The plaster is made into a thin paste with water, and is spread out with a knife on a large glass plate to an even thickness of inch. Grooves are cut across the plaster, so that it can easily be broken, when set, into plates measuring 4 x 12 inches. The smooth surface produced next the glass is used for the encrustation, and a small hole is cut in it near one end for the assay, which is mixed with hydriodic acid. Messrs. Wheeler and Luedeking* use a mixture of 40 per cent. iodine and 60 per cent. sulphur added in excess to the assay, which is much simpler, and which gives equally striking and brilliant encrustations of metallic iodides on plaster of Paris. Dr. Goldschmidt† uses mica or glass plates, resting on the charcoal, to receive encrustations. To prevent cracking, the latter are heated before use. The sublimate can thus be removed, and tested conveniently in the wet way. A A beautiful example of the oxidation of a metal before the blowpipe occurs in the cupellation of lead containing silver. The metallic bead is obtained from the ore by fusion with sodium carbonate on charcoal, and is removed and cleaned. cupel of bone-ash is then made, about 2 cm. in diameter, being shaped in a hollow cut in charcoal, or struck in a mould, such as those provided in Letcher's blowpipe-cabinets. The cupel should be gently dried, and supported in a hollow of the charcoal. The metallic bead is placed on it in a little hollow near one side, and is treated in O.F. The lead oxidises, forming exquisite iridescent films on the bead itself, and red stains, which rapidly grow richer, on the bone-ash beyond. The cupel finally absorbs all the lead as oxide, and similarly any copper present; and silver, if present, remains behind as a gleaming and non-iridescent bead. This process requires some experience and repetition; the detailed account in Plattner's work (English edition of 1875, p. 369) shows to what perfection it may be carried. * Trans. St. Louis Acad. Sci., vol. iv. (1886), p. 676. (ii.) TREATMENT OF RESIDUAL ASSAY WITH NITRATE OF COBALT. This If the residue of the assay after oxidising is white or greyish, cobalt nitrate should be dropped upon it, and it should be strongly heated in O. F., as in the case of encrustations. test is of especial service where no conclusive reaction has been hitherto obtained. On completely cooling, the colour may or may not have changed. The assay must be examined with a lens to ascertain whether fusion has taken place, since a blue colour after fusion has no determinative value. Similar reactions can be conveniently obtained by powdering the mineral, mixing with a drop of the solution, and taking up on a loop of platinum wire for treatment in the O. F. The colours useful in determination are: (iii.) REDUCTION TO METAL IN THE REDUCING FLAME Many ores can be reduced to metal in R. F. on charcoal, their volatile constituents going off as gas or becoming deposited as oxides on the charcoal. Other more refractory substances become reduced by being powdered with about three times their bulk of sodium carbonate; the mixture is then treated in R. F. on charcoal until all effervescence in the sodium carbonate has ceased, when often metallic globules will have separated out and have become distinctly visible in the flux. In many cases, however, the soda-slag must be cut out with a little of the charcoal under it and pounded out in water in an agate mortar; wash lightly with water, pressing firmly on the heavier residue with the pestle; metallic beads will often be found streaked out on the pestle, or in the mortar when the lighter admixture has been washed away. The metallic beads obtained must, as a rule, be subjected to further tests. They should be treated by themselves on charcoal, to observe their purity and to see if they form an encrustation of any characteristic oxide; their malleability and sectility should be examined; and they should be rubbed over a surface of white paper, under pressure from the clean base of the agate mortar, to determine whether or no they leave a streak such as is given by lead. Some bodies yield mere fused globular residues, which are brittle and can be referred to none of the undermentioned metals. Such residues can often be further decomposed; but alloys may be formed that are difficult to determine. The bead or powdered residue from treatment in R. F. should always be tested with the magnet, and any matter that is attracted should be examined in a borax bead. Iron, cobalt, and nickel can be thus extracted and distinguished. Precautions. Search for possible small beads in the residue from fusion with sodium carbonate, as above described. Rub beads in the mortar to clean off any oxide that may have formed during cooling, concealing their true metallic colour. NOTE.-Arsenic, antimony, and zinc are volatilised on reduction, and are recognised as encrustations and not as beads. Mercury and arsenic should be obtained by heating their compounds with sodium carbonate in a closed tube. Iron, cobalt, and nickel yield black magnetic residues, which should be tested in borax on platinum wire. Certain reactions in which the use of sodium carbonate plays an important part will be described under the head of the substances of which they are characteristic. The test for sulphur alone need be mentioned here. Because a substance is a sulphate or even a sulphide, it by no means follows that evidence of sulphur will be given either in the closed or open tube. The decisive determination is made as follows:-Fuse thoroughly some of the powdered mineral with about three times its bulk of sodium carbonate in R. F., until effervescence ceases. Cut out the slaggy residue and the patch of charcoal below it, and crush on the surface of a clean silver coin with a drop of water. Allow it to lie for about ten seconds and wipe it off lightly. If sulphur has been present in any form, sodium sulphide will have resulted, which decomposes on the coin, leaving a brown or black stain of silver sulphide. This test is delicate and unfailing, and can be performed as a natural sequel to any good reduction with sodium carbonate, a portion of the slaggy mass being reserved for this purpose. Precautions.-The reduction must be very thorough. The charcoal below must be cut out, owing to its absorption of the sodium sulphide formed. The sodium carbonate must itself be tested for sulphur; common gas also gives a slight reaction; but the stains thus produced are ordinarily quite slight compared with those yielded by minerals, especially if the mass is not allowed to rest long upon the silver coin. CHAPTER VI. SIMPLE AND CHARACTERISTIC REACTIONS OF THE CONSTITUENTS OF COMMON MINERALS. Borax bead. Closed tube. O. tube coal. Soda Sodium carbonate. Add. = = Open tube. Ch. On char- = Additional reactions. The most useful and characteristic reactions are preceded by an asterisk. Details must be looked for on pp. 45 to 60. Aluminium. * Ch.-Alumina becomes blue with cobalt nitrate; if the surface is fused, the reaction is indecisive. Dissolve the |