or Assaying, in chemistry and metallurgy, signifies the examination of a small quantity of any ore or mineral by fire, in order to discover its contents. This is very necessary for those who intend to deal largely in metallurgical operations, in order to avoid unnecessary expense, by becoming previously acquainted with the nature of the ore.

The first attempts in this way were no doubt extremely rude; but succeeding trials have advanced it to the form of a science or art practised by numbers of people under the title of assayers. No treatise was published on this subject till after the middle of the 16th century; and the first book we have upon it is attributed to Lazarus Ercker, which appeared in 1574. Agricola, however, in his seventh book De Re Metallica, published in 1576, described both the instruments and processes, illustrating the whole with plates; and there is incontrovertible evidence that this treatise had been presented to the elector of Saxony in 1567, though it did not appear to the world till after the publication of Ercker's book. Since that time, the art has been greatly improved; but the operations in the dry way are not materially different from those described under Metallurgy. The Blow-pipe likewise affords an excellent method of examining small quantities of metal in the dry way; but the greatest improvement hitherto made in it is that of assaying by the moist way introduced by Mr Bergman.

This celebrated chemist observes, that in the Docimasia Sicca, or assaying in the dry way, three things are requisite: 1. That the metal contained in the ore be all reduced to a complete form; for such part of it as is deficient in that respect cannot be united with the elutiated metal. 2. That the whole be collected into one mass; for when it is dispersed in numerous small grains, some of them are very easily scattered, and diminish the weight. 3. That the metallic form be preserved; for the extracted regulus must inevitably be diminished more or less by calcination. All these requisites are frequently effected conveniently enough in a crucible by fusion with proper strata of charcoal, provided the ore is free from sulphur and other volatile mixtures, and is entirely without a matrix, or united to one that can be melted by a moderate degree of heat; but if the matrix be refractory, notwithstanding the most subtle pulverisation, it will cover many of the metallic particles, and thus the reduction and fusion will be in some measure prevented. When this happens to be the case, we must add such other substances as not only promote fusion, but make the matter flow sufficiently thin to allow the reguline particles to fall to the bottom. These substances, which from the effect they have on the matter are called fluxes, are of a saline nature, and must therefore necessarily corrode the metals more or less; and hence the scoria, which are almost always tinged, contain a quantity of calcined metal. But as long as we are destitute of a sure method of measuring intense degrees of heat, and as long as it is necessary to perform the operation in close vessels to prevent the access of air, the force and proper continuance of the fire will be uncertain (A).

Now, by every excess or defect in this point some part of the regulus is lost; so that any judgment of the goodness of the ore, formed from the weight of the regulus, must be fallacious, or at least somewhat inaccurate.

Hence we may understand, that experiments upon ores made in the dry way, are liable to many faults and imperfections; to which we may add the following, viz., way of extracting that a given quantity of ore subjected to trial almost always exceeds in weight the regulus to be extracted from it. Now, since it is impossible to avoid a certain loss both during calcination and fusion, this loss will be the more remarkable, as the mass to be weighed becomes ultimately lighter. The case is quite otherwise with experiments made in the moist way; for here the weighty sediment, from which the quantity of the contents is judged, is never less, but often greater, than that obtained by fire.

In the attempts made to assay ores in the humid Method previous to those of Mr Bergman, both methods being used, the metallic part being extracted by a menstruum, and afterwards reduced by fire. Our author, however, has now shown a method of performing the operation without either calcination or fusion. "It must indeed be confessed (says he), that experiments in the humid way often require more care and pains than the other; but if accurate conclusions are thereby obtained, we ought not to grudge the slowness. Besides, in many cases this method is more expeditious than the other; and indeed almost always, if we content ourselves with such discoveries as can be made by the common calcinations and fusions; nay, sometimes the dry method is obviously insufficient, when the metallic content is either very small or volatile; but particularly if it be inflammable, as is the case with zinc."

In this method the ores to be examined should be reduced to a very subtile powder by pulverization and calcination. In dissolving such ores as contain fulphur, we ought to employ the vitriolic or marine acid; for the nitrous, by long continued heat, destroys the sulphur. Too great heat also dissipates some of it in vapours, or melts it into globules containing heterogeneous matters; therefore boiling ought to be avoided where it can be done. All the precipitates must be carefully collected, washed, and dried. Distilled water ought constantly to be used, and all the menstrua carefully depurated. Vitriolic acid our author calls diluted, when its specific gravity is below 1.3, the nitrous when below 1.2, and the marine when below 1.1. The precipitations should be carefully made in glass vessels; so that nothing may remain either through the deficiency of the precipitant, or be redissolved through its too great quantity. The clear liquor is to be decanted from the precipitate, water poured on in its place, the vessel shaken, and then suffered to stand; the water again decanted off, and more poured on in its stead, until it will no longer affect certain precipitants by which it must be examined. The sediment is then to be collected on a filter, the latter being previously weighed,

(A) The newly-invented thermometer of Mr Wedgwood has furnished us with a method of measuring intense degrees of heat; but we have not yet heard how far this has been found useful in practice. weighed, and made of paper not impregnated with alum. It is to be dried at first with a gentle heat, but afterwards exposed for five minutes to a heat of 100°. On cooling, it is to be weighed together with the filter; the known weight of which must afterwards be subtracted. The sediment is best washed in a bottle; for a filter when once impregnated with saline matter cannot be freed from it again without great difficulty, especially if an interval of some hours intervenes.

The alkali made use of in Mr Bergman's experiments, was that of soda saturated with aerial acid. His phlogisticated alkali is made by deflating equal weights of pure nitre and cream of tartar intimately mixed together; the residuum is the common white flux. Half an ounce of this is dissolved in half a quadrant of distilled water. To this he adds, in a digesting heat, two ounces of Prussian blue, carefully avoiding such an effervescence as may throw anything over, which easily happens if the quantity be too large. The pigment soon loses its beautiful blue colour, growing not red but black; which shows that a decomposition has taken place. The Prussian blue used in his experiments contained in 100 parts only 23 of the pigment and 77 of the clay; so that if we employ the blue made without any alum, 221 grains of it will saturate the half ounce of alkaline salt more completely than the two ounces of the kind already described. But in whatever manner the operation is performed, after the addition of the last quantity, the whole must be exposed to a stronger digesting heat, and stirred with a wooden spatula. If the liquor be too much diminished by evaporation, the defect must be supplied by adding more water. When the liquor becomes clear, the residuum must be collected upon filtering paper, and gradually washed with warm water until all the soluble part is extracted; when, if the operation has been properly conducted, the filtered liquor amounts to a whole quadrant, of a brownish yellow colour, and so well saturated with colouring matter, that it does not change the colour of paper tinged with Brazil wood. This lixivium, however, contains a small quantity of Prussian blue, about 4 lb. to a cwt. of the alkali. These should be previously separated by an acid, or, which is better, corrected by subtracting from the weight of the sediment 16 effay pounds for each quadrant of the lixivium. When we wish to examine the colour of the precipitate exactly, however, the lixivium we employ must necessarily be well depurated; for by neglecting this precaution we may easily persuade ourselves that any metal precipitated by the lixivium has a blue colour. When we only wish to ascertain the weight, the lixivium, having the small proportion of Prussian blue intermixed, may be employed: but still the proper correction must ultimately be made use of; for the precipitating acid is wont to impair the qualities of the lixivium, and even to destroy them altogether, especially in a warm temperature. Calcareous earth, whether in its mild or caustic state, is also capable of abstracting a coloured substance from iron and other metals.

