solving the assay [No. 68] on Ch in boracic acid and forcing into the glass, when a good fusion is effected, a piece of fine steel wire; a good R Fl is then given. The iron is oxidized at the expense of the phosphoric acid, causing the formation of a borate of the oxide. of iron and phosphide of iron, which fuses at a sufficiently high temperature. The bead is then taken from the Ch, enveloped in a piece of paper, and struck lightly with a hammer, by which means. the phosphide of iron is separated from the surrounding flux. It exists as a metallic-looking button, attractable by the magnet, frangible on the anvil, the fracture having the color of iron. If the substance under assay contained no phosphoric acid, the iron wire will keep its form and metallic lustre, excepting at the ends, where it will be oxidated and burnt. The substance to be assayed ought not to contain sulphuric acid, arsenic acid, or any metallic oxides reducible by iron. Phosphate of lead exhibits the peculiarity of crystallizing on cooling after having been fused on Ch; the crystals have frequently large facets of a pearly lustre. Potassa. § 97. The violet color of the flame is sufficiently characteristic for potassa (v. § 33). But being altogether prevented or, at least, made very indistinct by the addition of a few per cent. of soda or lithia, it can only in a very few cases be made use of. For the detection of potassa in silicates it is almost entirely unavailable, because these compounds almost always contain some soda. § 98. If the base of a compound consists essentially of potassa, the following method may be advantageously employed for its detection: Some Bx, to which a little boracic acid has been added, is melted into the hook of a platinum wire and so much protoxide of nickel added that the glass on cooling shows a distinct brownish color. A small piece of the substance under examination [No. 15] is made to adhere to the glass and the whole fused together with the O Fl. If the assay-piece contained no potassa, the color of the glass, after perfect cooling, will have remained unchanged; but if potassa was present in sufficient quantity, the glass will appear bluish. Selenium. § 99. The reactions of selenium are very characteristic. In nonvolatile compounds, which do not give the red sublimate mentioned § 11, the selenium is detected by heating a small piece of the substance [No. 87] on Ch in O Fl, when the peculiar odor is evolved; if much selenium is present, a Ct is deposited, v. § 28. Selenites and selenates are treated on Ch with Sd in R Fl, when a reduction takes place and the selenium vaporizes with the characteristic odor. Silica. § 100. Pure silica [No. 54], when treated with Bx on platinum wire, dissolves slowly to a transparent glass which fuses with difficulty. Treated with S Ph in the same manner, only a small quantity is dissolved, the rest floating in the liquid bead as a semitransparent mass. The behavior to Sd see § 39. With a little So Co it assumes a pale bluish color which, on addition of a large quantity of the reagent, turns dark-gray or black; very thin splinters may be fused by a great heat to a reddish-blue glass. § 101. Silicates [No. 61], when treated with S Ph on platinum wire, are decomposed; the bases unite with the free phosphoric acid to a transparent glass in which the silica may be seen floating as a gelatinous cloudy mass. The bead ought to be carefully observed while hot, since many silicates form a glass which on cooling opalizes or becomes opaque, when, of course, the phenomenon can no longer be seen. The experiment is best performed with a small splinter of the substance under examination, and only when this does not appear to be affected by the flux, the finely pulverized substance should be used. If but a very small quantity of silica is present, the glass will appear perfectly transparent. Its presence in this case cannot be detected by means of the Blp. § 102. Silicates containing at least so much silica that the quantity of oxygen in the acid is twice that of the oxygen in the base, dissolve, when treated with Sd on Ch, with effervescence to a transparent glass which remains so when cold. When less silica is present decomposition also takes place, but the glass turns opaque on cooling, the amount of silicate of soda which is formed not being sufficient to dissolve the eliminated bases. Silver. The reactions of silver, see § 27, and Table II, 20. § 103. When in combination with metals which are volatile at a high temperature, ex. gr. bismuth, lead, zinc, antimony, the substance is heated alone on Ch, when, after evaporation of the foreign metals, a button of pure silver remains behind and a feeble reddish Ct is deposited on the Ch. If associated with much lead or bismuth, these metals are best removed by cupellation, a process which is executed in the following manner: Finely pulverized bone-ash is mixed with a minute quantity of soda and made with a little water into a stiff paste; a hole is now bored into the Ch, filled with the paste, and its surface smoothed and made slightly concave by pressing on it with the pestle of the little agate mortar. The mass is then dried by the flame of a common spirit lamp. On this little cupel the assay [No. 51] is placed and so long heated with the O Fl until the whole of the lead or bismuth is oxidized and absorbed by the cupel. The silver or, if gold is present, the alloy of silver and gold remains as a bright metallic