STATICS. fite to the overfeer of one of the emperor's beft mines, he presently examin'd it, by a peculiar way, with hopes to find in it fome gold or filver; but, instead thereof, obtain'd a parcel of running mercury, which he prefented me. Notwithstanding the caution here given, I do not deny, but that 'tis poffible, for a skilful artist, to make a profitable ufe of marcafites, either by fixing the volatile gold, or filver, to be found in fome of them ; by graduating filver by their means, &c. But that for which I much more value them, is, what I defire should be well remark'd, that I am persuaded, if they were fimply, and, of themfelves, but dextrously handled, they may afford very noble and uncommon medicines, particularly excellent in continual fevers; tho' their ufual operation be almost infenfible, unlefs by their good effects.

Directions to find the best flux-powders, for fofils.

Directions relating to the ma

6. I must not here omit, that tho' many, who make trials of ores, value their own flux-powders, or fuch as are cry'd up by others; yet they commonly feem to expect nothing from those they prefer, more than that they should best facilitate the fufion of the ore; as that which being once done, the metalline part will feparate by its own weight, or, as it were, fpontaneously. But yet, having purpofely examin'd the matter more nicely, and compared the quantities of metal obtain'd from two portions, of equal weight, of the fame ore, we found thofe proportions confiderably differ'd; tho' that which yielded leaft metal, was flux'd down with a coftly, and well-adapted powder. And, I doubt not, but from other metalline ores, a greater quantity of pure metal is obtainable, by fome flux-powders, that are but little employ'd, or known; than by others, that are much more famous and common. Thus two equal portions of the fame lead-ore, clear of fpar; being, the one reduced with a due weight of nitre and tartar fulminated together, afforded me a much lefs proportion of malleable lead, than the other, by means of half, or a quarter the quantity of filings of iron. And, to inftance in a much more precious mineral than lead-ore, I try'd the like with fome ounces of good native cinnabar, finely pulveriz'd; by adding to one half a fix'd alkali of tartar, and to the other, a different flux-powder, we obtain'd from the former, twice as much mercury, as we did from the latter, tho' diftill'd with a fix'd alkali even of a mineral nature.

7. Before we proceed farther, 'tis neceffary the reader be acquainted nagement of the With the method of weighing heavy bodies in water; which being liable to feveral contingencies, 'tis proper in this place to take notice of the more confiderable ones.

hydrostatical balance.

Fig. 26.

But, firft, to give a general notion of the thing, I defire the figure of the hydroftatical balance itfelf, with all its apparatus, may be view'd; wherein A A, is the beam of the balance, B B, the difhes, CC, the frame whereon the beam is fufpended; with D, its fliding focket; E, the arm; with F, a pulley; over which, and G, another pulley pafles H, a line faften'd to I, a moveable weight; by which the beam is elevated and deprefs'd. K, is a hair wherewith to fufpend L, the body to be weigh'd in the water of M, the glafs ciftern. N, is the bucket for liquors, O, the box of grains, P,

the

the forceps, wherewith to manage them, Q, the pile of weights, R, the STATICS. handle of the balance, and SSSS, the table.

The folid body to be examin'd, fhould be ty'd about with an horfe-hair, of a competent length, and faften'd at its other end, to one of the fcales, of an exact and tender balance; that the body, having been carefully weigh'd in the air, may be immerfed in a proper veffel, almoft full of fair water, fo as to hang freely therein, and be on every fide encompafs'd with the fluid. This done, put weights into the oppofite fcale, to balance the body hanging in the water; when the beams of the fcale will lie horizontal, and the weights employ'd give that of the body in water. Deducting, therefore, this weight from the weight of the fame body in the air; by the remainder, that is, the difference of the two, divide the whole weight of the given body in air; and the quotient will fhew the proportion in fpecific gravity, between the folid, and an equal bulk of water. To illuftrate this by an example, we put a fine piece of white marble, which feems the moft pure of any common opake ftone, into an exact balance, and took its weight in the air 1169 grains; then a horfe-hair being ty'd about it, and the other end of the fame hair, faften'd to one of the fcales; under which, at a convenient diftance, was placed a deep glafs, almoft full of fair water; we fuffer'd the ftone to hang freely beneath the furface of the fluid; and, in the oppofite fcale, placed weights enow to bring it to an equilibrium with the other. These weights amounted to 738 grains, which gave us the weight of the marble in water; which being fubftracted from the weight of the fame ftone in air, there remain'd 431 grains for the weight of the equal bulk of water: by this remainder, we divided the weight of the marble in air, viz. 1169, and found the quotient to be 2.7 nearly, for the fpecific gravity of white marble to wa

ter.

'Tis here manifeft, from the nature of the thing, that the body propos'd to be weigh'd, must be heavy enough to fink in water; otherwife its weight in the fluid cannot be fignificantly deducted from its weight in air; but if there be occafion to weigh in water, a body lighter in Specie than water, it may be done, tho' with difficulty, by joining to it another body, that is able to fink it.

8. A horfe-hair is made choice of for hydroftatical operations, becaufe 'tis thought to be of the fame fpecific gravity with water; and tho' that be not ftrictly true, yet a horfe-hair is fitter to be employ'd in thefe trials, than any other ftring I know of; and its weight ufually differs fo little from that of water, that the difference may be fafely neglected: but if the folid propofed, be too heavy to be fuftain'd by a fingle hair, feveral of them may be twifted together.

9. I fometimes had occafion to examine bodies hydroftatically, which, by reafon of their roundnefs, or other inconvenient figure, could not well. be faften'd to a hair; in which cafe I ufed hairs interwoven into a kind of a fmall hoop-net, the meafhes whereof would not fuffer the body to flip thro' them.

10. And

STATICS.

10. And here let it be noted, once for all, that whenever any hydroftatical trial is made, by means of a horfe-hair, there must be put into the oppofite fcale, as much of the fame hair as feems equal to what fsustains the body, and appears above the furface of the water; for that fluid only takes off the weight of as much of the hair as is immers'd in it: fo that its unimmers'd part adds to the weight of the pendulous folid; and, therefore, ought to be compenfated by an equal weight in the other fcale.

11. When I referv'd a balance for hydroftatical trials, I found it convenient, on feveral occafions, to take off one of the fcales, with the ftrings belonging to it, and fubftitute, in its ftead, a piece of metal, of a conical, or other convenient figure, and equal in weight to the oppofite scale; as alfo, at the fame end of the ftring, to faften one end of the horse-hair that tied the body to be weigh'd in water. And fometimes, likewise, when I did not take off one of the fcales, I caufed it to be perforated in the middle, without leffening its weight; that fo the body to be immers'd, might hang perpendicularly from the centre of the scale.

12. Care must be taken, that the body to be examin'd, hang freely in the water, fo that no part of it, any where, touch the bottom, or fides of the veffel, or reach above the upper furface of the water it contains; for, when any of thefe circumftances are neglected, the true weight of the folid is fomewhat alter'd: and if any part of the body, or horfe-hair, 'tis tied with, appears above the water, it adds to the weight thereof. And, as nothing but the water is to touch the hanging body, fo no part of the fluid muft touch the fcale from whence it hangs. I have feveral times obferved, that immers'd bodies have been concluded to weigh more in water than they really do, from a neglect in obferving, that if the ftring be too fhort, or the veffel too full, the vibrating motion of the balance will, at one time or other, carry down the fcale to which the fufpended body is tied, fo as to make fome part of it touch the furface of the water; a few drops whereof will readily ftick to it; which, because they adhere to its under part, may lie conceal'd from a heedlefs eye, fenfibly add to the weight of the fcale, and make the body be thought heavier than it naturally is: an error very prejudicial, when exact experiments are required.

13. But the moft ufual caufe of mistakes, in hydroftatical trials, proceeds from hence, that the given folid, and the ftring whereto 'tis faftned, carry down with them particles of air; which, perhaps too, may extricate others that lay conceal'd in the pores of the liquor, in the form of bubbles. These aerial particles faften themfelves to the little afperities they meet with, on the furface of the immers'd body, and like fo many little bladders of air, endeavour to buoy up the body they adhere to ; and, therefore in proportion to their number and bulk, leffen the due weight of the immers'd body, in water. Great care, therefore, must be had, in nice experiments, to fhake the ftring, and warily knock the body againft the fides of the glafs, that the adhering bubbles may thereby be difplac'd, and emerge to the top of the water. And, on fome occafions, this caution should be ufed more than once in the fame experiment; because I have often ob

ferv'd,

ferv'd, that after the immerfed body has been freed from the firft bubbles STATICS. that appear❜d about it, others fucceeded in their ftead, before the experiment was finish'd.

I have been the more circumftantial in explaining the way of weighing bodies in water, because experience hath taught me, that the practice of it is not fo eafy as might, at first fight, be expected.

Having obtain'd the weight of the given body, firft in the air, and then in water, according to this method; it will not be difficult to difcover, practically, the proportion in weight between the folid and the fluid. The foundation of our practice, is this theorem of Archimedes. "A body heavier than water, weighs lefs in that, than it does in air, by the weight of an equal bulk of water." Whence we deduce a rule for our prefent purpose. For if the weight of the propofed body, whilft it is every way furrounded with water, be fubftracted from that of the fame body in air, the remainder gives the aerial weight of a quantity of water equal in magnitude to the folid; fo that having the weight both of the folid and fluid, divide the greater by the lefs, and the quotient compared to an unit, will be the antecedent of the proportion fought between the folid and the water. And this rule holds equally, mutatis mutandis, for other liquors, as well as water.

To apply this doctrine first to ores.

cal balance ap

Many learned men have been of opinion, that, properly fpeaking, there The hydrostati were no fuch things as gold-mines; and I confefs myfelf to have been long lied to ores, and kept in fufpence as to this particular: for, notwithstanding all the inquiry I first to gold-ore. could make after mines, wherein gold was the predominant metal, I could not find a man who would fay he had feen any. At length, I met with fome ore, prefented to King Charles II. which I judged to be genuine ; and alfo received from an unknown hand in the Eaft-Indies, another piece, in the clefts whereof, and a little beyond them, there appear'd fome lumps, wherein, by their colour and other figns, it feem'd manifest that gold was the predominant metal. But the largest piece, and that which was beft furnish'd with metalline parts, weighing about an ounce and a quarter, contain'd fo great a proportion of fpar, that its specific gravity to water, was but as 2.21 to 1. Its metalline portion, however, feem'd to be all gold; for there appear'd no fign of any other metal therein, nor, in fome leffer pieces that I receiv'd along with it. Its fpar, or ftony matter, wherein the true ore is immediately lodg'd, did not refemble the fpar of lead-ore, or that of any other of our English metals; but feem'd, at first view, to be a kind of white marble, with a dash of yellow. And, upon trial, I found it differ'd ftill more from the fpar of lead-ore, which, with us, is ufually white, and almost femi-diaphanous. For our fpar of lead-ore is often fo foft, that it may eafily be cut with a knife; but the fparry portion of this gold-ore was a folid ftone; and fo hard, that when ftruck against a piece of fteel, it would yield fparks of fire. And, farther, the fpar of lead-ore will eafily, and fuddenly, calcine to a kind of lime; but our golden-fpar, tho' kept for fome hours red-hot in a crucible, did not appear to be at all calVOL. II. cined

Tt

STATICS. cined. The fpar of lead-ore will, alfo, diffolve in fome acid menftrua, and even in diftill'd vinegar: yet I could not find that this golden fpar, when kept for feveral hours in fpirit of falt, Aqua fortis, or Aqua regia, was manifeftly wrought upon by any of them.

A piece of fpar, that feem'd fcarce to contain any gold at all, being hydroftatically examin'd, we found its specific gravity to water, as 2.65

to I.

Thefe obfervations, imperfect as they are, may, perhaps, be ferviceable to fuch as have never feen true gold-ore; and, in particular, to those who fearch after gold-mines in Jamaica: where, as general Venables, the perfon who conquer'd it for the English, inform'd me, at his return from thence, the Spanish governor of the island, whilft his prifoner, confefs'd there was mineral-gold; tho' the Spaniards, for want of workmen, could not dig deep enough in queft of it.

By what I have faid of the true gold-ore, I would not have any one difcouraged from feeking for gold in other metals; becaufe, I know, it may fometimes be found blended with predominant minerals, as appears from the copper-mines of Cremnitz, whence a confiderable quantity of gold is yearly obtain'd. And I have feen an English tin-ore, wherein there lay, in little cells, a number of small leaves or chips of gold; and tho' the tinmen, being unable to feparate them to profit, ufually melted both the metals together, and fold the mafs for mere tin; yet the owner of the mine affured me, that one of his work-men advantagiously employ'd his own children to pick the gold out of the ore, when skilfully broken. There is, alfo, a place in Scotland, where, over a lead-mine, near the furface of the ground, they often find large grains of native gold free from fpar; fome of which I thought worthy to be prefented to that curious examiner of ores, Prince Rupert; and I ftill have a piece of native metal by me, that came from the fame place, in weight above forty grains, and wherein gold is the predominant metal.

I have found a grain of natural Scotch gold, without any adhering fpar, to weigh three drams, twenty-one grains; another grain of the fame, that had a little fpar fticking to it, weigh'd three drams, three grains; fo that, allowing for the heterogeneous fubftance, it weigh'd about three drams. And, laftly, a grain of Scotch gold weigh'd, in air, 43 grains; in water, 39 grains; whence its proportion to water is as 12 to 1.

It often happens, that, among the leffer grains of gold, properly call'd fand-gold, there are pieces, fingly big enough to be ty'd about with a horfe hair, and weigh'd in water; to which, therefore, our hydrostatical way of examining ores may be usefully apply'd. For, fince pure gold is to water of the fame bulk, as 18 or 19 to 1; it will readily appear, whether the fragment propofed be perfectly pure, or not.

We frequently receive from the maritime parts of Africa, fmall fragments of gold, in the form of fand or gravel, which feem to have been wafh'd away from hidden veins by the violence of the waters; to eftimate the genuineness and degrees of purity whereof, our hydroftatical method

of