In the precipitation of metals by metals, it is to be observed, that the acid of the solution ought to be somewhat predominant; but any considerable excess must be corrected occasionally, either by alkali, water, or spirit of wine.

Vol. VI. Part II.

In the following experiments an essay cwt. was always employed, unless where it is expressly mentioned otherwise: conclusions sufficiently accurate may indeed be obtained from 25 lb. nay sometimes from smaller quantities. In these cases our author mentions Bergman's usual quantity; applying to them those formulae of calculation which are founded on the mutual proportions of the proximate principles constituting metallic salts. By an easy substitution, the same formulae may be used by those who employ ¼ or ½ cwt. We now come to describe the method of essaying the ores of the particular metals.

1. Ores of Gold. This metal occurs in the bowels of the earth native, possessing a complete metallic form, although in general the small particles of it are so fine as to be imperceptible in various matrices, that they are entirely invisible. It is also found mineralized, or united with sulphur by means of iron or some other metal. These two species of ore we shall consider separately.

Native gold is very seldom, if ever, free from heterogeneous matters; the most usual mixtures are cop-per, silver, and sometimes iron. The first of these remains in the menstruum, and may be separately collected by dissolving the gold in aqua-regia, and precipitating it by martial vitriol: the second falls during the solution, yielding a salted silver; which, being washed and dried, shows the weight of the silver contained; and the iron may be discovered by phlogisticated alkali. The precipitate occasioned by martial vitriol is pure gold in its metallic state, but very subtilly divided, and therefore its weight requires no correction.

Hence it appears how small a portion of gold inherent in the ores of other metals may be extracted; besides, a solution containing the most minute particle of gold instantly produces the purple precipitate of Caffius, with a solution of tin properly prepared.

As to the ore which contains gold adhering to and surrounded by foreign particles, 1. We must reduce a determined weight to an impalpable powder by trituration and led. Then let the powder, weighed beforehand, be boiled in aqua regia, as long as anything is taken up by the menstruum; after which, let the exhausted ore, well washed, be collected, excinced to ignition, and weighed. Let the clear solution (the colour of which, in some degree, affords a method of judging) be precipitated in the usual way by martial vitriol; the precipitate well washed, dried, and weighed, shows the gold, which, added to the weight of the exhausted ore, ought to be equal to the original weight, unless somewhat has been dispersed by the pulverisation, or unless some of the matrix has entered the menstruum. The former of these is discovered by comparing the weights before and after pulverisation; the latter by precipitants.

When grains of gold are mixed with loose earthy particles, they are sometimes easily separated by mechanical application of water.

When the metal is mineralised by sulphur, as in the gold mines, let one or more essay cwts. reduced to powder be gently boiled in the nitrous acid, or rather sulphur, digested in a heat of 50°—80°, lest the sulphur should be destroyed. It is even necessary to employ a more gentle heat for this purpose, that the sulphurous par- ticles, gradually separating, may remain in their natural state; for if they melt, the heterogeneous particles, which ought to have been removed, will be inclosed in the melted mass. The menstruum ought to be added in several portions, about five times the quantity of the ore at each turn. The pyrites is acted upon by this menstruum; an effervescence ensues, which continues for some time; after which a fresh quantity of the acid is to be added, until the sulphur is obtained pure and of its proper colour. From 12 to 16 parts of the acid are usually required to one of the ore. The purity of the sulphur is easily ascertained by caustic alkali.

The matrice, if insoluble in the menstruum, remains at bottom, together with the gold; which is distinguished by its peculiar colour and splendor, and may be separated from the matrice by careful elutriation. The particles of gold assume the form of very small grains, yet such as have angular points discernible by a good eye; and their appearance gives some reason for supposing that they have rather been intimately mixed with the pyrites than dissolved in it. The clear solution, which is generally green, must be evaporated, made red hot, and then weighed. Any other metals that happen to be present besides iron, may be extracted by suitable menstrua; as copper by the volatile alkali, manganese by dilute nitrous acid, with the addition of a little sugar: zinc is scarce ever met with in gold pyrites; but if it should happen to exist, may be extracted by any menstruum; and silver by pure nitrous acid. Calcareous earth, when it happens to form the matrice, unites with nitrous acid, and clay with that of vitriol. The sum of the weights of all the ingredients ought to be equal to the original weight of the ore; and unless any loss has been sustained during the operation, any deficiency may be attributed to the consumption of the sulphur.

2. Ores of Platina. The only metal with which platina is known to be alloyed is iron. This may be separated in a great measure by boiling the grains of platina, reduced to as fine a powder as possible, in marine acid, by which the original weight of the grains is generally reduced by about 0.05 of the whole. The depurated platina, dissolved in aqua-regia, easily discovers itself by precipitation with martial vitriol, if any gold be present: and, on the other hand, if platina contains a small quantity of gold, the latter may be discovered by any neutral salt containing vegetable or volatile alkali.

3. Ores of Silver. This metal, when found in its native state, is generally alloyed with gold or copper, or both. The silver and copper will be taken up by nitrous acid, leaving the gold at bottom in the form of a black powder, which may be made to assume a more metallic appearance by solution in aqua-regia and precipitation by martial vitriol. The copper remaining in the solution may then be collected by means of iron or aerated alkali.

Silver united with sulphur alone (the glassy ore of silver) is of a black colour. To discover the contents of this ore, let it be divided and powdered as much as possible, and then gently boiled for an hour in 25 cwt. of diluted nitrous acid; then after decanting the liquor, the operation is to be repeated with an equal quantity of the menstruum; and even a third time, unless the pure sulphur be now separated. The last particles of the silver adhere obstinately to the sulphur. If any gold be present, it remains undissolved at the bottom of the vessel. The decanted liquors being collected, are to be deprived of the silver by adding common salt; then if we suppose the precipitate when collected, washed, and dried, to be = a, the silver required will be \( \frac{100a}{129} \). The weight of the sulphur added to the above ought to be 100 lb. if the operation has been rightly performed, and no decomposition of the sulphur taken place. The clear liquor, which passes in filtering the luna cornea, easily discovers any other metal which may originally have been mixed with the silver; after which, the earth may be precipitated by means of a common fixed alkali.

It is difficult to separate the remains of the matrice from the sulphureous particles. To effect this, however, let the sum of the weights be first observed; then pour on caustic lixivium, which will dissolve the sulphur by a gentle digesting heat; the matrice then remains alone, and by its weight we can determine that of the sulphur; but we must not continue the digestion longer than is necessary to dissolve the sulphur, lest some of the siliceous earth should also be taken up, tho' Mr Bergman thinks there is no great reason to apprehend any inconvenience of this kind.

The red ore of silver may be examined by reducing Red silver to a very subtle powder, and boiling it twice gent. orely in diluted nitrous acid. A part of the menstruum being decanted off, wash the residuum well, then precipitate the silver by means of sea-salt; boil the abovementioned white powder quickly in aqua-regia until the arsenic be dissolved and the sulphur appear pure. The yellow solution, cautiously decanted, lets fall a very white powder on the addition of a proper quantity; and the final quantity taken up by the water may be obtained by evaporating to dryness. The sulphur separated in this manner, though it seems pure, yet contains some silver which the nitrous acid could not dissolve on account of the arsenic contained in the ore; but when this is taken away by the aqua-regia, the remaining parts of the silver adhere to the marine acid entangled among the sulphureous particles. This luna cornea may be freed from the sulphur by caustic volatile alkali diluted with water, and kept in a well-closeted vessel for some days. A weight of alkaline liquor equal to that of the sulphur is sufficient. By weighing the sulphur both before and after the operation, we know the weight of it as well as the luna cornea. Iron may be discovered by means of the phlogisticated alkali.

