In the assay of rich alloys, the proportion to the total amount of silver is very small, but notable; and it has been calculated for the alloys of copper employed in the arts at 3000th; but in the assay of poor ores, such as galena and other minerals treated in the large way, the loss is very great, for it is usually as high as th.

ड

By extracting the lead from cupels used in this class of assay, the metal furnished contains from about 300000th to 300000th of silver. The following experiment will give an idea of the influence of the proportion of lead on the loss. of silver 100 grains of commercial litharge were fused with 10 grains of black flux, and gave 27 grains of lead, and a slag; this was pulverised and reduced in the same crucible with 15 grains of black flux, and a second button was produced weighing 45 grains. These two buttons being cupelled separately, gave, the first 0035 and the second 001 only of silver. Three new quantities of 100 grains of the same litharge were fused; the first with

a part of

starch, the second with 2, and the third with 10 of the same reducing agent. The resulting buttons of lead weighed respectively 5.28 and 79 grains. These buttons were cupelled, and furnished 0035, 0035, and 003 respectively. From these experiments it will be seen that when the litharge is not reduced completely, there remains a notable proportion of silver in the scoriæ; but, nevertheless, in order to extract the largest possible quantity, the whole must not be reduced. Indeed, but a twentieth part need only be reduced, because more precious metal is lost in the cupellation of a large quantity of lead than remains in the portion not reduced. The loss of silver in large cupellations is less than that which takes place in an assay, because in the large way the litharge, or the greater part of it, is run off; whilst in an assay the cupel totally absorbs it, so that the latter presents, relatively to the same mass of lead, a very much smaller surface in the large than in the small way: now it can be readily seen that the quantity of silver lost by absorption into the pores of the cupel must be proportioned to its surface, all things being equal.

It has been ascertained by experiment that a cupel

absorbs about its own weight of litharge; so that from this fact a cupel of the proper size may be chosen, when the weight of lead to be cupelled is ascertained. It is always better to have the cupel about or as heavy again as the

lead to be cupelled.

as heavy

The various metals found in an alloy, which can be submitted to cupellation, scorify in proportion to their oxidisability. Those most oxidisable scorify with the greatest rapidity, and vice versa; so that those which have the greatest affinity for oxygen accumulate in the first portions of litharge formed, which, by that means becoming less fusible, sometimes lose the property of penetrating the cupel; hence the reason why cupellations always present more difficulties at the commencement of the operation than towards the end when the litharge formed is nearly pure oxide of lead, and can contain only oxide of copper.

The appearance of the cupel used in an assay will give indications of the metals the alloy contained. Pure lead colours the cupel straw-yellow, verging on lemon-yellow. Bismuth, straw-yellow passing into orange-yellow. Copper gives a grey, dirty red, or brown, according to its proportion. Iron gives black scoriæ, which form at the commencement of the operation, and are generally found at the circumference of the cupel. Tin gives a grey slag. Zinc leaves a yellowish ring on the cupel, producing a very luminous flame, and occasioning losses by carrying silver in its vapour, and by projecting it from the cupel in its ebullition. Antimony and sulphate of lead in excess give lithargeyellow scoriæ, which crack the cupel; but, when not produced in too great a proportion, are gradually absorbed by the litharge. If the lead alloy submitted to cupellation is found to produce this effect, a fresh portion must be mixed with its own weight of lead and scorified: the button so obtained can now be cupelled.

Amalgamation. There are a certain number of argentiferous matters which can be assayed by amalgamation, as they are treated in the large way by that method. Amongst these are native silver chlorides, sulphides, and arseniosulphides, which contain neither lead nor copper.

But this process is seldom had recourse to, because it is long, troublesome, and less exact than those just described.

Substances of the Second Class.

Native silver.

Alloys of copper and silver.

Alloys of other metals and silver (artificial).

Antimonide of silver.

Arsenide of silver.

Telluride of silver (AgTe).

Auriferous telluride of silver (see gold).

