NAT.HIST. glafs a number of diftin&t cryftals, long, tranfparent, and curiously

ihaped.

Perhaps 'twill be faid, that the petrefcent juice, when broken, often appears to abound on the infide with narrow ftreaks like thole of antimony; and that I my felf have obferved fome gems to be made up of thin plates; which internal figuration feems much more difficult to be accounted for, without a plaftic form, than the external.

But, for ought I find, many known falts wou'd, when broken, appear to be geometrically figured, even in their leffer corpufcles, as well as in their entire bulk; if we had eyes tharp enough to difcern the fhapes of the minute, as well as of the larger bodies. And we have great inducement to think, that the fmall vifible concretions, of which the bigger grains of falt confift, are as well as the greater, of a cubical figure. Befides, there are feveral bodies fo luckily fhap'd, that upon. a flow coalition, they will convene into a multitude of manifeft concretions; fome of which consist of streaks, and fome of flakes. Thus in the common fal-armoniac, tho' confeffedly a factitious body, upon breaking the bigger maffes, great multitudes of ftreaks are obfervable, like thole we ufually find in the broken Stiria of petrifying water. And I have prepared concretions, fome of which confifted of falts alone, and others of falts and minerals, fuch as ftones or antimony, which look'd very like talc, being white bodies made up of a multitude of very flender ftreaky particles, lying lengthwife one upon another. And as I have taken out of the earth many concretions, outwardly fhap'd like a rhombus, and compos'd of a multitude of flat and extremely thin plates; fo have I fometimes imitated fuch concretions by art. And tho' à folution of filver in purified Aqua fortis, ufually affords only a greatcompany of fmall, thin, and feemingly fimple flakes, like fcales of fish; yet having diffolv'd a large quantity of the metal together, and fuffer'd it to fhoot leifurely, and with due circumftances, I obtained many cryftals, which were geometrically figured without; and confifted of a multitude of exceeding thin flakes, orderly adhering to one another. And for a yet more pregnant experiment, to clear this objection; tin-glafs, tho' a compact and ponderous body, confifts of many fhining, polifh'd flakes; yet, I found, that if this mineral were melted, and fuffer'd to cool of it felf, the difpofition of its component particles wou'd determine them to stick to one another in broad fhining flakes; whereof many will lie upon the reft, and fome across each other, at various angles, according as the matter happen'd in its feveral portions to be refrigerated. There are factitious bodies alfo, which afford us the like inftances; as I have obferved in mixtures of copper, iron, and other minerals; and very confpicuoufly in good Regulus martis ftellatus, whole internal parts may be found, by breaking it, to confift of flat,. fhining, polish'd fakes.

If it be urg'd, to ftrengthen the foregoing objection, that fome petrefcent juices concrete, even whilft men are looking on, and yet our

tony

tony Stiria have in fome places a freaky, and in others an an- NAT.HIST. gular configuration of parts; I anfwer first, that I have feen feveral concretions of that kind, which, as far as appear'd to the eye, were made up of parts confufedly jumbled together. And, fecondly, I have found by trials, that if there be a due difpofition in the component corpufcles of bodies to fuch configurations, they may be brought to concrete accordingly, in a far fhorter time than one wou'd imagine. Having fometimes warm'd fix or feven ounces of Aqua fortis, glutted with fine filver, till the mixture was wholly brought into a transparent liquor; and then put the clear, ftrong glafs which contain❜d it into cold water, that the menftruum might be the more haftily refrigerated; I obferved, that when once the diffolv'd metal began to fhoot, the coagulation into figur'd cryftals proceeded fo faft, that the naked eye cou'd fee its progrefs. And having fometimes put a quantity of falt and flow, or fome other ftrongly refrigerating mixture, into a convenient glafs, and wetted the outfide with a ftrong folution of fal-armoniac, or fome urinous fpirit, whereby, tho' it wou'd be coagulated in lefs than a minute; yet the falt into which it fhot had ufually a curious and determinate figure, according to the nature of the liquor that afforded it; as I have often experienced.

Perhaps you will fay, thefe inftances are taken from faline bodies, which are, for the most part, difpofed to convene into smooth furfaces, and angular fhapes, and to be eafily wrought on by the external cold; and that it may yet feem ftrange, what in fome cafes muft have happened, if our hypothefis be admitted, that external circumftances and accidents, fuch as the figure of a mould, or womb, the coldness of the ambient medium &c. fhould vifibly, and fometimes greatly diverfify even the internal figuration of clofe and folid minerals and gems; without excluding all thofe that are fuppofed to be of a quicker concretion.