The white ore of silver, consisting of the metal united with sulphur, arsenic, and copper, is assayed in the following manner. Let 1 cwt. of the ore, reduced to powder, be gently boiled for an hour in a little more than 12 times its weight of diluted nitrous acid. The dry powder becomes black, foul, and sends forth the smell of hepatic sulphuris. Part of it is dissolved, and a white residuum remains at length at the bottom. The liquor cleared by subduing or filtration, contains the silver and copper; the former cannot be precipitated alone by sea-salt, because the marine acid attracts the copper more strongly. A white precipitate indeed, consisting of small needle-like crystals, is thrown down; but it is found on examination to consist of a peculiar combination of marine acid, silver, and copper. The silver therefore must be precipitated by a determined weight of copper, and the latter may be afterwards separated by iron or mild fixed alkali; but from the ultimate weight we must subtract that of such part of the precipitant as has entered the menstruum. The white menstruum must next be boiled in marine acid, and precipitated by water; by which means we obtain the arsenic, along with a small quantity of marine acid which it retains obliquely. After the separation of the arsenic, it remains only to prove the purity of the sulphur by volatile alkali, in order to determine whether it still contains any luna cornea, or copper.

Silver mineralized by sulphur sometimes contains antimony also; and this ore often appears in the form of capillary threads of an hoary brown colour. To analyse this, let it be gently boiled, or rather digested, for an hour, in five times its weight of diluted nitrous acid, until the silver is thoroughly dissolved, and all the antimony reduced to a white calx; which, after decanting the liquor, may be separated from the sulphur by marine acid, and precipitated by water. The solution of silver may be precipitated by sea-salt, and 1 cwt. seldom contains more than four ounces. Sometimes there is present in this kind of ore a little copper and iron besides the sulphur and antimony; in which case we may conduct the experiment in the same manner, only with the addition of a double portion of acid. All the metals are easily obtained by precipitating the silver by copper, and the iron by zinc or an alkaline salt.

The cornous silver ore, in which the metal is mineralized by the marine and vitriolic acids, has two remarkable varieties; one of which may be cut, and is somewhat malleable; the other brittle, and containing some sulphur besides the acid. An hundred parts of the former, reduced to a fine powder, is to be digested for one day in marine acid, shaking the mixture from time to time. The liquor is then to be decanted clear, and the residuum, previously well washed in water, added to the liquor. A solution of terra ponderosa is to be gradually dropped into the liquor, until it ceases to occasion any precipitation. Suppose the weight of the precipitate, washed and dried, = a; now vitriolated terra ponderosa, whose weight is a, contains of acid 0.15a, which corresponds with vitriolated silver 0.48a; for from 100 lb. of vitriol of silver, 68.75 of metal is obtained by reduction. But as all the silver is not precipitated from nitrous acid by mineral alkali combined with vitriolic acid, the luna cornea will therefore be 100 - 0.48a.

In the former salt, the silver contained is expressed by 0.33a; in the latter, by 75.19 - 0.36a; and therefore the sum required for the 100 will be 75.19 - 0.03a. The brittle cornous ore likewise contains sulphur; but the saline part may be extracted by volatile alkali, and the quantity of metal afterwards ascertained by the method already described. Or this compound may be reduced in the following manner: Let the mass be mixed with an equal bulk of alkaline salt in a glass mortar, and be formed into a globule by means of a few drops of water: let this globule be put into a crucible, the bottom of which has been previously fireproofed with sal soda, compressed, and covered with the same alkali. On applying a melting heat, the whole of the metal will then be reduced if the luna cornea has been properly collected.

4. Ores of Mercury. Native quicksilver is seldom mixed with any other metals than gold, silver, and bismuth. The first remains at the bottom on dissolving the fluid mass in nitrous acid; the second is discovered by sea-salt, which at the same time precipitates the mercury combined with sea-salt; and the third, though it is taken up by the spirit of nitre, is yet precipitated by the mere affluence of water.

The combination of quicksilver with sulphur (native Cinnabar, cinnabar) cannot be decomposed either by vitriolic, nitrous, or marine acid. Our author has even attempted in vain to disunite them by boiling for many hours in a solution of caustic fixed alkali in water. There are, however, he tells us, two ways of effecting a perfect decomposition; one by gently boiling for an hour the cinnabar with eight times its weight of an aqua regia, one fourth of which is marine acid; the other by boiling it in marine acid, with the addition of one tenth of the weight of the cinnabar of the black calx of manganese; but the former method is preferable, as no heterogeneous matter is thus added to the mercury. The menstruum is the same in both, viz., the deplogisticated marine acid; the only difference is, that in the former method it is deplogisticated by the nitrous acid, and in the latter by the manganese. In whatever manner, however, the sulphur be separated, it may be collected by a filter, and the mercury precipitated by zinc: copper precipitates mercury from the marine acid in a more imperfect manner.—If the ore under examination be very much entangled in the matrice, it must be mechanically freed from it by lotions; after which the soluble parts of the matrice being taken up by the nitrous, marine, or vitriolic acid, the metal itself is separated by aqua-regia.

When mercury is mineralized by the vitriolic acid, it may be separated by the help of the marine acid by mineralization or digestion, and the metal precipitated by vitriolic acid. Terra ponderosa dissolved in nitrous acid; after which the weight of the new earthy salt a being given, we can easily learn the quantity of metal contained; yet, as solution of mercury in nitrous acid is not totally precipitated by Glauber's salt, we must not here depend on the weight of the precipitate. By another process, therefore, our author obtained from 100 lb. of vitriol of mercury 33.899 of pure metal, and from an equal weight of corrosive sublimate 75.5; from whence a calculation is easily deduced in the following manner. Let the quantity of vitriolic acid be = 0.15a; the vitriol of mercury containing this, = 0.44a; and the combination of mercury with marine acid, = 100 - 0.44a. In the former salt the mercury constitutes 0.29a, and in the latter 72.5 = 32a; so that the whole metallic content in 100 lb. is 72.5 - 0.03a. The scarcity of this ore, however, renders it still uncertain whether this combination of mercury with marine acid approaches to the nature of corrosive sublimate or mercurius dulcis. In the latter case the calculation comes out different; for mercurius dulcis contains above 0.91 of metal, and the whole content is expressed by 91.18a + 0.29a - 0.40a = 91.18 - 0.11a.

N.B. The weights on which all these calculations are are founded, may be found in Bergman's table of precipitates under the article Chemistry.

5. Ores of Lead. This metal, if ever found native, may be easily examined as to its purity by means of nitrous acid, which discovers copper both by its blue colour and precipitation by iron; and silver is discovered by the addition of copper.

When lead is mixed with sulphur, and freed from any matrice, it is to be reduced to a fine powder, and then boiled in nitrous or marine acid until the sulphur is obtained pure, which may be ascertained by the caustic fixed alkali. The solution is then to be precipitated by mild mineral alkali, when the lead is either alone or mixed with silver. In the former case, if \(a\) be the weight of the precipitate, that of the lead will be \(\frac{100a}{32}\). In the latter, the silver is to be extracted by volatile alkali, and the residuum multiplied by \(\frac{100a}{132}\) will give the weight of the lead. The aerated silver is known by the diminution of weight; and if this be called \(b\), then the silver in a metallic state will be \(\frac{100b}{129}\).

