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either in making thefe or other experiments of affinity to them. By way of proof to fuch as may diftrufl this, from their never having made fuch experiments themfelves, two very eminent teftimonics may be produced. That molt in- duftrious and diligent mathematician Merfennus candidly ac- knowledges, that when he has occafion to mention the ex- periments of the learned Ghetaldus, and of the accurate French engineer Monfieur Petit, with his own, there will be often a variety of weight of fome few grains ; but he fays, it is only like the variety of aflronomical obfervations, which do almoft always differ alio by a few minutes or feconds ; and to this he afterwards adds, that he expected not an exact uniformity between the obfervations of Ghetaldus, &c. al- ready made, and the trials of any new experimenter, who would go through the work again. And our famous expe- rimenter the lord Verulam confeffes, that it is not to be doubted but that many of the bodies which he has fet down in the table he has given, of their dimenfions and weights, differ in the fame fpecies or denomination, fome being hea- vier than others, and that therefore there is fome contin- gency in the affair ; fo that tis not neceffary that the indivi- duals he made his trials upon mould be exact ftandards of the weight of their refpective fpecies, or fhould, which makes directly to our preient purpofe, agree altogether to a tittle with the experiments of other men. Boyle, Hydroft. Expe- rim. in Pref.
But this fcarce evitable imperfection of hydroft atical experi- ments does not hinder, but that by their help we may make good eftimates of the weights and bulks of very many bodies ; and thefe eftimates will be found not only preferable to thofe that can be made of the fame bodies by geometrical inft.ru- ments, but accurate enough to be very ufeful on a great number and variety of occafions.
Mbtals/w Specula. In the making reflecting telefcopes, the greateft article of all is the hitting upon a proper Metal for the fpecula. The common fearch is for a white and hard Metal, that {hall be bright and durable ; but Sir Ifaac Newton ad- vifes, that at the fame time great care muft be taken that the Metal be not full of holes or pores, which, though fo fmall as to be only difcoverable by the microfcope, yet will greatly impede the office of the fpeculum. This Metal may feem to take a very fine polifh, but, in reality, the edges of thefe fmall pores will wear away fafter in the polifliing, than any other part of the Metal, and then however polite the Metal may feem, yet it will not reflect with fuch an accurate regu- larity as it ought to do. Thus tin-glafs, mixed with the common he\\-Metal, renders it more white and capable of reflecting a greater quantity of light than it other wife would do ; but then the fumes of this femi-Metal raifed in the fufion like fo many aerial bubbles, fill the Metal with microfcopic pores. White arfenic both blanches the Metal, and renders it rather more denfe than before, giving it no microfcopical pores, unlefsthe fufion has been too violent. The ftellate regulus of Mars is recommended by the fame gentleman to farther trials.
When the proper mixture is hit upon for the making fuch a Metal, there requires yet great caution in the polifhing it. Whatever this is done with ought to be of the moft extreme finenefs, for if otherwife, its particles will fcratch the face of the Metal, and make fuch microfcopical holes as the tin- glafs gives in the other inftance. The proportion of arfenic is to be judged by trials, but fomething between a fixth and an eighth part of the weight of the copper feems to be the juft proportion ; when there is much lefs than an eighth, the Metal wants of its whitenefs and brightnefs ; and when much more, it is rendered too brittle. The method of making the mixture is this: The copper is firft to be melted alone, and then the arfenic is to be put in ; but while this mixture is ftirred, great care muft be taken to avoid the poifonous fumes. When the copper and arfenic are mixed, the tin is to be put in; this foon melts, and then the whole is to be ftirred together, and poured off. Some add filver ; but it is found, on trial, that this does as much harm in rendering the Metal foft, as it does good in rendering it white and lu- minous. Six ounces of copper, two ounces of tin, and one ounce of arfenic, feems one of the beft proportions. It is to be carefully managed in the fire, and not kept too long in fufion, left it grow porous by that means. Phil. Tranf. N°. 81.
Colours from Metals. As metals have a ftrong texture in their metalline form, fo they preferve their natural co- lours durably, unlcfs corroded or difl'olvcd by particular men- ftruums, after which their folutions ftrike particular durable colours, or afford the ftron^eft ftains.
Iron diffolved in ftale fmalf beer gives the beautiful yellow ufed in calhco printing ; when fublimed with fal armoniac it alfo affords a yellow : And the common iron-moulds made by ink are owing to the iron dtflblved in the copperas of which ink is made.
Copper melted with zink appears of a gold colour ; diffolved in aqua fortis, it affords a beautiful green ; and in any alkali, a beautiful blue. And thefe folutions may be reduced to dry colours by cryftalHzation or evaporation ; and the fame Me- &uppl. Vol. II.
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ial, precipitated out of aqua fortis with common falts, give's the turquoife colour to white glafs.
