Micrographia. Robert Hooke
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averse thereunto. Hence we find, that two small drops of water, on any superficies they can roul on, will, if they chance to touch each other, readily unite and mix into one 3d drop: The like may be observed with two small Bowls of Quick-silver upon a Table or Glass, provided their surfaces be not dusty; and with two drops of Oyl upon fair water, &c. And further, water put unto wine, salt water, vinegar, spirit of wine, or the like, does immediately (especially if they be shaken together) disperse it self all over them. Hence, on the contrary, we also find, that Oyl of Tartar poured upon Quick-silver, and Spirit of Wine on that Oyl, and Oyl of Turpentine on that Spirit, and Air upon that Oyl, though they be stopt closely up into a Bottle, and shaken never so much, they will by no means long suffer any of their bigger parts to be united or included within any of the other Liquors (by which recited Liquors, may be plainly enough represented the four Peripatetical Elements, and the more subtil Æther above all.) From this property 'tis, that a drop of water does not mingle with, or vanish into Air, but is driven (by that Fluid equally protruding it on every side) and forc't into as little a space as it can possibly be contained in, namely, into a Round Globule. So likewise a little Air blown under the water, is united or thrust into a Bubble by the ambient water. And a parcel of Quick-silver enclosed with Air, Water, or almost any other Liquor, is formed into a round Ball.
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