During this operation the solution in marine acid deposits a large quantity of plumbum corneum, which is to be dissolved in water before the precipitation. If antimony happens to be present, it is so much deplogificated by the concentrated nitrous acid, that it is calcined and falls to the bottom: the given weight of this multiplied by \(\frac{100}{138}\) shows the quantity of regulus dissolved in marine acid, which falls spontaneously upon being dropped into water, and the plumbum corneum is taken up in its place.

Iron is seldom found in galena; however, in case it should happen to exist, its presence may be discovered in the following manner. Let the solution in marine acid be first so far saturated with fixed alkali, that the acid may predominate only a little, and yet all precipitation be carefully avoided. The lead will then be precipitated by a polished plate of iron added during boiling; as will also the silver, which almost always exists in lead. The iron is then to be precipitated by aerated or phlogisticated alkali, and its weight corrected by the part of the metallic plate which is dissolved during precipitation.—When the ore contains any matrice, this is either soluble, and may at first be separated by vinegar; or else is insoluble in common acids, and is found collected at the bottom.

When this metal is mineralized by fixed air, and deprived of all heterogeneous soluble mixtures, it may be dissolved in nitrous acid, and precipitated by aerated mineral alkali; which being done, the quantity of lead is known by the weight of the precipitate as before. But if the matrice be soluble, we must employ the marine acid, and precipitate the metal by iron, as already directed.

Lead has lately been found mineralized by acid of phosphorus. An hundred pounds of this in powder is dissolved in nitrous acid by means of heat, excepting a few martial particles which commonly remain at the bottom. On adding the vitriolic acid, the dissolved lead falls in the form of a snow-white precipitate; which, when washed, collected, and dried, we may suppose to weigh \(a\); in which case the corresponding lead \(=\frac{100a}{143}\). The liquor remaining after precipitation yields, on being evaporated, a phosphoric acid.

6. Ores of Copper. This metal, when native, readily dissolves in nitrous acid. Gold, when mixed with it, falls untouched to the bottom in form of a black powder. Silver is soon precipitated by copper; and iron, by boiling the solution for some time, and insufflating to dryness, is gradually calcined and falls to the bottom.

Copper mineralized by sulphur is to be powdered, and gently boiled to dryness in five times its weight by sulphur of concentrated vitriolic acid. The residuum must then be well washed with water, until all the metallic part has entered the menstruum. The quantity of water used for the solution ought to be in some degree proportioned to the goodness of the ore; that which contains 0.05 of copper requires about 0.08 of water, and so on. A polished plate of iron, about twice the weight of the copper, is then to be immersed in the solution properly diluted, and the boiling continued until all precipitation ceases. If the quantity of water be too small, the precipitated metal adheres very obstinately to the surface of the iron plate; which, however, may always be freed by making use of a proper quantity of liquid. The precipitated copper, after being well washed, is to be speedily dried; "but yet (says our author) with such a degree of heat as to make the surface of the metal of different colours, which instantly and sensibly increases the weight."

Sometimes the precipitated copper is found mixed with iron, especially in a poor ore; in which case the iron from precipitate must be redissolved in order to obtain a rich precipitated solution; and this deposits pure copper, if the operation has been properly conducted. A similar circumstance also takes place in the precipitation of silver by copper; a rich solution yielding the metal pure, but a poor one affording it mixed with copper. When the precipitated copper is alloyed with other metals, they may easily be separated by solution in the nitrous acid. Gold, as has already been observed, remains at the bottom in form of a black powder, and silver is precipitated on a copper plate.

During this process almost all the sulphur is dissipated by the intense heat necessary for evaporating the vitriolic acid to dryness; however, we may judge of its quantity from the sum of the weights of the other ingredients, compared with that of the whole; or a solution in aqua-regia may be made on purpose for collecting the sulphur.

The beautiful green ores of copper called malachites, Malachites, in which the metal is mineralized by fixed air, are totally soluble in acids, and may be precipitated by iron or aerated fixed alkali. In the latter case, supposing the weight of the precipitate to be \(a\), that of the copper will be \(\frac{100a}{194}\). Calcareous earth, when any happens to be present, may be thrown down by aerated alkali, and the metal precipitated by phlogisticated alkali.—Blue calciform copper, in which the metal is also mineralized by aerial acid, is to be analyzed in the same way. Calciform red copper is also totally or in great part dissolved with effervescence, though somewhat weaker than the other. Mr Bergman has examined by many different ways the red quartz of Mr Cronstedt, supposed to contain a red calx of copper. None of this metal, however, was extracted either by volatile alkali, or boiling the vitriolic acid to dryness upon it. As the siliceous matrices, however, cannot easily be dissolved by the common menstrua, a quantity of mineral fluor was added to the vitriolic acid. The fluor acid has the property of dissolving the particles of quartz, and setting at liberty those of copper which might be entangled among them; but though this experiment always succeeds when copper is present, yet in this substance not the smallest sign of metal could be discovered, and therefore it is probable that Mr Cronstedt was mistaken.

7. Ores of Iron. Though some traces of this metal are found almost everywhere in the mineral kingdom; yet the ores which contain it in considerable quantity, have it either mineralized by sulphur or more or less calcined. Ores of iron are frequently found in Sweden so perfect that they obey the magnet, or are themselves magnetic. These attractive and magnetic ores, though they do not contain much sulphur, are yet seldom entirely without it, though more can be extracted by menstrua. Those saturated with sulphur are called sulphurous pyrites, nothing but sulphur being extracted from them; for though they sometimes contain the metal in sufficient quantity to pay the expense of smelting, it is always brittle and untractable in the fire, and is easily corroded by rust on exposure to the open air.

All the ores of iron, when reduced to a very subtile powder, and repeatedly boiled in marine acid, part with their metal; the solution of the pyrites is accelerated by the addition of a small quantity of nitrous acid. In order to obtain the metal by itself, we must precipitate it by phlogisticated alkali; when, if we suppose the weight of the precipitate to be \(a\), the corresponding quantity of metal will be \(\frac{a}{6}\); but this must be corrected according to the quantity of the precipitant. That ore which is naturally soluble by vitriolic acid requires nothing but water to precipitate by means of phlogisticated alkali.

Manganese, which is frequently mixed with iron, may easily be discovered by immersing the blue sediment (carefully weighed) in water sharpened by nitrous acid; by which means the part arising from the manganese is dissolved. Other metals sometimes enter the ore of iron in larger quantities; which for the most part render the former useless, by imparting bad qualities to the smelted iron.

8. Ores of Tin. The examination of native tin by the humid method is attended with no difficulty: for the addition of nitrous acid quickly deprives it far off its phlogiston, that it is reduced to the form of a white calx; the iron and copper, if any be present, remaining in the liquor. An hundred parts of tin corroded by nitrous acid, washed and dried, yielded 140 of calx. Arsenic may be separated by washing with large quantities of warm water; for little enters the acid menstruum. The other metals are but rarely united with native tin.

The pure ore, is commonly called, according to the magnitude of its crystals, zinngrauen or zwitter, by the Germans. These forms cannot be examined in the moist way without great difficulty, as they are not acted upon effectually either by vitriolic, nitrous, or marine acid, or even by aqua-regia. The reason of this insolubility is, that the calx, being well dephlogisticated, is either not taken up at all, or in very small quantity; and besides, being involved in strong particles, the menstrua can scarce have access to it. The following method is recommended by our author as one by which this process may be nearly effected.