button on the cupel. § 104. When combined with metals which are not volatile, but which are easier oxidized than silver, the presence of this metal may in some cases be detected by simply treating the alloy with Bx or S Ph on Ch. Copper, nickel, cobalt, &c., become oxidized and their oxides dissolved by the flux, while silver remains behind with a bright metallic surface. But when these metals are present to a considerable extent, another course has to be pursued, a course which may always be taken when a substance is to be assayed for silver, or silver and gold. 105. The assay-piece [No. 86] is reduced to a fine powder, mixed with vitrified Bx and metallic lead (the quantities of which altogether depend upon the nature of the substance, and for which, therefore, no general rule can be given), and the mass placed in a cylindrical hole of the Ch. A powerful R Fl is given until the metals have united to a button, and the slag appears free from metallic globules. The flame is now converted into a O Fl and directed principally upon the button. Sulphur, arsenic, antimony, and other very volatile substances, are volatilized; iron, tin, cobalt, and a little copper and nickel become oxidized and are absorbed by the flux; silver and gold and the greater part of copper and nickel remain with the lead (and bismuth, if present). When all volatile substances are driven off, the lead begins to become oxidized, and the button assumes a rotary motion; at this period the blast is discontinued, the assay is allowed to cool, and when perfectly cold the lead button is separated from the glass by some slight strokes with a hammer. It is now placed on a cupel of bone-ash and treated with the O Fl until it again assumes a rotatory motion. If much copper or nickel is present, the globule becomes covered with a thick infusible crust, which prevents the aimed-at oxidation; in this case another small piece of pure lead has to be added. The blast is kept up until the whole of the lead and other foreign metals, viz., copper and nickel, are oxidized; this is indicated by the cessation of the rotatory movement, if only little silver is present, or by the appearance of all the tints of the rainbow over the whole surface of the button, if the ore was very rich in silver; after a few moments it takes the look of pure silver. The oxides of lead, copper, &c., are absorbed by the bone-ash, and pure silver, or an alloy of silver with other noble metals, remains behind; the button may be tested for gold, &c., after the method given in § 79. Sulphur. § 106. The presence of sulphur in sulphides may in many cases be detected by heating in a glass tube (v. §§ 11, 14), or on Ch with the O Fl. § 107. A very delicate test for the presence of sulphur, in whatever combination it may be contained in the substance, and which possesses moreover the advantage over all other methods of being very easily performed, is to mix the pulverized assay [No. 4] with some pure Sd or, better still, with a mixture of two parts of Sd and 1 of Bx, and to treat it on Ch with the R Fl. The fused mass is removed from the Ch, powdered, the powder placed on a silver foil or a bright silver coin, and a drop of water added. If the substance under examination contained any sulphur, a black spot will be formed on the silver foil, owing to the formation of sulphide of silver from the decomposition of the sulphide of sodium, which, in its turn, resulted from the decomposition of the sulphide or sulphate, or other sulphur-compound of the assay-piece, under the influence of Sd, Ch, and a high temperature. Selenium shows the same reaction; it is readily recognized by the peculiar odor which it emits when heated on Ch alone. § 108. To decide whether the reaction obtained in the experiment was owing to the presence of a sulphide or to that of a sulphate, the finely-pulverized substance [No. 76] is fused in a small platinum spoon with some hydrate of potassa. The spoon with the contents is then placed into a vessel containing some water, and a piece of silver foil inserted into the liquid. If the silver remains perfectly bright, a sulphate was present, if it turns black, a sulphide. The absence of substances which might exercise a reducing influence is required. Tellurium. In § 109. The presence of tellurium in mineral substances is detected by the tests given §§ 11, 18, 29. In presence of lead or bismuth the reactions in the open tubes and on Ch are not quite pure. this case we may subject the assay to the following treatment: The substance is mixed with some Sd and charcoal-powder, the mixture introduced into a glass tube closed at one end, and heated to fusion; after cooling, a few drops of hot water are poured into the tube; if tellurium was present, telluride of sodium has been formed, which dissolves in hot water with a purplish-red color. This test is applicable to show the presence of tellurium in a great many compounds, even in such where it occurs in the oxidized state. Tin. The reactions of tin and its compounds, see §§ 12, 26, 45, and Table II, 22. § 110. The presence of tin is indicated by its Ct when the substance [No. 13], alone or mixed with Sd, is exposed to the R FI on Ch. When the substance under examination is an alloy, a little Bx is conveniently added, which absorbs the oxide of tin in the measure as it is formed, and allows the presence of those metals which are more volatile, ex. gr. antimony, lead, bismuth, to be recognized by |