is hindred from uniting with any dissimilar, fluid, or solid Body.
This last property, any one that hath been observingly conversant about fluid Bodies, cannot be ignorant of. For (not now to mention several Chymical Spirits and Oyls, which will very hardly, if at all, be brought to mix with one another; insomuch that there may be found some 8 or 9, or more, several distinct Liquors, which swimming one upon another, will not presently mix) we need seek no further for Examples of this kind in fluids, then to observe the drops of rain falling through the air and the bubbles of air which are by any means conveyed under the surface of the water; or a drop of common Sallet Oyl swimming upon water. In all which, and many more examples of this kind that might be enumerated, the incongruity of two fluids is easily discernable. And as for the Congruity or Incongruity of Liquids, with several kinds of firm Bodies, they have long since been taken notice of, and called by the Names of Driness and Moisture (though these two names are not comprehensive enough, being commonly used to signifie only the adhering or not adhering of water to some other solid Bodies) of this kind we may observe that water will more readily wet some woods then others; and that water, let fall upon a Feather, the whiter side of a Colwort, and some other leaves, or upon almost any dusty, unctuous, or resinous superficies, will not at all adhere to them, but easily tumble off from them, like a solid Bowl; whereas, if dropt upon Linnen, Paper, Clay, green Wood, &c. it will not be taken off, without leaving some part of it behind adhering to them. So Quick-silver, which will very hardly be brought to stick to any vegetable body, will readily adhere to, and mingle with, several clean metalline bodies.
And that we may the better finde what the cause of Congruity and Incongruity in bodies is, it will be requisite to consider, First, what is the cause of fluidness; And this, I conceive, to be nothing else but a certain pulse or shake of heat; for Heat being nothing else but a very brisk and vehement agitation of the parts of a body (as I have elswhere made probable) the parts of a body are thereby made so loose from one another, that they easily move any way, and become fluid. That I may explain this a little by a gross Similitude, let us suppose a dish of sand set upon some body that is very much agitated, and shaken with some quick and strong vibrating motion, as on a Milstone turn’d round upon the under stone very violently whilst it is empty; or on a very stiff Drum-head, which is vehemently or very nimbly beaten with the Drumsticks. By this means, the sand in the dish, which before lay like a dull and unactive body, becomes a perfect fluid; and ye can no sooner make a hole in it with your finger, but it is immediately filled up again, and the upper surface of it levell’d. Nor can you bury a light body, as a piece of Cork under it, but it presently emerges or swims as ’twere on the top; nor can you lay a heavier on the top of it, as a piece of Lead, but it is immediately buried in Sand, and (as ’twere) sinks to the bottom. Nor can you make a hole in the side of the Dish, but the sand shall run out of it to a level, not an obvious property of a fluid body, as such, but this dos imitate; and all this meerly caused by the vehement agitation of the conteining vessel; for by this means, each sand becomes to have a vibrative or dancing motion, so as no other heavier body can rest on it, unless sustein’d by some other on either side: Nor will it suffer any Body to be beneath it, unless it be a heavier then it self. Another Instance of the strange loosening nature of a violent jarring Motion, or a strong and nimble vibrative one, we may have from a piece of iron grated on very strongly with a file: for if into that a pin screw’d so firm and hard, that though it has a convenient head to it, yet it can by no means be unscrew’d by the fingers; if, I say, you attempt to unscrew this whilst grated on by the file, it will be found to undoe and turn very easily. The first of these Examples manifests, how a body actually divided into small parts, becomes a fluid. And the latter manifests by what means the agitation of heat so easily loosens and unties the parts of solid and firm bodies. Nor need we suppose heat to be any thing else, besides such a motion; for supposing we could Mechanically produce such a one quick and strong enough, we need not spend fuel to melt a body. Now, that I do not speak this altogether groundless, I must refer the Reader to the Observations I have made upon the shining sparks of Steel, for there he shall find that the same effects are produced upon small chips or parcels of Steel by the flame, and by a quick and violent motion; and if the body of steel may be thus melted (as I there shew it may) I think we have little reason to doubt that almost any other may not also. Every Smith can inform one how quickly both his File and the Iron grows hot with filing, and if you rub almost any two hard bodies together, they will do the same: And we know, that a sufficient