"To a very subtile powder of the crystalline tin ore obtained not only by levigation but elutriation, let there be added a quantity of concentrated vitriolic acid, and let this be exposed to a strong digesting heat for several hours; then pour on a small portion of concentrated marine acid; and upon agitating it, a vehement effervescence immediately begins, with a considerable heat arising from the marine acid, which is partly deprived of its water by the vitriolic, and generates a marine acid air. By this method the forces of the two are conjoined: water is to be added in about an hour after, and the clear liquor decanted after the sediment has fallen. This operation is to be repeated with the residuum until the acids can dissolve no more. What remains finally undissolved is nothing more than the stony matrice. Let the solution precipitated by means of aerated alkali \(=a\), and the quantity of regulus will be \(\frac{100a}{131}\). The subtile atoms of the crystalline ore, intimately mixed with any matrice, may, after due pulverisation, be separated by washing from a given portion, as the crystals are nearly of six times the specific gravity of water; so that they not only exceed the gravity of the earthy particles, but that of the ores of other metals, and approach even to the lighter metals themselves. The crystalline particles, after being separated, are exposed to the trial above described. The larger distinct crystals can seldom be employed; the most common ore contains particles of them very much dispersed."

The adventitious metals usually found in tin are copper and iron.

9. Ores of Bismuth. This femimetal, when native, Bismuth, is easily taken up by nitrous acid, and may then be precipitated by water; after which any other metals that happen to be mixed with the bismuth remain in the liquor, and may be separated by the methods already frequently described. When mineralized by sulphur, the ore is decomposed by flight boiling in the same menstrum; so that the sulphur may be at last obtained; which when washed and collected is to be examined as to its purity and quantity. The solution of the metallic part precipitated by water leaves a white calx; and supposing its weight \(=a\), that of the corresponding metal will be \(\frac{100a}{113}\).

Iron is sometimes met with in these ores, which may easily be discovered after the separation of the bismuth.

Bismuth in form of a calx, whether alone or mineralized by aerial acid, is also soluble in nitrous acid, and may be precipitated by water, upon which the sediments heterogeneous matters remain in the liquor. The presence of cobalt is discoverable by its communicating a red colour. Ores of Nickel. This substance, when found native, may be dissolved by the nitrous acid; and when precipitated by aerated alkali, yields a calx which almost always contains iron, arsenic, and cobalt, in the same proportions in which they usually accompany the regulus obtained in the common way. If silver and bismuth happen to be present, which, however, is very seldom the case, the former is precipitated by common salt before the latter is employed. Sulphur may be separated and collected during solution.

Nickel, mineralized by vitriolic acid, is scarcely ever without iron. A great part of the latter, however, is separated by long and violent boiling in water. Aerated alkali throws down a greenish white precipitate; and if we suppose the weight of this = \(a\), that of the reguline nickel is \(= \frac{100a}{135}\). The same metal mineralized by aerial acid is dissolved by spirit of nitre, and may be precipitated by means of mild alkali.

Ores of Arsenic. The purity of native arsenic may be examined by dissolving it in four times its weight of aqua-regia, and the solution slowly evaporated without any separation of the metal. The arsenic is then to be precipitated by water, and collected upon a filter; the heterogeneous metals will be contained in the clear liquor which passes through the filter. If any silver be present, it falls to the bottom in conjunction with the marine acid. Iron is hardly ever absent altogether, and is frequently in such quantity, that the mass has a polished appearance, most commonly crystalline, and is commonly known by the name of milspickel.

Arsenic mineralized by sulphur is to be dissolved in marine acid, with the addition of the nitrous occasionally, in greater or lesser quantities, so that the sulphur may be separated free from all metallic matter. The sulphur collected, washed, and weighed, indicates the quantity of the arsenical part. This, however, ought to be precipitated separately by water, and weighed; a step which is always necessary where great accuracy is required. Arsenic dissolved by marine acid may also be precipitated in its metallic form by zinc; the solution being previously weakened by spirit of wine. When sulphur alone is united to the arsenic by its different proportions, it produces different colours, from a dilute yellow to an intense red. But if a considerable portion of iron also enters the composition, a white colour is generated, and a very different species of pyrites formed, which is called the arsenical pyrites. This may be analyzed by solution in marine acid in the manner already described.

In analyzing arsenical ores in general, we must take care not to add too much nitrous acid, as we would thus take away the whole of the phlogiston, and disengage the arsenical acid. The smallest quantity sufficient for solution ought therefore to be employed; otherwise water will occasion no precipitation; and even with all our caution, it is scarce possible to prevent a small portion of the arsenical acid from being disengaged, especially if the boiling be long continued. This may be recovered by evaporating to dryness, though rarely alone, but united either with the alkaline earths or the metals which are present. Some of the arsenic easily flies off.

Ores of Cobalt. This semimetal, when native, almost always contains iron, arsenic, and frequently nickel; whence no doubt it is, that some authors have said that vitriolated cobalt is of a green colour, as well as the other salts containing this semimetal; but the truth is, that they are of an obscure red, unless the nickel be in large quantity. To separate these metals from one another, dissolve the compound mass in water, evaporate to dryness, and extract the cobalt with vinegar. Let the weight of the precipitate be \(a\), and that of the corresponding regulus will be \(= \frac{100a}{160}\). If the arsenic be abundant in the evaporated solution, it may perhaps be precipitated by the effusion of water. Cobalt united with sulphur may be treated in the same way, as it differs from the native cobalt only in containing a small quantity of sulphur, which is to be separated and collected.

Cobalt has been discovered by Mr Brandt in a state mineralized by union with vitriolic acid, along with a large quantity of iron, and without any arsenic. This may be examined by solution in aqua regia. The solution is yellow, with scarce any redness, on account of the great quantity of iron. By boiling it affumes an obscure green, and resumes its former colour; a property by which the existence of cobalt is always known. The ore does not appear to contain any sulphur; but a few drops of solution of terra ponderosa dissolved in marine acid immediately discovered the vitriolic acid. Scarce any vestige of arsenic was to be met with. The vitriolic acid, however, though present in such abundance, was yet so far dephlogisticated, that it could not unite with the semimetal into a vitriolated cobalt, and therefore must be considered only as an impurity.

The trichetes of the Greeks, which is found in the Trichites mines of Herngrund and Idua, adhering to an argillaceous stone, is found to contain a real cobalt, besides cobalt, the clay and vitriolic acid. It can only be precipitated by the phlogisticated alkali.

Cobalt frequently exhibits beautiful red efflorescences, sometimes more dilute, and sometimes of a deeper cobalt colour. Sometimes it appears like a loose powder, sometimes concrete, and at times forming most beautiful crystals radiating from a centre like a star. These substances always show some vestiges of arsenic; but as this substance is incapable, either in its reguline or calcined state, of imparting a red colour to arsenic, it is reasonable to suppose that it is done by the arsenical acid itself, as all acids have the property of communicating a red colour to cobalt. To determine this point, Mr Bergman made the following experiments.

1. Having artificially combined the acid of arsenic with cobalt, he found an exact resemblance betwixt this compound and the natural crystals above mentioned.

2. On account of the scarcity of the latter substance, he extracted the pure acid of arsenic, first separating it by vitriolic acid, and then absorbing the latter by highly rectified spirit of wine, which takes up only the superfluous acid, leaving the vitriolated cobalt untouched. Natural arsenicated cobalt is scarcely soluble in water, unless the latter be sharpened by an acid; and when thus dissolved it should be precipitated by mild alkali, to discover the quantity of semimetal. Cobalt artificially combined with arsenical acid, and dried, shows the same properties with the natural.

The black calx of cobalt is generally found concen- ted into an hard mass, known by the name of the glaify ore of cobalt. This, when pulverized, may be dissolved in the marine acid or aqua-regia, and examined like the former.

