given for Lead Assays which require to be roasted previous to reduction.

If, on the other hand, the acidiferous lead ores contain other metallic salts, particularly sulphates, or if combinations of metallic sulphides and arsenides be mixed with them, a perfect roasting with charcoal is necessary.

A decigramme of the substance, so roasted, should be dressed with

Soda, 100 milligrammes;

Borax Glass, 40 to 50 milligrammes.

The further treatment of such an assay remains, however, the

same.

If acidiferous lead ores are mixed with other metallic salts, the bases of which are reduced with the lead, the latter must be examined for silver, copper, and antimony, in the manner above described.

In the reduction of oxide of lead from lead salts, the soda, with the surrounding charcoal, serves as a reducing agent for the oxide of lead and the acids. Thus, for example, arsenic acid is reduced to metallic arsenic, which volatilizes; sulphuric acid, to sulphur, which combines with the sodium of a portion of the soda, forming sulphide of sodium; chromic acid, to oxide of chromium, which permeates the slag. Borax, however, serves only, as with pure galena, to prevent the absorption of the soda by the charcoal support; as it fuses with soda, and probably with reduced portions of the ores, to a bead, while the metallic lead, in various sized globules, collects on its surface.

In the reduction of the oxide of lead, contained in a roasted assay composed of lead salts, contaminated with other substances, the soda plays the same part as in the preceding case, in addition to its solvent action, which it probably exerts on traces of silica; but an excess of borax, however, principally serves as a solvent for the non-reducible metallic oxides.

c. METHOD OF ESTIMATING THE VALUE OF LEAD, IN PRODUCTS OF

SMELTING WORKS,

And other Artificial Substances containing Lead in a pure oxidized state, or only combined with Vegetable Acids.

To this class belong litharge, abstrich, cupel grounds, pure roh and blei slags, all kinds of plombiferous glass, white lead, sugar of lead, et cetera.

The bodies of this class do not require to be roasted; but in dressing them for the reduction of their oxide of lead, it should be ascertained whether the body under examination is a simple oxide of lead, or a combination of oxide with other substances.

As the preceding substances may be at once subjected to the reduction process, they should be dressed, the pure oxide of lead, or such oxides of lead as are merely combined with organic acids, with

and those in which difficultly fusible ingredients predominate, as for example, cupel grounds, roh and blei slags, with a quantity of borax amounting to about 50 per cent.

If such products contain, besides lead, other easily reducible metals, the resulting lead button, after being weighed, must be examined for such metals in the manner already given.

d. METHOD OF ESTIMATING THE VALUE OF LEAD, IN MINERALS CONTAINING THIS METAL ALLOYED WITH OTHERS.

Black Tellurium Yellow Tellurium, Selenide of Lead, Selenide of Lead and Copper, and Selenide of Lead and Mercury, come under this class.

Of these minerals, the German author had only an opportunity of examining selenide of lead quantitatively for its metal

before the Blowpipe, and the method by which its value in this mineral was found, can only be given.

The assay succeeds most readily, and most accurately, when 100 milligrammes of the finely powdered mineral are introduced into a pipe clay crucible, with a small piece of iron of 30 to 50 milligrammes in weight, covered with a mixture of 150 milligrammes of soda, 30 milligrammes of borax glass, and 20 milligrammes of tartaric acid; on which mixture, about 50 milligrammes of dried chloride of sodium should be shaken, and the fusion then proceeded with in the manner described at pages 370, 371. If the selenide of lead contains an admixture of sulphide of lead, it will be decomposed; the selenium, however, combines with the radical of the soda, forming selenide of sodium, from which a small portion of the selenium volatilizes, and the lead, combined in one globule, deposits on the iron, at the bottom of the crucible. When the fusion is finished, and the fused assay has cooled, the crucible is broken, and the reduced lead freed from the iron and slag. The malleability of the lead button should be tested, and its weight ascertained.

When required, the value of the silver present may be ascertained by cupelling the button on bone ashes, and weighing the resulting silver button, when sufficiently large; if too small for the balance, it should be measured on the scale.