degree of heat causes fluidity, in some bodies much sooner, and in others later; that is, the parts of the body of some are so loose from one another, and so unapt to cohere, and so minute and little, that a very small degree of agitation keeps them always in the state of fluidity. Of this kind, I suppose, the Æther, that is the medium or fluid body, in which all other bodies do as it were swim and move; and particularly, the Air, which seems nothing else but a kind of tincture or solution of terrestrial and aqueous particles dissolv’d into it, and agitated by it, just as the tincture of Cocheneel is nothing but some finer dissoluble parts of that Concrete lick’d up or dissolv’d by the fluid water. And from this Notion of it, we may easily give a more Intelligible reason how the Air becomes so capable of Rarefaction and Condensation. For, as in tinctures, one grain of some strongly tinging substance may sensibly colour some hundred thousand grains of appropriated Liquors, so as every drop of it has its proportionate share, and be sensibly ting’d, as I have try’d both with Logwood and Cocheneel: And as some few grains of Salt is able to infect as great a quantity, as may be found by præcipitations, though not so easily by the sight or taste; so the Air, which seems to be but as ’twere a tincture or saline substance, dissolv’d and agitated by the fluid and agil Æther, may disperse and expand it self into a vast space, if it have room enough, and infect, as it were, every part of that space. But, as on the other side, if there be but some few grains of the liquor, it may extract all the colour of the tinging substance, and may dissolve all the Salt, and thereby become much more impregnated with those substances, so may all the air that sufficed in a rarify’d state to fill some hundred thousand spaces of Æther, be compris’d in only one, but in a position proportionable dense. And though we have not yet found out such strainers for Tinctures and Salts as we have for the Air, being yet unable to separate them from their dissolving liquors by any kind of filtre, without præcipitation, as we are able to separate the Air from the Æther by Glass, and several other bodies. And though we are yet unable and ignorant of the ways of præcipitating Air out of the Æther as we can Tinctures, and Salts out of several dissolvents; yet neither of these seeming impossible from the nature of the things, nor so improbable but that some happy future industry may find out ways to effect them; nay, further, since we find that Nature does really perform (though by what means we are not certain) both these actions, namely, by præcipitating the Air in Rain and Dews, and by supplying the Streams and Rivers of the World with fresh water, strain’d through secret subterraneous Caverns: And since, that in very many other proprieties they do so exactly seem of the same nature; till further observations or tryals do inform us of the contrary, we may safely enough conclude them of the same kind. For it seldom happens that any two natures have so many properties coincident or the same, as I have observ’d Solutions and Air to have, and to be different in the rest. And therefore I think it neither impossible, irrational, nay nor difficult to be able to predict what is likely to happen in other particulars also, besides those which Observation or Experiment have declared thus or thus; especially, if the circumstances that do often very much conduce to the variation of the effects be duly weigh’d and consider’d. And indeed, were there not a probability of this, our inquiries would be endless, our tryals vain, and our greatest inventions would be nothing but the meer products of chance, and not of Reason; and, like Mariners in an Ocean, destitute both of a Compass and the sight of the Celestial guides, we might indeed, by chance, Steer directly towards our desired Port, but ’tis a thousand to one but we miss our aim. But to proceed, we may hence also give a plain reason, how the Air comes to be darkned by clouds, &c. which are nothing but a kind of precipitation, and how those precipitations fall down in Showrs. Hence also could I very easily, and I think truly, deduce the cause of the curious sixangular figures of Snow, and the appearances of Haloes, &c. and the sudden thickning of the Sky with Clouds, and the vanishing and disappearing of those Clouds again; for all these things may be very easily imitated in a glass of liquor, with some slight Chymical preparations as I have often try’d, and may somewhere else more largely relate, but have not now time to set them down. But to proceed, there are other bodies that consist of particles more Gross, and of a more apt figure for cohesion, and this requires somewhat greater agitation; such, I suppose
, fermented vinous Spirits, several Chymical Oils, which are much of kin to those Spirits, &c. Others yet require a greater, as water, and so others much greater, for almost infinite degrees: For, I suppose there are very few bodies in the world that may not be made aliquatenus fluid, by some or other degree of agitation or heat.