13. Ores of Zinc. If ever this semimetal occurs in a native state, its purity may be easily determined, as it is readily soluble in all the acids; and whatever heterogeneous metal is present may be precipitated by zinc.

The pseudo-galena which contains zinc mineralized by sulphur, together with iron, must be carefully treated with nitrous acid, in order to extract the metallic part without decomposing the sulphur. If no other metal than iron be present, it may be precipitated by zinc; but if others also are combined with it, the iron must be calcined, by repeatedly abstracting nitrous acid to dryness, and a new solution, made by vinegar or any other acid, examined.

To analyze the combination of vitriolic acid and zinc, dissolve the salt in water, and precipitate the solution with mild fixed alkali; when, if the weight of the precipitate be \(a\), that of the regulus will be \(\frac{100a}{193}\). When iron is present, as is usually the case, it ought to be precipitated by a known weight of zinc.

This semimetal, mineralized by aerial acid, ought to be dissolved in some of the mineral acids, and then precipitated by phlogisticated alkali or mild fixed alkali. When the former is employed, the weight of the sediment must be divided by 5, in order to ascertain that of the metallic part.

14. Ores of Antimony. The purity of this semimetal, when found native, may be examined by reducing it to a calx with strong nitrous acid; in which case, if it has been entirely pure, there will remain only a small part dissolved in the water, and which will separate on the addition of water. When mineralized by sulphur, the metallic part is taken up by aqua-regia, and the sulphur remains pure. The solution, by boiling with strong nitrous acid, lets fall a calcined antimony; which being separated, the remaining liquor may be examined by phlogisticated alkali or otherwise at pleasure.

By the addition of a certain quantity of arsenic, crude antimony grows red, frequently exhibiting beautiful fasciculi of filaments radiating from a centre. The presence of arsenic may be discovered by gently boiling the powder in aqua-regia, until the sulphur be obtained pure. The arsenic and antimony are contained in the clear solution, and may be separated in the following manner. Let concentrated nitrous acid be poured on, and the antimony reduced to a white calx by boiling. Let this be collected on a filter; and the liquor that passes through affords arsenic by evaporation, but generally deprived of phlogiston, or reduced to the state of arsenical acid. As the caustic alkali also takes up both sulphur and antimony, it may be advantageously employed, especially for the separation of silver, or other metals which do not yield to this menstruum. A hepar sulphuris is indeed produced; but in this case it dissolves little or nothing.

15. Ores of Manganese. This semimetal accompanies most of the ores of iron, though it has likewise ores of its own in which it predominates, but seldom to be met with. It has never been found native or mineralized by sulphur, but commonly occurs in the form of a calx, generally alone and black, though sometimes mineralized by the aerial acid. These ores, after being reduced to a subtile powder, must be immersed in any acid, particularly one of the mineral kind, together with a small piece of sugar, in order to supply the phlogiston necessary for dissolving the manganese. Fresh acid is to be poured repeatedly on the calx with sugar, until no more can be extracted by a digesting heat; after which the solution is to be precipitated by mild alkali; and if we suppose the weight of the sediment \(=a\), that of the corresponding regulus will be \(\frac{100a}{180}\). The insoluble residuum at bottom either contains heterogeneous mixtures or belongs to the matrix.

To separate the iron from calx of manganese combined with aerial acid, nitrous acid is to be repeatedly with aerial acid abstracted from the ore, and the heat, after each addition, increased to ignition; after which the manganese will be obtained pure, or at least contaminated with iron in a much smaller degree than before. It may then be separated by strong concentrated vinegar or diluted nitrous acid. Manganese, when precipitated from superabundant nitrous acid by phlogisticated alkali, totally dissolves in distilled water; which property affords likewise a method of separating it perfectly from iron.

Besides the foregoing kind of operations which relate only to the ores of metals, effaying is used in metallurgical operations to signify the method of determining how much gold or silver is contained in any mass of metal already melted from its ore.

1. Effay of the Value of Silver, to examine its purity, or the quantity of alloy mixed with it. The common method of examining the purity of silver, is by mixing it with a quantity of lead proportionable to the quantity of imperfect metals with which it is supposed to be alloyed; by testing this mixture; and afterwards by weighing the remaining button of silver. The loss of weight which the silver suffers by cupellation shows the quantity of imperfect metals which it contained.

We may hence perceive, that the effay of silver is nothing else than the refining of it by cupellation. The only difference between these two operations is, that when silver is tested merely for the purpose of refining it, its value is generally known; and it is therefore mixed with the due proportion of lead, and tested, without any necessity of attending to the loss of weight it suffers during the operation; whereas, in the effay, all possible methods ought to be employed to ascertain precisely this loss of weight. The first of these operations, or the mere refining of silver, is made in the great, in the smelting of silver ores, and in mints for making money*. The second operation is never made but in small; because the expenses of small operations are less than of great, and in the requisite accuracy is more easily attended to. The last operation is our present object, and is to be performed in the following manner.

We suppose, first, that the mass or ingot of silver of which an effay is to be made, consists of 12 parts perfectly equal; and these 12 parts are called penny-weights. Thus, if the ingot of silver be an ounce weight, each of these 12 parts will be \(\frac{1}{12}\) of an ounce; or if it be a mark, each of these will be \(\frac{1}{12}\) of a mark. Hence, if the mass of silver be free from all alloy, it is called silver of 12 penny-weights; if it contains \( \frac{1}{12} \) of its weight of alloy, it is called silver of 11 penny-weights; if \( \frac{3}{4} \) of its weight be alloy, it is called silver of 10 penny-weights; and these 10 penny-weights or parts of pure silver are called fine penny-weights.

We ought to observe here concerning these penny-weights, that assayers give also the name penny-weight to a weight equal to 24 real grains; which latter real penny-weight must not be confounded with the former, which is only ideal and proportional; and such a confusion is the more likely to take place, as this ideal penny-weight is also, like the former, divided into 24 ideal grains, which are called fine grains.

An ingot of fine silver, or silver of 12 penny-weights, contains then 288 fine grains; if this ingot contains \( \frac{1}{12} \) part of alloy, it is said to be silver of 11 penny-weight and 23 grains; if it contains \( \frac{3}{4} \) of alloy, it is called silver of 11 penny-weight and 22 grains; if it contains \( \frac{1}{2} \), it is called silver of 11 penny-weight and 10 grains; and so on. Lastly, the fine grain has also its fractions, as \( \frac{1}{2}, \frac{1}{4} \) of a grain, &c.

As assays to discover the value of silver are always made in small, assayers only take a small portion of an ingot for the trial; and the custom in France is to take 36 real grains for this purpose, which is consequently the largest weight they employ, and represents 12 fine penny-weights. This weight is subdivided into a sufficient number of other smaller weights, which also represent fractions of fine penny-weights and grains. Thus 18 real grains, which is half of the quantity employed, represent five fine penny-weights; three real grains represent one fine penny-weight, or 24 fine grains; a real grain and a half represent 12 fine grains; and \( \frac{1}{2} \) part of a real grain represents \( \frac{1}{4} \) part of a fine grain, which is only \( \frac{1}{7} \) part of a mass of 12 penny-weights.

We may easily perceive, that weights so small, and assay-balances, ought to be exceedingly accurate. These balances are very small, suspended and inclosed in a box the sides of which are panes of glass, that they may be preserved from dust, and that their motion may not be affected by agitated air, so as to disorder their action.

