Page:Cyclopaedia, Chambers - Supplement, Volume 2.djvu/485

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and from this it arifes in gravity to twenty five, and twenty! fix pennyweights the port.ee, and fometimes to twenty fe-j ven and twenty eight ; but even thefe weights may be dif- penfedwith, provided that the other qualities be good, that is, that it be well wrought, even, and clean. When the filk is more than twenty eight pennyweights the portee, it muft always be proportionably cheaper. Philof. Tranfaft. N 252. p. 186. $iLK-worm> bombyx. This infect confifts of eleven rings, and each of thefe of a great number of other fmaller ones, joined to each other; and the head, which terminates thefe rings, is furnifhed with two jaws, which work and cut the food, not by a perpendicular, but a lateral action. .The humours, found in the body of this creature, all feem approaching to the nature of the filk which it fpins, for on being rubbed in the hands, they leave a hard or folid cruft behind them. Under the fkija there is always found a mucous rofy-co- Joured membrane, enveloping the animal, and fuppofed to he the new fkirj in which it is to appear, on throwing off the old one. The heart of this creature reaches from the head to the tail, running the whole length of the body : it is indeed rather a feries of many hearts connected together, than one. The motion of fyftole and diaftole is very evi- dent in this whole chain of hearts ; and it is an elegant fight to obferve the manner of the vital fluid's pafling from one of them to the other. The ftomach of this animal is as long as the heart, reaching, like it, from one end of the body to the other. This large receptacle for food, and the fudden paflage of it through the animal, are very good reafons for its great voracity.

In the fides of the belly, all about the ventricle, there are depofited a vafr. number of veflels, which contain the filky juice ; thefe run with various windings and meanders to the mouth, and are fo difpofed, that the creatures can difcharge their contents at pleafure at the mouth ; and according to the nature of the juices, that they are fupplied with, furnifh different forts of filk from them, all the fluid contents of ' thefe veflels hardening in the air into that fort of thread,
 * that we find the web, or balls of this creature confift of.

Thefe creatures never are offended at any ftench of what- ever kind, but they always feel a fouthern wind, and an ex- tremely hot air always makes them fick. Malpghi de Bombyce. Ghryfalis of the SihK-zuorm. See the article Fe ve. SILVER (Cycl.) — Siher works, as fpurs, wrought hilts, fcrV. are boiled in fait, alum and argol, to give it a whitenefs and clearnefs. Boyle's Works Abr. Vol. I. p. 135. Silver burnt on a glafs-plate tinges it of a fine yellow or golden colour. Boyle's Works, vol. I. p. 147. See alfo p. 458. and Vol. II. p. 64.

Chemifts have made frequent attempts to 6\\iTo\ve Jtlver in vegetable acids, but with little fuccefs, according to Mr. Marggrave ; who fays, that he himfelf, at laft, fucceeded in the attempt.

The chief art required, is to find a proper precipitate of fiher. Mr. Marggrave's is formed by precipitating a folution of filver, in the belt fpirit of nitre, with that fait of urine which he fays is the bafts of phofphorus. See Salt fujibk of urine.

This precipitate reduced to a fine powder, and digefted in a fand-heat with diftilied vinegar, well concentrated by freez- , ing, will in part be difiblved thereby. But if this precipitation of the fiber be made with fait of tartar, and then dried and pulverized, a confiderable quan tity of it may be difiblved in diftilied vinegar, in juice of lemons, in Rhenifh wine, and in other vegetable acids. The like may be done with mercury. Mem. de l'Acad. de Berlin, 1741. See Mercury. Purified Silver. The moft commodious and regular manner of purifying fiher is this : put a large teft, inclofed in an iron ring, into a furnace ; when the teft: has been red-hot about half an hour, put into it the fiver to be purified, wrapt up in pieces of cloth or paper, and divided into fmall portions ; then fill the orifice of the furnace with burning coals, and blow with a pair of hand-bellows, till the fiver is in fufion; then add, at fever al times, fome lead, reduced to globules of a determined weight; continue adding lead, and keep the fire in fuch a degree, as is juft fufficient to keep the metal .in fufion, and continue thus, till the fiver is rendered quite pure. This may be partly guefTed, by the before made trial of its degree of impurity on the touch- ftone, and by the quantity of lead, judged neceifary for the operation, having been confumed ; and farther known, by trial of it on a wire thruft into it while in fufion, arfd the examining what is found adhering to it when it is taken out. When the operation is finifhed, pour water in a fmall ttream on the fiver to make it grow folid, and taking it out of the teft, clean it from all impurities that may adhere to its furface with a brufh made of fmall brafs wire. Cramer's Art of Allaying, p. 204. 208.