To clear this difficulty, I fhall fubjoin an experiment, devifed to fhew, that if the corpufcles of a body be fo fhap'd, as to be fitted, by their coalition, to conftitute fmooth, gloffy planes, tho' they be varioufly fhuffled and difcompofed as to their priftine order; yet if kept for a while in a state of fluidity, they will prefently convene again into fmooth, fhining planes; and the fituation of thofe planes, with respect to one another, will be exceeding uniform and regular, confidering the concretion is haftily made; while their internal contexture will be much diverfify'd by circumftances, as particularly, by the figure of the veffel or mould, wherein the fluid matter concretes. Confidering then, that if, as was lately observed, we break the lumps of tin-glafs, it will discover a great many bright and fmooth planes, which fometimes meet, and fometimes crofs one another, at very different angles; I thought it probable, that a body which had been melted, and was apt to convene into fuch planes, wou'd not only do the fame upon a fecond fufion, but might have the order and magnitude of thefe planes

diver

.

NAT.HIST. diverfified by the figure and capacity of the vessel, chosen for the purpose. Having, therefore, powder'd a fufficient quantity of tin-glafs, when 'twas well melted, and caft it into a good pair of iron moulds, whofe cavity was an inch in diameter, we thence obtained a bullet, which, being warily Broken, feem'd, as we expected, to be made up of a multitude of little fhining planes, fo fhap'd and placed, that they appeared orderly to decreafe, as they were further removed from the fuperficies of the globe; they were, likewife, fo rang'd as to feem made up of a multitude of these rows of planes, reaching every way, almoft like fo many radij of a sphere, from the center to the circumference. But if tin-glafs be melted in a crucible, and fuffer'd to cool there, the matter, when taken out and broken, will appear, indeed, full of fmooth planes, but, as was lately intimated, very irregularly and confufedly affociated or ranged.

This experiment we have, alfo, varied by cafting bullets of fome other bodies, and particularly of the fimple regulus of antimony, and with fuccefs; tho' the texture hereby produced was not fo uniform as in tin-glafs. And by cafting melted fulphur into a spherical body of about five or fix inches diameter, and breaking it warily; tho' one wou'd think this an unlikely mineral to make any other than a confufed concretion; it prefented us great fibres almoft like little straws; whofe number and orderly fituation afforded a confiderable inftance for my prefent purpose.

And having broken feveral marcafites of a peculiar fort, that were either roundish or almoft of a cylindrical figure, to observe their internal ftructure and qualifications; I found in fome of them a great many rows of little planes, or glittering corpufcles, reaching from the innermoft parts to the external furface, and in thofe that were cylindrical, to the outfide. These ranks of gold colour'd particles, in the feveral planes of the broken mineral, feemed like femi-diameters, iffuing from a row of phyfical points of an imaginary line, lying al moft like the axis of a cylinder; as if the cavities of the chalk or clay, where these marcafites were found, had made the foil like a mould, wherein the matter of them being detained, whilft 'twas in a fluid form, afterwards concreted like the bullets of our tin-glass, &c.

As to the uniformity of fhape admir'd in gems, and which is thought to demonftrate them to be form'd by a feminal and geometrical principle; I cannot find, upon a more attentive furvey, that it is near fo great as men ufually imagine.

In feveral tranfparent gems it manifeftly appears, as I lately noted, that the fhape was, in a great measure, owing to the figure of the womb or mould, wherein the matter, whilft liquid or foft, happened to fettle. But in fome other tranfparent and well figured gems, of the fame denomination, and, fometimes, growing very near oneanother, I have found, by a diligent infpection, a manifeft, and fometimes a very confiderable difference of fhape, either as to the number, fi

gures

gures, or bignefs of the fides that made it; by comparing thefe par- NAT.HIST. ticulars with those of a stone of the fame kind perfectly figured. This I took notice of particularly, in two forts of ftones, granats, and Cornish diamonds. A confiderable number of the former, that grew in America, was brought to me in one lump of matter; but growing in diftin&t parts of it, and without touching one another. Among thefe I took notice of a manifeft difparity of fhape; fo I did in fome African ones, as alfo in others that were European; one of which, being of an extraordinary large fize for a figured gem of the tranfparent kind, and weighing above eleven drams and a half, I confider'd, with a parti cular attention; and found, that tho' it seemed to have been coagulated in a fluid medium, and to confift of twelve planes, at the concourse of two or three whereof it feem'd to have been broken off from the root; yet it was very far from the geometrical dodecahedron; for as this confifts of twelve equilateral and equiangled pentagons, almoft all the planes of our granats were quadrilateral, and very different from what they should regularly have been, not only in magnitude, but in fhape; for one of them feemed to have five fides; and for the reft, fome neareft to a rhombus, others to a rhomboides; but the moft were trepezia. And, by the way, a curious perfon fhewed me a pearl very irregularly fhaped, and of an enormous fize. It was fo artificially fet in gold, that by the help of a little of that metal, fitly placed, the whole jewel reprefented a lion; yet I made a fhift to meafure it exactly with collapar compaffes, and found the length to be an inch and a half, and the greateft breadth, where it was of a proportionable thickness, to be inch. The colour was orient enough, except one dark spot, which by its fize, figure, and fituation, I guefsed to be the remains of that part whereby 'twas faften'd to the fhell of the fish which produced it. And thus much for the first fort of gems, whole figures I obferv'd to be irregular.