When an assay of a mass or ingot of silver is to be made, the custom is to make a double assay. For this purpose, two fictitious semi-marks, each of which may be equal to 36 real grains, are to be cut from the ingot. These two portions of silver ought to be weighed very exactly; and they ought also to have been taken from opposite sides of the ingot.

Persons accustomed to these operations know pretty nearly the value of silver merely by the look of the ingot, and still better by rubbing it on a touchstone. By the judgment they form of the purity of the ingot, they regulate the quantity of lead which is to be added to it, as this quantity must be always proportionate to the quantity of imperfect metal mixed with the silver.

Nevertheless, this proportion of lead to the alloy has not been precisely determined. Authors who treat of this subject differ much. They who direct the largest quantity of lead say, that thereby the alloy is more certainly destroyed; and others who direct a small quantity of lead, pretend, that no more of that metal ought to be used than is absolutely necessary, because it carries off with it always some portion of silver. Every assayer uses his own particular method of proceeding, to which he is attached.

To ascertain these doubtful points, three chemists of the Academy of Sciences at Paris, Messrs Hellot, Tillet, and Macquer, were appointed by the French government. They were directed to ascertain everything concerning the assay of gold and silver by authenticated experiments, made under the inspection of a minister whose superior knowledge is equal to his desire of public good, and in presence of the officers of the mint.

The experiments made by these chemists, and the consequent regulation, have determined that four parts of lead are requisite for one part of silver of 11 penny-weight and 12 grains, that five parts of lead are requisite for silver of 11 penny-weight, eight parts of lead for silver of 10 penny-weight, 10 parts of lead for silver of nine penny-weight, and so on in the same progression.

Two cupels of equal size and weight are to be chosen. The custom is to use cupels of such a size that their weight shall be equal to that of one half of the lead employed in the assay; because such cupels have been found capable of imbibing all the litharge formed during the operation. These cupels are to be placed together under a muffle in an assay-furnace. The fire is to be kindled, and the cupels are to be made red-hot, and to be kept so during half an hour at least before any metal be put into them. This precaution is necessary to dry and calcine them perfectly; because if they contained any moisture or inflammable matter, an ebullition and effervescence would be occasioned in the assay. When the cupels are heated so as to become almost white, the lead is to be put into them; the fire is to be increased, which is done by opening the door of the ash-hole so as to admit air, till the lead becomes red, smoking, and is agitated by a motion of its parts called its circulation, and till its surface becomes smooth and clear.

Then the silver, previously beaten into small plates for easier fusion, is to be put into the cupels; the fire is to be continued, and even increased, by putting hot coals at the mouth of the muffle, till the silver shall have entered the lead, that is, till it have melted and mixed with the lead. When the melted matter circulates well, the heat is to be diminished by taking away partly or entirely, the coals put at the mouth of the muffle, and by closing more or less the doors of the furnace.

The heat ought to be regulated so, that the assays in the cupels shall have surfaces sensibly convex, and shall appear ardent, while the cupels are less red; that the smoke shall rise almost to the roof of the muffle; that undulations shall be made in all directions upon the surfaces of the assays, which are called circulations; that their middles shall be smooth, and surrounded with a small circle of litharge, which is continually imbibed by the cupels.

The assays are to be kept in this state till the operation is finished, that is, till the lead and alloy have soaked into the cupel; and the surfaces of the buttons of silver being no longer covered with a pellicle of litharge, become suddenly bright and shining, and are then then said to lighten. If the operation has been well conducted, the two effays ought to become bright nearly at the same time. When the silver has been by this operation well refined, we may see, immediately after it has brightened, the surface of the silver covered with rainbow colours, which quickly undulate and cross each other, and then the buttons become fixed or solid.

The management of the fire is an important article in effays. For if the heat be too great, the lead is scorified and imbibed by the cupel so quickly, that it has not sufficient time to scorify and carry along with it all the alloy; and if the heat be too little, the litharge is gathered upon the surface, and does not penetrate the cupel. The effayers say then, that the effay is choked or drowned. In this case the effay does not advance; because the litharge covering the surface of the metal defends it from the contact of air, which is absolutely necessary for the calcination of metals.

We have above related the marks of a successful effay. The heat may be known to be too great, from the convexity of the surface of the melted metal; from the too strong circulation; from the too vivid appearance of the cupel, so that the colours given to it by the litharge cannot be distinguished; and, lastly, by the smoke rising up to the roof of the muffle, or not being at all visible from its being so ardent and red-hot as not to be distinguishable. In this case, the heat must be diminished by shutting the door of the ash-hole. Some effayers, for this purpose, put round the cupels small, oblong, cold pieces of baked clay, which they call insulators.

If, on the contrary, the melted metal have a surface not very spherical, relatively to its extent; if the cupel appear dark-coloured, and the smoke of the effay do only creep upon the surface; if the circulation be too weak, and the scoria, which appears like bright drops, have but a dull motion, and be not soaked into the cupel; we may be assured that the heat is too weak; much more may we be assured of it when the metal fuses, as the effayers call it. In this case, the fire ought to be increased by opening the door of the ash-hole, and by placing large burning coals at the mouth of the muffle, or even by laying them across upon the cupels.

As soon as the lead is put into the cupels, the fire is to be increased, because they are then cooled by the cold metal; and the lead ought to be quickly melted, to prevent its caking from collecting upon its surface in too great quantity before it be formed into litharge; which it would do, and be difficultly fused, if the heat were too weak.

When the silver is added to the lead, the heat must be still increased; not only because the silver cools the mass, but because it is less fusible than lead. And as all these effects ought to be produced as quickly as possible, more heat is at length given than ought to be continued; and therefore, when the silver has entered the lead, the heat is to be diminished till it becomes of a due intensity for the operation.

During the operation, the heat ought gradually to be augmented to the end of it, both because the metallic mixture becomes less fusible as the quantity of lead diminishes; and also because the lead is more difficultly scorifiable, as it is united with a larger proportion of silver. Hence the effays must be rendered very hot before they brighten.

Vol. VI. Part II.

When the operation is finished, the cupels are left in the same heat during some seconds, to give time to the last portions of litharge to be entirely absorbed; because, if any of it remained under the buttons of silver, it would stick to them. The fire is then allowed to extinguish, and the cupels to cool gradually, till the buttons have entirely fixed, particularly if they be pretty large; because if they cool too quickly, their surfaces fix and contract before the internal mass, which is thereby so strongly compressed as to burst through the external solid coat and form vegetations, or even to be entirely detached from the rest of the mass, and dissipated. This is called the vegetation of the button. It ought to be carefully prevented, because small bits of silver are sometimes thrown out of the cupel.

Lastly, when the buttons are thoroughly fixed, they are to be disengaged from the cupels by a small iron utensil while they are yet hot; otherwise they could not be disengaged clean and free from part of the cupels, which strongly adhere to them when the heat is much diminished.

Nothing then remains to complete the effay, but to weigh the buttons. The diminution of weight which they have sustained by cupellation will show the purity or value of the ingot of silver.

We ought to observe, that as almost all lead naturally contains silver, and that after cupellation this silver is mixed with the silver of the ingot in the button of the effay; before we employ any lead in this operation, we ought to know how much silver it contains; that we may subtract this quantity from the weight of the button, when we compute the fineness of the silver of the ingot effayed. For this purpose effayers generally cupel a certain quantity of their lead separately, and weigh accurately the button of silver it yields; or, at the same time when they effay silver, they put into a third cupel, in the muffle, a quantity of lead equal to that employed in both their effays; and when the operation is finished, and the buttons are to be weighed, they throw the small button produced from the lead alone into the scale which contains the weights; and as this exactly counterpoises the small portion of silver which the effay buttons have received from the lead employed in the cupellation, the weights will show precisely the quantity of silver contained in the ingot, and thus the trouble of calculating is prevented. The small button of silver procured from the cupellation of lead alone is called the witness. But to prevent this trouble, effayers generally employ lead which contains no silver, such as that from Willach in Carinthia, which is therefore procured by effayers.

