operation: this takes place with the sulphuret of anti

mony.

Pearlash and native soda act more powerfully as sulphurets than the carbonate of potash obtained with nitre and charcoal, or the artificial carbonate of soda, because they always contain a part of the alkali in the caustic state.

Nitre, Saltpetre, Nitrate of Potash, has a very powerful action on the sulphurets; in fact, if not modified by the addition of some inert substance, as an alkaline carbonate or sulphate, explosion may take place, and a portion of the contents of the crucible thrown out. Where an excess of nitre is used, all the sulphur is converted into sulphuric acid, and every metal but gold and silver oxidated. When only the exact quantity of nitre is employed, that is to say, just as much as is sufficient to burn all the sulphur in the sulphurets of those metals which are not very oxidizable, as those of copper, silver and lead, the metal is obtained in a state of purity, and the whole of the sulphur converted into sulphuric acid, but with the sulphurets of the very oxidizable metals, the oxygen of the nitre is divided between the sulphur and the metal.

Nitrate of Lead possesses the combined properties of nitre and litharge. It is not much used.

Sulphate of Lead is not used as a re-agent, but is often formed in the assay of lead ores. It decomposes the sulphuret of lead by burning the sulphur. It acts on many other sulphurets in a similar manner.

SULPHURATING AGENTS.

1. Sulphur.

2. Cinnabar, or sulphuret of mercury.
3. Galena.

4. Sulphuret of antimony.

5. Iron pyrites.

6. The alkaline persulphurets.

Sulphur fuses at 226o, and at 284° is very liquid. It has very powerful affinities, combines with all the gases, excepting nitrogen, and with the greater part of the metals. That kind generally known as flour of sulphur ought to be employed; and before use, the presence or absence of earthy matters ought to be ascertained, by burning a portion.

It is principally used in the preparation of the alkaline sulphurets, and in the assay of some of the noble metals, because it does not enter into direct combination with gold, &c.

Cinnabar is decomposed by many of the metals, and it is better for use as a sulphurating agent than sulphur itself, as it is less volatile.

Galena.-Many of the metals, as iron, copper, &c., separate sulphur from lead, while some others, as silver, gold, &c., do not; so that if galena be heated with an alloy of various metals, some of which decompose it, and some do not, the former are transformed into sulphurets, and the latter combine with the metallic lead which is produced. It is often employed for this purpose. It is a common ore, and readily procured. The samples employed must contain no sulphuret of

antimony, and all the matrix must be carefully separated by sifting and washing.

of

Sulphuret of Antimony yields its sulphur to many the metals, but it cannot be used in the purification of gold or silver.

Iron Pyrites is a persulphuret which loses half its sulphur at a white heat. It is much employed in metallurgical operations, but not in assaying.

The Alkaline Sulphurets can support a tolerably elevated temperature without losing sulphur, but they have a great tendency to do so, to which their sulphurating power is due. By their means every metal can be made to combine with sulphur. When an alkaline persulphuret is heated with a metal, or an oxide of a metal mixed with charcoal, a fused compound, or a mixture of the sulphuret of the metal, and an alkaline sulphuret is obtained.

When they are in combination, they are held together by very feeble affinities, and their decomposition is generally effected by the mere action of water, which dissolves the alkaline sulphuret and leaves the other perfectly pure. But with gold, molybdenum, tungstenum, antimony, &c., the compound is stable and soluble in water; and it is from this fact that the alkaline sulphurets are sometimes employed in the assay of auriferous substances.

In order to effect a sulphuration by means of the alkaline sulphurets, it is much better to use equivalent mixtures of sulphur and alkaline carbonates than to prepare them beforehand. To obtain persulphuret of potassium, 46 parts of carbonate of potash, and 54 of flour of sulphur must be employed; and for persulphuret

of sodium, 40 parts of fused carbonate of soda, and 60 parts of sulphur.

When the mixture is fused in a plain crucible, sulphate of potash, or sulphate of soda is formed, because part of the alkali is reduced to the metallic state by its affinity for the sulphur, giving up its oxygen to a portion of the sulphur, which becomes sulphuric acid; but when lined crucibles are used, the carbon combines with the oxygen of the alkali, and not a trace of sulphate is produced.

FLUXES.

Fluxes are used for the purpose of causing fusion, as their name indicates, and are employed for the following reasons:

1. To cause the fusion of a body, either difficultly fusible, or infusible by itself.

2. To fuse foreign substances mixed with a metal, in order to separate the latter by its difference of specific gravity.

3. To destroy a compound into which an oxide enters, and which prevents the oxide being reduced by charcoal. The silicate of zinc, for instance, yields no metallic zinc with charcoal, unless it be mixed with a flux capable of combining with the silica.

4. To prevent the formation of certain alloys, and consequently the combination of some metals with others, as in the case of a mixture of the oxides of manganese and iron with a suitable flux, the iron is obtained in a state of purity, whereas if no flux had been added, an alloy would have been obtained. Gold

and silver can be separated from many other metals by means of a flux.

5. To scorify some of the metals contained in the substance to be assayed, and obtain the others alloyed with a metal contained in the flux, as gold or silver with lead.

6. And lastly, a flux may be employed to obtain a single button of metal, which otherwise would be obtained in globules.

Fluxes are divided into non-metallic and metallic; and the non-metallic fluxes are:

1. Silica.

2. Lime.

3. Magnesia.

4. Alumina.

5. Silicates of lime and alumina.

6. Glass.

7. Boracic acid.

8. Borax (biborate of soda).

9. Fluor spar (fluoride of calcium). 10. Carbonate of potash.

11. Carbonate of soda.

12. Nitre (nitrate of potash).

13. Common salt (chloride of sodium).

14. Black flux and its equivalents.

15. Argol (bitartrate of potash).

16. Salt of sorrel (binoxalate of potash).
17. Soap.

The metallic fluxes are:

18. Litharge (oxide of lead), and ceruse (carbonate

of lead).