Tin, a white or colourlefs Metal, affords alight blue colour, when fluxed with antimony and nitre. The fame Metal is neceffary in ftriking the fcarlet dye, with aqua fortis and co- chineal; and its calx, by ftrong inftifion, turns to a glafs of an opal colour.
Lead, ccrroded by the fumes of vinegar, gives the fine white ccrufs ; burnt in a ftrong naked firej it becomes the ftrong red lead or minium ; and melted into a glafs with fand, is of the hyacinth colcur. Shaw's Lectures, p. 171. Silver being diffolved in aqua fortis, if chalk be put to the folution, turns of a beautiful purple or amethyft colour; and its own folution, though pale as water, durably ftains the nails, fkin, or hair, brown or black.
Quickfilver mixed with brimftone makes a black mafs, and this,_by fublimation, affords the beautiful red pigment, cal- led cinnabar, or Vermillion ; and the folution of quickfilver being precipitated with common fait, yields a mow-white powder, which alfo turns black by being mixed with ful- phur.
Gold, diffolved in aqua regia, affords a fine yellow liquor, Which ftains animal fubftances beautifully purple ; and if the folution be fufficiently weakened with water, and mixed with a folution of tin, a fine red or purple powder may be pro- cured, very ufeful for ftaining of glafs and partes to a beau- tiful red.
Fluxes 0/ Metals. See the article Fluxes.
Granulation of Met als. See the article Granulation.
Medicines from Metals. All medicines that have lead in their composition, are fcarce to be trufted for internal ufe ; for we have numerous inftances of the poifonous nature of this Me- tal, in the workers at the white lead or cerus-houfes, and indeed in all who work on that Metal : And it were enough to bar.ifh the internal ufe of faccharmn fatumi, and the tinclura antlphthlfica, to read in the Mlfcellanea Curiofa y Decad. 3. Obf. 30. how many men were poifoned at a time by drinking of prick' d wine, which had been rendered fweet by litharge.
Metals, however, do not act in the body unlefs diffolved ; whence large quantities of crude mercury being fwallowed, prove harmlefs tor want of a proper menftruum to diflblve it in the ftomach : But when crude mercury is diffolved in its proper menftruum, as in the making corrofive fublimate, we fee it becomes one of the ftrongeft poifons. Lead, Iron, and copper, being much more eafily diffolved than quick- filver, are feldom taken without fome remarkable effect, which muft always prove the greater, the more of an acid humour, or in cafe of copper, even an alkaline is lodged in the ftomach or interlines. The internal ufe of the cry- ftals of filver, though fonietirn.es fuccefsfully given in dropii- cal cafes, fhould hardly be trufted, on account of their cor- rofive acrimony, which has fometimes occafioned bloody ftools, and extreme weaknefs. The common tinctures of copper and folutions of blue vitriol, can fcarce be taken inter- nally with fafety ; as being not only emetic, but in fome de- gree corrofive and poifonous. Even the aurum fulminans can- not be fafely given till thoroughly wafiied from its falts ; and even then if it meet with an acid folvent in the ftomach, it may do very great mifchief. Iron and tin have not been found fo pernicious in the body, but rather beneficial, efpecially iron, when properly prepared and ufed. Metallic bodievcorroded by acids, generally erode when ap- plied to fores. Some of thefe, fublimate mercury, for inftance, and particularly arfenic, have fhaken the whole frame of the body, when applied externally ; and mercurial preparations do fometimes enter the blood, and produce a falivation. Med. Eff. Edinb. vol. 5. art. 24.
Salts of Metals. The falts of the feveral Metals have been medicines long known in the world, and are of excellent ufe in the cure of many difeafes : But they might be of yet infi- nitely more fervice to the world, if a happier way of extract- ing them could be found than the general one now in ufe, of doing it by acids, and fuch ftrong and corrofive menfiruums. Befide the mifchiefs thefe menftruums bring as corrofives, which is affuredly much lefs than is vulgarly fuppofed, becaufe they lofe the greater part of this in acting on the A'letal, they produce a great number of bad effects, and render the falts unfit for a great number of ufes, by making them all vitriols, that is, by converting them into falts, which, like the com- mon native vitriols, have a feptic and violently acrimonious quality. The cryftals, or fait of filver, called by the che- mifts, lunar vitriol, are a plain inftance of this. Borrichius and Kunkell deviled another method to be ufed for obtaining the falts of Metals. This was the ufe of fire inftead of acids, as the menftruum that fhould prey upon the Metal, and feeking the falts of all Metals from the mere action of a reverberatory furnace ; but this method, though a very laudable attempt, yet is by no means calculated to be of general ufe, becaufe the quantity of falts thus obtained is very fmall, and that obtained with great pains and expence. Kunkell has given the world an account of the effects of an- other method of treating Metals, of which he relates wonders ; N but