In the second place, we shall observe, that a certain quantity of silver always passes into the cupel, as refiners in the great have long observed, and which happens also in effaying small quantities. The quantity of silver thus absorbed, varies according to the quantity of the lead employed, and the matter and form of the cupels; all which objects will undoubtedly be determined by the above mentioned chemists.

The cupellation which we have now described is exactly the same for effays by which the produce of a silver ore, or of an ore of another metal containing silver, is determined. But as these ores contain frequently gold, and sometimes in considerable quantity, when these effays are made, the buttons of silver obtained by the essays ought to be subjected to the operation called parting. See Silver, Refining, &c.

M. Tillet has published a memoir, showing that essays of silver made in the common method are uncertain and not to be depended upon; and that this uncertainty proceeds from the different quantities of silver absorbed by the cupel in different essays, according as the heat and other circumstances happened to vary. He therefore proposes, in order to render essays accurate, to extract from the cupel the quantity of silver it has absorbed during the operation, and to add this particle of silver to the button, as these two contain the whole quantity of silver in the matter essayed.

The variations in the different results of different essayers, or of the same essayer at different times, upon the same mass of silver, are sufficient proofs of the uncertainty mentioned by M. Tillet. These variations are occasioned, according to that author, principally from the following causes: 1. From the inaccuracy of the balances and weights employed. 2. From the faulty fusion of the mass to be essayed; by which means the contained alloy may be unequally diffused. 3. From the impurity of the lead, especially from its containing silver, which is not always equally diffused through its mass. 4. From the different proportions of lead used by different essayers. 5. From the difference of the intensity of heat: for if the heat be not sufficiently intense, the silver will still contain a portion of alloy; and if the heat be too intense, too much of the silver will be imbibed by the cupel. 6. From the want of care in picking the small particles of silver, which frequently adhere to the sides of the cupel separately, from the principal button. 7. From the spouting which sometimes happens unobserved by the essayer; and which may further falsify the essays of other pieces included under the same muffle, by the falling of the particles thrown out of one cupel into others adjacent. But, with all the attentions to avoid these causes of error, the author obtained different results from different essays of the same mass of silver. Nor could he, by any method, make his different essays consistent with each other, but by adding to each button the particle extracted from the cupel; and this method he found by accurate experiments to be perfectly exact.

M. Tillet observed, that the quantity of lead directed in the regulations established in consequence of the report made by Messrs Macquer, Hellot, and Tillet, is not sufficient to purify the silver perfectly from its alloy. He nevertheless approves of the said regulation; and considers the weight of the alloy retained by the button, as some compensation for the weight of the silver absorbed by the cupel. And as it is a constant fact, that the more lead is used, the greater is the loss by the absorption of the cupel, he remarks, that a regulation, directing a larger proportion of lead for France than is used in other countries, would be disadvantageous to that kingdom; as thereby the silver of the same denomination would be required to be finer in that than in other countries where a less proportion of lead was employed. He observes, that the above mentioned rule, "that the more lead is used, the greater is the loss by the absorption of the cupel," does not extend to quantities of lead much above double the usual quantities. Thus 32 parts of lead to one of silver, will not occasion more absorption than 16 parts of lead. For the refining scarcely takes place till the extraordinary quantity of lead be gone, and the silver is only or chiefly carried into the cupel along with the copper. Accordingly, he found, that he could render the silver finer by using four parts of lead at first, and afterwards adding two more parts when the irides began to appear, than by employing all the six parts of the lead at once. By this method of dividing the quantity of lead, the loss of silver by absorption was greater. M. Tillet did not find, that, by employing bismuth alone, or mixed with lead, his essays were more certain than when lead alone was used. He observed, however, that the addition of bismuth made the silver purer, but occasioned a greater absorption by the cupel.

2. Essay of the Value of Gold. The fictitious weights used to determine the purity of gold, and to essay this metal, are different from those of silver. See the preceding article. A mass of gold perfectly pure, or which contains no alloy, is ideally divided into 24 parts, called carats; this pure gold is therefore called gold of 24 carats. If the mass or ingot contains \(\frac{1}{24}\)th part of its weight of alloy, the gold is then of 23 carats; and if it contains \(\frac{1}{24}\)th or \(\frac{1}{24}\)th of alloy, it is gold of 22 carats, &c. Hence we see, that the carat of gold is only a relative and proportional weight, so that the real weight of the carat varies according to the total weight of the mass of gold to be examined. If this mass of gold weighs a mark, the real weight of the carat will be \(\frac{1}{24}\)th of eight ounces, which is equal to a mark. If the mass weigh an ounce, the carat will be \(\frac{1}{24}\)th part of an ounce, or 24 grains. If it is only a penny-weight or 24 grains, the real weight of a carat will be one grain; and so on.

For greater accuracy, the carat of gold is divided into 32 parts, which are relative and proportional weights, as the carat itself is. Thus \(\frac{1}{32}\)d of a carat of gold is \(\frac{1}{32}\)d of \(\frac{1}{24}\)th, or the \(\frac{1}{768}\)th of any mass of gold; and the gold which contains an alloy equal to the \(\frac{1}{768}\)th part of the whole mass is called gold of 23 carats, and \(\frac{3}{4}\); gold which contains \(\frac{3}{768}\)th of alloy is gold of 23 carats and \(\frac{3}{4}\), and so on.

The real weight now generally used in the operation for determining the purity of gold is six grains. This weight then represents 24 carats. The half of this weight, or three real grains, represents 12 carats. According to this progression, we shall find that \(\frac{1}{4}\)th of a real grain represents one carat, and the \(\frac{1}{768}\)th part of a grain represents the \(\frac{1}{32}\)d of a carat, or the \(\frac{1}{768}\)th part of a mass of gold to be essayed.

As these weights are exceedingly small, some essayers employ a weight of 12 grains, which must be very convenient.

When a mass or ingot of gold is to be essayed, six grains are to be cut off, and exactly weighed: also 18 grains of fine silver are to be weighed. These two metals are to be cupelled together with about ten times as much lead as the weight of the gold. This cupellation is conducted precisely like that of the essay to determine the purity of the silver, excepting that the heat must be raised a little more towards the end of the operation when the essay is going to brighten. Then the gold is freed from all alloy but silver. If the quantity of copper or other alloy destructible by cupellation be required to be known, the remaining button is accurately weighed. The diminution of weight from the The button containing gold and silver is then to be flattened upon a polished piece of steel, and care must be taken to anneal it from time to time, to prevent its splitting and cracking. By this method it is reduced to a thin plate, which is to be rolled up, in order to be parted by aquafortis*. The diminution found after the parting from the original weight of the gold assayed, shows the whole quantity of alloy contained in that gold.

The assay for determining the purity of gold is then made by two operations: the first, which is cupellation, deprives it of all its imperfect metals; and the second, which is parting, separates all the silver from it. By antimony also gold may be purified, which is a kind of dry parting. By this single operation, all the imperfect metals, and silver with which gold is alloyed, are separated. See Purification, Gold, Silver, Refining.

Essar-Hatch, is the miners term for a little trench or hole, which they dig to search for lead or ore.