with such a proportion of black flux, containing only the requisite proportion of carbon to reduce the oxide of lead, or with an equivalent mixture of carbonate of soda and charcoal. Pure sulphate of lead fused with one part of carbonate of soda and four per cent. of charcoal gives 66 of lead; but in order to employ this method the richness of the ore must be known, and the dry way is then useless, excepting for the estimation of the silver these substances always contain.

Assay by the wet way.-Lead is generally estimated in the state of sulphate of lead in this process. As the sulphuret of lead is the only ore in general use, I shall give its method of analysis by humid re-agents.

The well pulverized galena must be treated by hot weak nitric acid (if strong nitric acid be employed, much sulphate of lead will be formed); when sulphur will separate and a solution of nitrate of lead formed, to which is added a solution of sulphate of soda until no further precipitation takes place. The whole is then to be thrown on a filter, and the residue carefully washed, dried, and weighed; it is sulphate of lead and contains 68.28 per cent. of metallic lead.

If the galena contain either iron pyrites or blende, the analysis is made exactly in the same manner, for solution of sulphate of soda throws down nothing but the lead, leaving the iron and zinc untouched.

CHAPTER IX.

ASSAY OF TIN ORES.

ALL substances containing tin, which are generally assayed, contain that metal in the state of oxide; such are the ores, either in a rough or washed state, and all varieties of slag.

Although oxide of tin is completely reducible by charcoal at a white heat, it has such an affinity for silica, that whenever that substance is present, the metal cannot be wholly reduced, but at the highest temperature of a wind furnace. Hence the necessity of making all tin assays, at the same temperature as those of iron.

When an ore of tin is heated with a reducing flux, at the temperature of a copper assay, a certain quantity of tin is separated, if the substance is rich; but a large proportion remains in the slag, the quantity of which increases in proportion to the poorness of the mineral; and if a very large quantity of flux were employed, some tin ores which are treated with profit in the large way, would give little or no tin by this method of assay. This was, however, the process employed by most of the older assayers. Schlutter fused at a lead assay heat, with 4 parts of black flux,

and part of resin. Bergman employed 2 parts of tartar, 1 part of black flux, and part of resin.

The following is the result of some of Berthier's experiments on this subject:

Pure oxide of tin gives 72 per cent. of metal, with 4 parts of black flux, in a plain crucible; and 76 per cent. with part of carbonate of soda, in a charcoal lined crucible. Less is obtained in naked crucibles, than in lined crucibles; probably because the alkali of the flux attacks the clay of the pot, forming a silicate, which combines with a certain portion of the oxide of tin employed.

An ore containing 70 per cent. of metallic tin, gave 62 per cent. with a part of carbonate of soda, in a charcoal crucible; 51 per cent. with a part of borax, in a similar pot; 57 per cent. with from 4 to 8 parts of black flux; 60 per cent. with 3 parts of tartar, or a part of carbonate of soda, and one sixth of charcoal, in a plain crucible.

From which experiments may be seen, the advantage of employing an excess of charcoal, and the smallest possible quantity of flux.

The following mixtures of the same ore and quartz, in powder:

heated with four times their weight of black flux produced. The first, 52 per cent of tin; the second, 43 per cent.; the third, 28 per cent.; the fourth, 10 per cent.; and the last, nothing. The slag contained

from one fourteenth, to one tenth, of oxide of tin, and more in proportion to the quantity of silica.

The slags produced in the treatment of tin ores in the large way, give no return with black flux. Hence this method of assay is defective, and ought never to be employed in the estimation of the quantity of tin.

The above experiments were made at a low temperature: the following results are those obtained by the employment of a high temperature.

Crammer, and after him Klaproth, assayed tin ores at a high temperature; but their processes are long and troublesome.

.

Before assaying a tin ore, it is usual to submit it to the same kind of operation it undergoes in the large way.

Most assayers roast it with about one fifth of its weight of charcoal, and wash the residue by decantation, until the liquid ceases to be troubled. The object of this operation, is to separate the pyritic substances, which would unite with the tin during its fusion, altering its weight and character; but the same end can be obtained in a more expeditious, simple, and exact manner.

For instance, all pyritic and arsenical substances, being soluble in aqua regia, it is sufficient to boil the pulverized ore with an excess of that liquid, in order to separate all such matters. After the ebulli

tion, dilute with water, throw the residue on a filter, and carefully wash it. The filter is then dried, and the substance removed and calcined, to free it from a little sulphur, which will doubtless be with it, pro

ceeding from the decomposition of the pyrites. By these means the ore is completely purified, without losing the smallest possible trace of tin.

When the ore has been prepared either by roasting and washing, or by ebullition with aqua regia, it is placed in a charcoal crucible, with the addition of a suitable flux, and assayed at the same temperature as a sample of iron.

The choice of flux depends upon the nature of the gangues. There are two, nevertheless, which may be used in all cases; they are borax and carbonate of soda, employed in the proportion of 30 to 40 per cent., more or less. Borax may be employed in all cases, because it has the property of forming fusible compounds with earthy substances of every kind. As to carbonate of soda, although it has not the same property, it can always be used as a flux for tin ores, because their gangues are very siliceous.

In general, a tin assay may be considered exactly like an iron assay, although the one metal is so much more fusible than the other. Both metals have one property in common, that is, of forming silicates, which are with difficulty reducible by charcoal. In tin assays, in order to be sure no oxides remain in the slag, it is necessary that it contain bases capable of saturating silica. The best flux for tin ores seems to be ths of borax, and th of lime; and the assay made in a charcoal crucible.

The assay of tin slags must be performed by mixing up a certain weight, as 400 or 500 grs., with from 10 to 15 per cent. of iron filings, and fusing the whole in a naked pot. The iron takes the place of the tin