The fecond confifts of thofe cryftalline ftones, fome of which are much harder than the Bristol diamonds, or, perhaps, than rock-crystal it felf; for 'tis cafie to write upon glafs with them. Having procured a large number of thefe ftones, I took notice, by comparing them together, that tho' fome of them were geometrically and curioufly fhap'd like rock-cryftal, having each fix fides, whereof two, that were oppofite, feem'd perfectly alike and equal; and tho' the ftone had a pyramidal termination, made up by feveral refembling and curiously figur'd planes, that ended in a folid angle; yet the greatest number, by much, of thefe diamonds, confifted of ftones whofe figures were far from exact and uniform. For tho' most of them had fix long planes, yet often the oppofite ones were neither parallel to one another, nor fimilar, but exceedingly unequal; and thofe planes that went to make up the apex, compared with one another, or with regular patterns, their figures, magnitudes, and manner of concurring, were fo remote from uniform, as to agree far better with our hypothefis than with

the

NAT.HIST, the other. And yet the want of room to coagulate freely, could not, with probability, be here pretended; for they feem'd to have been form'd feparately in a fluid ambient, except at the bottom, where they were faftened to the rock; as appeared by an opake root which ftill adhered to most of them. And, if I mifremember not, I have more than once, on diamonds newly brought from the Indies, and fome of them very fair ones, obferved great irregularities in the area's of the fuperficial planes, or in their figures, or both; and, fometimes, in the very number as well as fituation of their folid angles.

I now proceed to fhew, that the figuration of gems not only confifts with our conjectures, but confirms them; for I have, more than once, taken notice in Cornish diamonds, that a small ftone, of the fame kind, has made, as it were, one body with a greater; not only adhering clofely to it, but being bedded in it; fo that when they were feparated, there remained a cavity in the larger, whofe figure exactly anfwered that of as much of the fmaller flone as chanc'd to be lodg'd therein. Thus, alfo, I have feen a greater grow to a much lefs; there being a cavity in the latter anfwerable to the part of the greater that had enter'd it; which feems to fhew, that the ftone to which the other grew, was firft form'd and hardened, fince it retain'd its own fhape; and that whilft this adhered to the rock or foil, more liquor began to coagulate, by faftening it felf to the folid body, which was already concreted. Thus by putting into a ftrong folution of pure nitre, or rock-alum, fome little fticks of wood or other folid body, that may be kept fteadily in the fame pofture, you will fee many coagulations begin to be made against them; and the cryftals, thus concreted, will neceffarily have their figures incompleat, and cavities correfpondent to thofe parts of the ftick whereto the faline corpufcles faftened. And tho' I inftance this inofculation of figured ftones but in Cornish diamonds, yet thofe are not the only tranfparent minerals wherein I have obferved it. And particularly, I found among a parcel of minerals, a fine tranfparent and neatly figured ftone, which feem'd to be pure cryftal; but lay coagulated about a kind of branching wire, whereof a great part was enclofed by the ftone, feem'd to grow out of a piece of ore, refembling that of filver; which the owner affirmed to be, together with the above mentioned branch, good filver, produced by nature in that form; which excellently agrees with the refemblance I juft propofed, betwixt the coagulation of diffolved falts and the liquid matter of gems, about ftable bodies immerfed therein. But to proceed. The fecond grand confideration, whereon our hypoSpecific gravi thefis is founded, arifes from the weight of tome gems, which, being ty in gems argreater than what feems to belong to them, as hard tranfparent ftones, gues their metalline or miwe may, probably, derive it from metalline or mineral mixtures. Gems are neral nature. eftimated by lapidaries as they weigh a determinate number of carrats, or of grains; for they compare only the weight of ftones, of the fame kind, to one another, as the greater or leffer weight argues a greater or lefs

A greater

bulk;