Hydrargyride of silver (amalgam), (Hg2Ag).
Auride of silver (see gold).

The following method of separating silver from galena is given in the Chemical News,' vol. ii. p. 239.

'Galena consists, as is well known, of the sulphide of lead, mixed with a variable proportion of the sulphide of silver, and both these substances fuse together, or melt at a bright red heat. Now, it so happens that, when sulphide of silver is fused with chloride of lead, what is called a double decomposition takes place; that is to say, chloride of silver and sulphide of lead are formed. Consequently, if we fuse together a quantity of argentiferous galena and chloride of lead, we shall remove the whole of the silver from the galena, and replace it by sulphide of lead. This, then, is the process: mix together the galena and chloride of lead in the proportion of 100 lbs. of galena, 1 lb. of chloride of lead, and 10 lbs. of chloride of sodium or common salt; or, if the galena be very argentiferous, add a larger amount of chloride of lead. The whole is then fused together, when the chloride of silver and common salt rise to the surface, and may be skimmed off, and the desilverised galena falls and may be run out from the bottom. The mixture of chloride of silver and salt may then be decomposed by lime and charcoal, or in any other manner, so as to reduce the silver and a portion of the surplus chloride of lead, by which a metallic mass will result, suitable for the operation of the cupel.'

General Remarks on the Assay of the Alloys of Silver and Copper. The assay of these alloys is nearly always accomplished (at least in England) by cupellation. This assay is

most important, as it is by the results obtained in the manner hereafter described that the price or value of all kinds of silver bullion is determined.

This class of cupellation is effected without difficulty, because the oxide of copper forms so slowly, that the litharge is always enabled to pass it into the body of the cupel. After having weighed the lead and placed it in the cupel, as soon as it is perfectly fused place in it the alloy to be assayed, wrapped either in blotting-paper or thin leaf-lead. It is essential, in this class of assay, to employ a sufficient quantity of lead to carry away all the copper. We may

always be sure of succeeding, whatever the alloy may be, by employing the maximum proportion of lead, that is to say, the quantity necessary to pass pure copper; but as the loss which the silver undergoes increases with the length of the operation and with the mass of the oxidised matters, it is indispensable to reduce this loss as much as possible by reducing the proportion of lead to that which is strictly necessary. Long experience has proved that silver opposes the oxidation of copper by its affinity, so that it is necessary to add a larger amount of lead in proportion to the quantity of silver present.

M. D'Arcet has obtained the following results by the most accurate experiments:

It is remarkable that below the standard of 500, the same proportion of lead must be employed, whatever that of copper. This fact is repeatedly verified by experiment. Whenever fine silver is fused in a cupel, it is always neces sary to add lead, in order to cause the button to unite and

form well. If less than ths of lead be employed, the button will be badly formed; the litharge cannot separate but by the action of a very strong heat, and a considerable loss of silver ensues. If, on the contrary, ths of lead is exceeded, the cupellation goes on well, but the loss is greater on account of the duration of the process. These proportions also ought to vary with the temperature. M. Chaudet has found, that to cupel an alloy containing ths of silver, 5 parts of lead are required in the middle of the muffle, 10 in the front, and only 3 at the back.

The proportion of copper carried off by litharge varies not only with the temperature, but even for the same temperature in relation to the amount of copper and lead the alloy contains. By cupelling 100 parts of copper with different proportions of lead in the same furnace, M. Karsten obtained the following results :

From which we see that the lead carried away from

th to 1th of its weight of copper. Much less lead can be employed in a cupellation by making the alloy maintain its richness of copper throughout the operation. This can be accomplished by adding to the alloy in the cupel small doses of lead, in proportion as that first added disappears by oxidation. If, for example, an alloy composed of 4 parts of copper and one of silver be fused with 10 of lead, by adding successive small doses of the latter, as already pointed out, but 7 parts will be consumed, although in the regular way from 16 to 17 would be employed.

The proportion of oxide of copper contained in the litharge increases each instant, and goes on incessantly increasing