According to Berthier, 1 part of the various metallic sulphides requires the following quantities of litharge for its decomposition:—

Litharge entirely free from silver can be prepared by oxidizing the purest Pattison or Villach lead by cupellation, or bringing such lead, after it has been granulated, into fused saltpetre; or by gradually strewing charcoal powder upon litharge fused in a refractory crucible, whereby some lead will be reduced which, in sinking down, withdraws the silver from the litharge.

3. Solvent agents.

a. Acid; such as powdered quartz; powdered glass' free from arsenic and lead. It should contain 60 to 70 per cent. or more of SiO2, 5 to 22 per cent. of alkalies, 6 to 25 per cent. of lime, 0.5 to 5 per cent. alumina, and its fusing point should be between that of borax and fluorspar, or about 1200° C. (2192° F.). Borax, NаBO,+ 10H2O, with 16.37 Na2O, 36.53 BO3, and 47.10 H2O. It should be completely dehydrated, or in the condition known as borax-glass. This is produced by fusing borax in a clay crucible, and then pouring it upon a bright metallic surface. It is more fusible than glass, and the boracic acid forms combinations with nearly all the bases as well as with silicic acid. Salt of phosphorus (sodiumammonium-hydrogen phosphate), or microcosmic salt,

'Analyses of Glass, in Poggendorf's Ann. 1879, vol. 6, p. 431. Dingler, ccxxxii. 348 (Weber).

NaH(NH)P + 4H2O, with 14.90 Na2O, 12.46 NHO, 4.29 basic water, and 34.32 water of crystallization, and 34.03 PO. In the anhydrous condition it is a more energetic solvent agent than borax (assays of cupreous nickel). Clay, such as kaolin, Al,Si,O,, with 46.40 SiO2, 39.68 Al2O3, 6.96 H2O, and 6.96 aq.; burned China clay contains 53 Si0, and 47Al2O3. Most varieties of clay contain over 50 per cent. of silicic acid.

b. Basic; such as potassium carbonate, K2CO3, with 68.09 K.O and 31.91 CO2. It should be as free as possible from sulphates, and, when mixed with carbon as black flux (p. 76), and with flour (p. 77), is a vigorous reducing, fluxing, and desulphurizing agent. Sodium carbonate, Na,CO3, with 58.58 Na2O and 41.42 CO2, acts somewhat less energetically than potassium carbonate, and consequently a larger quantity of it must be used. It is less deliquescent, more fusible, and cheaper. A mixture of 13 parts of dry potassium carbonate and 10 parts of calcined sodium carbonate, furnishes a flux very easily fusible.-Caustic alkalies act more energetically than carbonates, but exert a very injurious effect upon the crucibles.-Calcium carbonate, CaCO3, with 56.29 CaO, in the form of chalk (or calcite), may be used for smelting operations at higher temperatures (for instance, in assays of iron, Cornish assays of copper).-Fluor-spar (calcium fluoride), CaFl2, with 51.54 Ca, is more easily fusible than calcium carbonate, and is especially effective for removing silicic acid. It readily fuses down with calcium phosphate, heavy spar, and gypsum.-Lead oxide (litharge, minium, white lead) readily fuses with silicic acid, the alkalies, and with most of the heavy metallic oxides, but less so with the earths and alkaline earths.-Ferric oxide, FO, with 69.3 Fe (in assays of copper).

4. Precipitating or desulphurizing agents.—Iron in the form of iron filings (assays of zinc blende, antimony, and murcury), and as pieces of wire 4 to 5 millimeters (0.15) to 0.19 inch) thick, 6 to 9 millimeters (0.23 to 0.35 inch) long, and weighing from 0.5 to 2 grammes (7.71 to 30.87 grains) (assays of lead and bismuth).—Potassium cyanide (p. 77) and potassium ferrocyanide (p. 77). The caustic alkalies and carbonates (p. 80) decompose metallic sulphides. The metal is separated, and sulphites, hyposulphites, and sulphates of the alkalies together with alkaline sulphide are formed. The latter forms a sulphosalt (for instance, with FeS, PbS, and Cu,S) with one part of the metallic sulphide, which, as a general rule, can be decomposed with iron. Carbon promotes desulphuration (black flux, potassium carbonate, and flour, p. 76).—Lead oxide (p. 78).-Saltpetre oxidizes metallic sulphides, while the metals (silver, copper, lead) are separated.