The way of purifyingy7/wr by means of nitre is this: re- duce _the fl ver to grains, or fmall thin plates, put it into a CmC i a and add onc °= uarter P art of nit re, well dried and powdered ; p Ut Upon this crucible another of a fmaller fize,

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having a hole as big as a pea in its bottom ; lute the joint of the two crucibles, and place them in a wind- furnace * put charcoal about them as high as the rim of the lower cru- cible, but no higher, then light the fire at top, and en- creafe it till the veflels are middlingly red-hot, then with a pair of tongs hold a burning coal dire&ly over the hole in the upper crucible* but at the diftance of one finger ; if you fee a fudden clear light produced near and about the coal* together with a crackling noife, this fhews that the fire has the right degree of ftrength ; but if this does not appear at all, or but very faintly, the fire muft be e'nereafed: but if, on the contrary, you hear a violent blaft of air from the hole, and a loud crackling noife, without holding the coal over, the fire is then too fierce, and muft be diminifhed* otherwife you will not only lofe a large quantity of fiver, but very often the veflels will burft.

When this is perfectly over, encreafe the fire fo far, as to be able to melt fiver without any addition; then take out the veflels, and when they are cooled break the under one, and you will find the regulus of fiver at the bottom, and an al- kaline fcoria, commonly green, at the top. If the fiher is not yet thoroughly purified, put it into another crucible, and this open into a wind-furnace, throwing upon it a little nitre, and as foon as it melts, pour it into a mould for in- gots; thus will it be purified from every thing but gold. Cramer's Art of Aflaying, p. 246.

When any quantity of this metal is contained in iron* it may, according to the rules of aflaying, be precipitated from it by fcarification, by the afliftance either of ftrong acid men- ftrua, or of crude antimony ; the latter method is the ea- fier, and is performed thus : put one centner of the iron reduced to very fine filings, and two centners of crude an- timony, into a fmall crucible, clofe it with a tile, and fet it in a ftrong fire, that it may melt ; when it has been in a perfect ftate of fufion for fix or eight minutes, take it out, and let it cool, break the crucible, and you will find a mafs, compofed of fcoria at top, and a regulus underneath ; throw away -the fcoria, and powder the regulus, mix it with twelve centners of granulated lead, and fcorify it in a continual, but not over-ftrong fire, till the lead is feen covered all over its furface with fcoria;, then take it out, and pour the mafs into a mould : the regulus fhould now be tough, and of a light lead-colour, both within and without ; if it be blackifh and brittle, it muft be returned into the teft, and fcorified again, and when brought to its proper ftate, and the antimony all confumed, let it be put into the coppel ; and when the bead of fiver is produced pure, fubtrac/t from its weight the before known quantity of the fiver contained in the lead ufed in the procefs, and the reft is the weight of the filver obtained from the iron. Cramer's Art of Aflaying, p. 223.

Silver, in medicine, is mightily cried up for its virtues by chemical writers; accordingly, they have endeavoured to introduce a train of lunar medicines, as they call them, fuch as argentum potabile, diaphoreticum lunare, bezoardium lunare, and fifty others, as pompous as infignificant. The only preparations of fiver, which keep up their credit in the fliops, are the lunar cryftals, and the lunar cauftic, called lapis infernalis ; both which are violent cauftics, eat- ing away the flefh, and even the bones, they are applied to. See Caustic.

Inflammable Silver, a chemical preparation of the lapis infer- nalis made by a fmall heat. The procefs is this : take an ignited piece of Dutch turf, after it ceafes to fmoak, place it with its upper flat furface parallel to the horizon, make a little cavity in the middle, and therein put a drachm of dry lapis infernalis ; it will immediately melt and glow, and fi- nally it will take flame, and hifs and fhine like nitre : after the flame ceafes, pure fiver will be found in the hollow, as much in quantity as was ufed in the making fo much lapis infernalis.

This curious experiment fhews the phyfical manner in which acids do but fuperficially adhere to filver \ and the manner wherein acids operate, when united to metals, while fur- rounding their metallic mafs, they arm the ponderous prin- ciples thereof with fpiculse: it (hews alfo the immutability of fiher diflblved in an acid, and the various ways in which it may be concealed, yet ftill have its action : it alfo fhews the difference of potable fiher, while exifting in a faline form, by means of an adhering acid, from that potable filver of the adepts, where the principles of fiver are fup- pofed converted into a fluid, that will mix with the juices of the body, and cannot be reduced to fiher again; but the great thing to be here obferved, is, that the acid fpirit of nitre, adhering in a folid mafs of filver, is in this ftate as inflammable, on coming in contact: with an ignited com- ' buftible body, as crude nitre itfelf : this feems to happen with filver alone, which is unchangeable with fpirit of nitre. Hence alfo we fee one way, by which fiher may be obtain- ed pure from other adhering matters, by bare burning: the acid here acts neither upon the mercurial part of the filver t nor on its fixing fulphur. Boerh. Chem. Part 2. p. 297.

SiLVER-om. Some of the ores of- fiher are rendered very re- fractory in the working, by means of a mixture of unme- tallic earths, from which no art can wafh them clean. The

method