V. DETERMINATION OF TIN.

The usual method of determining tin by the dry way, which has been for so long a time employed, and is so still in most assay laboratories, is far from being accurate. There are a great many different methods of determining quantitatively the value of tin in an ore, by the dry way; but if it be a ferruginous tin ore, by one method pure tin will be obtained, but generally in too small a quantity, while by another, it appears in excess, but never pure. It must be here observed, that in practice, where it is certain that too much, and therefore impure tin, is obtained, the value is

taken at a few per cents. less than that obtained by experiment. The cause of this is, probably, that with a sufficient quantity of carbon, the reduced tin acts during the fusion as a reducing agent on the sesquioxide of iron, by which a portion of the latter is not only reduced to protoxide, but even to the metallic state, which latter combines with the reduced tin, affording a hard tin button, which, when weighed, gives too high a result.

The German author, after convincing himself, by a number of tin assays made according to different methods, of the above source of error, concluded, that these discrepancies could not be avoided in all the control assays made according to the usual methods, and for this reason he introduced a new one.

For this purpose, various quantities of pure peroxide of tin were mixed with a number of pulverized minerals, -as Iron Pyrites, Sulphide of Arsenic, Blende, et cetera, which generally occur along with tin ores;-and the value of tin which each should afford, calculated; they were then submitted to the action of the Blowpipe, with the view to determine this value by a number of experiments. The assays made with these mixtures fully convinced him that the same difficulties are to be encountered with the Blowpipe, as with the usual tin assays by the dry way, made on a large scale. From one to two, and once even four per cent. too much,-that is, of ferriferous tin, were obtained. A number of other experiments, in which all possible means to slag perfectly as protoxide in the dry way by fusion the peroxide, which would be formed during the roasting, were employed but in vain; in no case was the resulting tin free from iron. At length, after trying every other way, the iron was separated in an oxidized state from the oxide of tin by a simple yet accurate means in the moist way, namely, by hydrochloric acid, and the oxide of tin left behind with the earthy ingredients was then submitted to reduction.

By the employment of this method, a pure malleable tin was always obtained, whose weight agreed well with that found by calculation. Mixtures of the pure tinstones with other ores, containing either copper or iron, yield exactly the same result.

It may be objected to the employment of hydrochloric acid for

the separation of oxide of tin from oxide of iron, that it does not belong to Blowpipe analysis; but if the difficulties which are to be encountered in the dry way with the Blowpipe, in order to purify peroxide of tin from the oxides of iron and copper, be compared with the easy and accurate method by the moist way, there is no doubt the latter will be chosen, as the best which can be employed in the quantitative examination of ferruginous and cupriferous minerals and dressed ores for tin.

In respect to the quantitative examination for tin ;-minerals, ores, and artificial products, in which this metal forms an essential ingredient, may be classed as follows:

a. Such as contain the tin combined with sulphur;

b. Those containing the tin in an oxidized state; and,
c. Those in which metallic tin is alloyed with other metals.

a.

DETERMINATION OF TIN IN MINERALS, ORES, AND ARTI-
FICIAL PRODUCTS, IN WHICH THE TIN IS COMBINED

WITH SULPHUR.

Besides Tin Pyrites, Tin Schlichs, dressed on the large scale, may be placed in this division; although they contain tin in the state of oxide, they very often, notwithstanding the roasting, exhibit traces of metallic sulphides and arsenical compounds.

Of the artificial products which belong to this class, Mosaic Gold-sulphur combined with excess of tin, will only be mentioned.

To determine the tin in any of the substances belonging to this class, an assay powder should be prepared according to the method. given at page 298,-100 milligrammes of which are to be weighed and roasted, in order to free it from volatile ingredients. But as the roasting of tin assays is exactly similar to those of copper with charcoal dust, any further notice of it here is

unnecessary.

If the volatile bodies contained in a substance to be roasted be confined to sulphur, or to sulphur with a trace of arsenic, or only

a

few per cents. of arsenic, which last is sometimes the case with