5. Sulphurizing agents.-Sulphur in the form of flowers of sulphur; or of iron pyrites (Cornish assay of copper).

6. Concentrating fluxes.-Lead in a granulated condition (assay lead). It is prepared by rocking lead in the form of thick liquid paste in a trough well covered with chalk and then sifting the mass. It is also used in the form of sticks of the purest Pattison lead for alloying with gold and silver. (Where the assayer is obliged to make his own granulated lead, as in sections where pure lead free from silver cannot be obtained, it will be necessary for him, after granulation in the manner above described, to sample well, and test about 30 to 50 grammes for silver by the scorification assay. In using this lead, the amount of silver contained in it must, in all cases, be deducted from the results obtained in making an assay.)—

Silver for alloying with gold (quartation).—Gold for collecting copper (assay of nickel and cobalt).-Antimony (antimony oxide) and arsenic for copper (refining).—Copper oxide for tin.-Iron pyrites as a collecting agent for copper (assay of matt).

7. Decomposing and volatilizing fluxes.-Charcoal and graphite for decomposing sulphates, arseniates, and antimoniates by roasting.-Ammonium carbonate (NH4)2CO3 for decomposing sulphates, especially copper sulphate, at red heat (p. 35), but less completely lead and bismuth sulphates. Common salt (sodium chloride) NaCl, with 39.66 Na and 60.34 Cl, for the volatilization of antimony and arsenic in refining black copper according to the Cornish method.

8. Air-excluding fluxes (covering agents).-Decrepitated common salt, as free from sulphates as possible, fuses easily, and, becoming very thin fluid, washes down particles of metal adhering to the sides of the assay vessels. It volatilizes at a red heat.-Glass (p. 79) in Berthier's assay of fuel and assay of matt.-Refined slag from charcoal iron blast-furnaces (in assays of zinc).

1 Preparation of pure silver: Cupreous silver or fine silver is dissolved in pure nitric acid This is diluted with distilled water, allowed to stand for some time, and then filtered. The filtrate is strongly diluted, and chlo ride of silver is precipitated by the addition of pure hydrochloric acid. This is washed by decantation and then boiled several times with diluted hydrochloric acid, but after each boiling it should be washed with distilled water. The moist chloride of silver (3 parts) is mixed with dry sodium carbonate (1 parts) and with pure saltpetre (part of the whole) The mixture is then dried in a porcelain dish and fused in a porcelain crucible; or 100 parts of chloride of silver are fused with 70 parts of chalk, and 4 parts of wood charcoal. (Mohr, Titrirmethode, 1874, p. 425. Fresenius' Ztschr. xiii. 179.)

23. REAGENTS FOR WET ASSAYS.

The following are principally used.

1. For assays by gravimetric and colorimetric analysis. -Acids: hydrochloric, sulphuric, nitric, and acetic acid, aqua regia (nitro-muriatic acid). Bases and salts: caustic alkalies, alkaline carbonates, potassium chlorate, ferrous sulphate, sodium sulphide, etc.-Metals for precipitation: iron in the form of wire-pins 30 to 35 millimeters (1.18 to 1.37 inches) long and 2 to 3 millimeters (0.079 to 0.12 inch) thick, or in a pulverulent condition, for copper; zinc in the form of such pins or of granules or in a pulverulent state,' as a reducing agent for iron solutions, etc.; copper (electrolytic copper is the purest); bromine for decomposing sulphurets, compounds of gold, etc.

2. For volumetric assays: Potassium permanganate (chameleon), KMnO4, with 29.8 K2O and 70.2 Mn2O7; sodium sulphide; potassium cyanide; barium chloride; potassium iodide, free and with dissolved iodine; sodium hyposulphite; ferric chloride; sodium chloride; potassium sulphocyanide, etc. As indicators: litmus tincture, tincture of Brazil wood, etc., for acids and alkalies; for sulphur: the salts of iron, nickel, and lead, and sodium nitroprusside; for iodine: starch paste; for iron oxide: potassium sulphocyanide, etc.

' Dingler, ccxxviii. 378.

2 Dingler, ccxix. 544.

3 Fresenius' Ztschr. 1875, p. 324 (Rothholz). Bericht der deutsch. chem. Ges. 1876, p. 1572 (Fluorescein).