Having therefore in short set down my Notion of a Fluid body, I come in the next place to consider what Congruity is; and this, as I said before, being a Relative property of a fluid, whereby it may be said to be like or unlike to this or that other body, whereby it does or does not mix with this or that body. We will again have recourse to our former Experiment, though but a rude one; and here if we mix in the dish several kinds of sands, some of bigger, others of less and finer bulks, we shall find that by the agitation the fine sand will eject and throw out of it self all those bigger bulks of small stones and the like, and those will be gathered together all into one place; and if there be other bodies in it of other natures, those also will be separated into a place by themselves, and united or tumbled up together. And though this do not come up to the highest property of Congruity, which is a Cohæsion of the parts of the fluid together, or a kind of attraction and tenacity, yet this does as ’twere shadow it out, and somewhat resemble it; for just after the same manner, I suppose the pulse of heat to agitate the small parcels of matter, and those that are of a like bigness, and figure, and matter, will hold, or dance together, and those which are of a differing kind will be thrust or shov’d out from between them; for particles that are similar, will, like so many equal musical strings equally stretcht, vibrate together in a kind of Harmony or unison; whereas others that are dissimilar, upon what account soever, unless the disproportion be otherwise counter-ballanc’d, will, like so many strings out of tune to those unisons, though they have the same agitating pulse, yet make quite differing kinds of vibrations and repercussions, so that though they may be both mov’d, yet are their vibrations so different, and so untun’d, as ’twere to each other, that they cross and jar against each other, and consequently, cannot agree together, but fly back from each other to their similar particles. Now, to give you an instance how the disproportion of some bodies in one respect, may be counter-ballanc’d by a contrary disproportion of the same body in another respect, whence we find that the subtil vinous spirit is congruous, or does readily mix with water, which in many properties is of a very differing nature, we may consider that a unison may be made either by two strings of the same bigness, length, and tension, or by two strings of the same bigness, but of differing length, and a contrary differing tension, or 3ly. by two strings of unequal length and bigness, and of a differing tension, or of equal length, and differing bigness and tension, and several other such varieties. To which three properties in strings, will correspond three proprieties also in sand, or the particles of bodies, their Matter or Substance, their Figure or Shape, and their Body or Bulk. And from the varieties of these three, may arise infinite varieties in fluid bodies, though all agitated by the same pulse or vibrative motion. And there may be as many ways of making Harmonies and Discords with these, as there may be with musical strings. Having therefore seen what is the cause of Congruity or Incongruity, those relative properties of fluids, we may, from what has been said, very easily collect, what is the reason of those Relative proprieties also between fluid bodies and solid; for since all bodies consist of particles of such a Substance, Figure, and Bulk; but in some they are united together more firmly then to be loosened from each other by every vibrative motion (though I imagine that there is no body in the world, but that some degree of agitation may, as I hinted before, agitate and loosen the particles so as to make them fluid) those cohering particles may vibrate in the same manner almost as those that are loose and become unisons or discords, as I may so speak, to them. Now that the parts of all bodies, though never so solid, do yet vibrate, I think we need go no further for proof, then that all bodies have some degrees of heat in them, and that there has not been yet found any thing perfectly cold: Nor can I believe indeed that there is any such thing in Nature, as a body whose particles are at rest, or lazy and unactive in the great Theatre of the World, it being quite contrary to the grand Oeconomy of the Universe. We see therefore what is the reason of the sympathy or uniting of some bodies together, and of the antipathy or flight of others from each other: For Congruity seems nothing else but a Sympathy, and Incongruity an Antipathy of bodies, hence similar bodies once united will not easily part, and dissimilar bodies once disjoyn’d will not easily unite again; from hence may be very easily deduc’d the reason of the suspension of water and Quick-silver above their usual station, as I shall more at large anon shew.
These properties therefore (alwayes the concomitants of fluid bodies) produce these following visible Effects: