10. And here let it be noted, once for all, that whenever any hydrostatical trial is made, by means of a horse-hair, there must be put into the oppofite fcale as much of the fame hair as feems equal to what fuftains 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 scale. 11. When I referv'd a balance for hydroftatical trials, I found it convenient, on several occafions, to take off one of the fcales, with the strings belonging to it, and substitute in its ftead a piece of metal of a conical, or other convenient figure, and equal in weight to the oppofite fcale; as alfo, at the fame end of the ftring, to faften one end of the horfe-hair that tied the body to be veigh'd in water. did not take off one of the fcales, I caufed it to be perforated in the midAnd fometimes, likewife, when I dle, 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 surface of the water it contains; for when any of these 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 most usual caufe of mistakes in hydroftatical trials, proceeds from hence, that the given folid, and the string whereto 'tis faften'd, 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. Thefe aerial particles faften themselves 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 fake the ftring, and warily knock the body against 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 fhould be ufed more than once in the fame experiment; because I have often ob ferv'd, that after the immerfed body has been freed from the first bubbles STATICS. that appear'd about it, others fucceeded in their ftead, before the experi ment was finish'd. I have been the more circumftantial in explaining the way of weighing bodies in water, becaufe experience hath taught me, that the practice of it is not fo eafy as might, at firft 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 less, and the quotient compared to an unit, will be the antecedent of the proportion fought between the folid and the waAnd this rule holds equally, mutatis mutandis, for other liquors, as well as water. To apply this doctrine firft to ores. Many learned men have been of opinion, that, properly fpeaking, there The hydroftawere no fuch things as gold mines; and I confefs my felf to have been long tical balance kept in fufpence as to this particular: for, notwithstanding all the inquiry and first to applied to ores, I could make after mines, wherein gold was the predominant metal, I could gold-ore. 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 East-Indies, another piece, in the clefts whereo', and a little beyond them, there appear'd fome lumps, wherein, by their colour and other figns, it seem'd manifeft 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 fpecific 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 some 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 firft view, to be a kind of white marble, with a dath 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 almoft femi-diaphanous. For our fpar of lead-ore is often fo foft, that it may easily 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 cal VOL. II. Tt cined. 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 spar, 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 person who conquer'd it for the English, inform'd me, at his return from thence, the Spanish governor of the ifland, 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; because, 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 separate them to profit, ufually melted both the metals together, and fold the mass 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 spar; fome of which I thought worthy to be prefented to that curious examiner of ores, Prince Rupert; and I still 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 sticking 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 horsehair, and weigh'd in water; to which, therefore, our hydroftatical 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 estimate the genuineness and degrees of purity whereof, our hydrostatical method of examination may be of fingular fervice. For the fpecific gravity of STATICS. perfectly refined gold to water, being known; 'tis hence eafy to examine its degrees of finene fs. And, when once the true fpecific gravity of a parcel of fand gold is known, together with its degree of fineness, gain'd by collateral trials; this fpecific gravity may be ufed as a ftandard, whereby to try the fineness of other parcels of the like native gold: whence the fraud of the Negroes may, in this cafe, be prevented; who often clandeftinely mix with the right fand-gold, filings of copper, or brafs; whofe fpecific gravity is not half fo great as that of fine gold. Aqua fortis, alfo, would immediately difcover this fraud, which will not work upon gold; but corrodes brafs, and thereby gains a colour betwixt blue and green; tho' if much filver be naturally mix'd with the gold, this proof, by Aqua fortis, will require skill: and, therefore, good fpirit of urine may be fubftituted in its ftead, occafionally; for this will readily work upon filings of copper or brafs in the gold, and gain from them a fine blue colour. And, to haften the operation of this liquor on filings of brafs or copper; 'tis but fpreading them thin, upon a piece of white paper, and moistening them throughly therewith, that the air may promote its action; for, by this means, I have often produced, in a few minutes time, a pleasant blue colour upon the paper. Spirit of hart's-horn, or other volatile alkalies, or even stale rank urine, will serve in time of need, and yield the fame phenomenon. But ifa folid fubftance be defired for this purpose, common fal-armoniac may be used, by making a strong solution thereof in water, and applying it as any of the former; for this alfo, will foon gain a greenish colour, or a blue, from brafs or copper, and without affecting the gold. amined, and, I have obferv'd fuch a variety of appearances and difguizes in metalline All minerals bodies, and other minerals, that I would advise the fearchers after mines bould be careto have their eyes always open, and ready to take notice of any un-fully exknown, or uncommon foffil; and not neglect to weigh it in their if ponderous, hands; and if it feem to exceed the weight of cryftal, or marble, to hydroftatiexamine it hydroftatically. For there are in England, and feveral other cally. countries, useful foffils, ufually overlook'd by the unskilful. I have found, in this kingdom, eagle-ftones even upon the high-ways; and fome other minerals that were not fufpected to be of English growth. And, I reinember, that paffing by a potter's work-house, and viewing the ground attentively, I made a difcovery of manganefe, a mineral very proper for glazing and colouring of earthen veffels, which the potter, afterwards, gladly used for that purpofe. The fame perfon fhew'd me a place that contain❜d great store of a foffil fubftance, unknown in England, that ran very far under ground, like a vein of metalline ore; by fome eafy trials I found it abounded with vitriolate falt, much more than any marcafite I had examin'd in the form of ftones: whence I concluded, it might more profitably be employ'd to make vitriol, than the marcafites ufed for that purpose in the works at Deptford, or elsewhere in England. STATICS. How to exa mine earths, ces, hydroftatically. Colour'd fands I remember, alfo, that a mineral of an odd appearance, being fent me, as unknown to the miners who dug it up; I found it to be the ore of bifmuth, tho' the vein that afforded it was very finall. But the chief reafon for which I give this particular caution, is, that an application of our general remark upon the fpecific gravity of foffils, may be extended to a new and confiderable ufe; for we need not confine ourselves to examine only thofe foffils, whereof we can obtain pieces fufficiently large to be fingly weigh'd in water; becaufe, not to mention the minerals that may be found ufeful to the phyfician, the drugfter, or the mineralift; the ores of metals may be often found difguiz'd in the form of earth, or mud, that is easily dry'd; which foffils, tho' unfit to be immediately fufpended by a horfe-hair, may conveniently be examin'd by means of a glafs jar-bucket, of a known specific gravity: for, this veffel being almost fill'd therewith, and that matter, carefully counterpois'd in air, and made thoroughly wet with water, and the whole warily let down in the fame fluid, and there kept suspended by a horse-hair, tied to a tender balance, the difference between the weight of the mineral, and veffel in air and water, will be obtain'd; the weight, therefore, of the veffel, in water, being fubftraeted from that difference, will give the weight of the foffil in water, and, confequently, its fpecific gravity in water. And to fhew that this method wants not proper fubjects, whereto it may beapply'd; we are told, that one of the beft forts of Swedish iron is often found in the form of a red mud, at the bottom of lakes, or other ftagnant water and I have obferv'd English okers to be richer in iron, than fome ores of that metal. And, an experienc'd writer, upon the gold and filver mines of America, takes notice, that gold, itfelf, is often found difguized in a reddish earth. Vannochio, alfo, a famous Italian mineralift, tells us, that a reddish fort of earth, fometimes, contains the richest metals. I, myself, have obferv'd fome finely figur'd cryftals, to grow in a red earth. And, lally, a traveller prefented me with a certain earth, which he affirm'd to come from the diamond-mines; and this alfo I found to be red. But a mineralift may make a more advantagious ufe of our hydroftatical and gravel. bucket, by employing it in weighing colour'd fands and gravel. And, to fhew how apt we are to over-look fands, for want of trying them by weight; I-have, fometimes, feen a fort, flighted as worthlefs, which being wash'd, and view'd thro' a microfcope, appear'd like a heap of fmall granats, and, perhaps, were really fo. But what is more extraordinary, having obferv'd, that the black fand, ufually employ'd to dry fresh writing, feem'd, manifeftly heavier than the common; I examin'd it by the hydrostatical bucket, which gave us its fpecific gravity to water, as about 4.6 to 1 ; and, by melting it down, with two or three parts of antimony, and cafting it into an iron cone, I found it of an irony nature; and, by applying it to a load-ftone, to be far richer in metal, than any of our English iron-ores; for, at least, feven parts in eight, would eafily be taken up by the magnet. But fuch obfervations as thefe, are not my chief motives to recommend the examination of fands and gravel to the mineralift. |