examined for chromium in the following manner. A small fragment of the mineral is first powdered as fine as possible, the powder mixed with three times its volume of saltpetre, and the mixture fused with the aid of the Blowpipe, either on the platinum wire or in a small platinum spoon. Chromate of potassa is thus produced, and may be dissolved in water in a porcelain capsule, over the spirit-lamp. If the clear solution be poured off from the insoluble metallic oxides into another capsule, supersaturated with acetic acid, and a crystal of acetate of lead, then laid in it, the latter dissolves, and the chromic acid instantly combines with the oxide of lead, forming a beautiful orange-yellow powder, which precipitates, and, after filtration, yields with borax or microcosmic salt, in the oxidating flame, a beautiful green glass. Even small traces of chromium may be detected in this manner. If the substance examined by this method for chromium should contain sulphur or sulphuric acid,—which is not, however, present in the natural compound, the chromate of lead will be contaminated with sulphate, and the yellow color will be lighter in proportion to the quantity of the sulphate present. Silicates that contain only a little chromium, and much iron or other metals yielding colored oxides, and which afford with fluxes only the peculiar tinge of iron or of the other metals, cannot be examined for chromium by the foregoing method, silicious combinations not being decomposed by saltpetre, but requiring a different treatment; such as the following:-The finely pulverized mineral is to be fused to a bead on charcoal, with from one to one and a half parts of soda, and half a part of borax; the bead pulverized, treated with hydrochloric acid, and evaporated to dryness. The resulting dry mass is to be dissolved in water, the silicic acid separated by filtration, a few drops of nitric acid added to the solution, in order to convert the protoxide of iron into the sesquioxide, and the oxides of chromium, iron, and alumina, et cetera, precipitated by ammonia. This precipitate, after being collected on a filter, must be fused with saltpetre; chromate of potassa is thus obtained, which may be decomposed by acetate of lead, in the manner already described.

The silicates colored blood-red by chromic acid, namely, the

Pyrope and Spinelle, from Ceylon, possess the property of becoming black and opaque by simple heating, but during the refrigeration, they become, by transmitted light, gradually yellowish on chrome-green, then almost colorless; and when perfectly cold they re-exhibit their original color. Those silicious combinations which are colored red, both by chromium and iron, are also rendered opaque by calcination, but, on cooling, immediately recover their redness and transparency.

§ 24. TELLURIUM-Te-Presence in the Mineral Kingdom.

Tellurium occurs in nature only in the metallic state; thus,a. As Native Tellurium [Te], which contains a little Gold and Iron ;

b. With Gold and Silver, in Graphic Tellurium, from Nagyag; with Lead, in Black Tellurium, mechanically mixed with Telluride of Gold, and sometimes Sulphides of Silver, Copper, and Antimony; with Bismuth, a little Sulphide of Bismuth, and Selenium, in Telluric Bismuth; with Silver, Lead, and Gold, in Yellow Tellurium; and in a compound of Bismuth, Lead, Copper, and a little Nickel, in the Needle Ore from Siberia.

Examination for Tellurium.

A substance may be examined for tellurium either on charcoal or in a glass tube, open at both ends. In the first case, a white sublimate, with a reddish border, is obtained by heating the mineral in either flame, and which disappears, with a beautiful bluish-green tinge, when the reducing flame is directed on it. When the mineral is rich in tellurium, it tinges the flame bluish, while subjected to the action of the reducing flame. If the assay, thus treated, gives off the odor of horseradish, it contains selenium. If the mineral contains lead or bismuth, and the operation be performed on charcoal, the pure sublimate of oxide of tellurium is never obtained when the blast is continued for a few seconds too long, but, on the contrary, will always be contaminated with. oxide of lead or bismuth. It will, in consequence, be safer to

pulverize the substance, mix it with an equal bulk of glacial boracic acid, place it on a broad charcoal support, and treat it in the reducing flame. The oxides of lead and bismuth thus formed, notwithstanding the action of the reducing flame, will be dissolved by the boracic acid, and will not produce a sublimate; while the tellurium volatilizes, and alone coats the charcoal. Should the mineral, at the same time, contain a large quantity of selenium, a portion of it will also be deposited on the charcoal, and the tellurium cannot then be well recognized. In such a case, it is always better to perform the operation in a tube open at both ends.

If the operation is performed in a glass tube in a similar manner to Antimony, a strong white vapor is obtained from telluriferous minerals, which deposits as a pulverulent oxide on the glass, and, by heating, fuses on the same part of the tube on which it is deposited to clear colorless drops, which may be more closely examined with a lens. Should selenium be present, however, a trace of selenite of tellurium will be formed, which also melts into clear colorless drops, incapable of being distinguished from pure oxide of tellurium by the unaided eye.

When the telluriferous metal contains much lead, a sublimate is obtained, which appears grey close to the assay, and white at a distance from it. The white portion can be fused to colorless drops, and is oxide of tellurium; the grey, on the contrary, does not fuse like oxide of tellurium, but exhibits a somewhat changed appearance, and forms only a semifused greyish coating on the glass. According to BERZELIUS, this substance is tellurate of oxide of lead.

If bismuth be also present in the telluriferous metal, it remains behind, while the tellurium volatilizes and deposits in the tube. By continued treatment in the flame, it oxidizes, but the surface of the bead is coated with fused brown oxide of bismuth.

§ 25. ARSENIC-As-Presence in the Mineral Kingdom, and in the products of Smelting Furnaces.

Arsenic is found very abundantly in nature. It occurs :a. Metallic, either per se, or in combination with other metals;

namely, as Native Arsenic [As], which generally contains traces of Antimony and Iron; with Cobalt, in Speisscobalt, which often contains traces of Iron, Copper, and Sulphur; with Nickel, in Arsenical Nickel - Kupfernickel-which sometimes contains traces of Iron, Lead, Cobalt, Antimony, and Sulphur; with a lesser proportion of Nickel, in Weissnickelkies, from Schneeberg; with Antimony, in Arsenical Antimony-Arsenik-Speisglanz ; -with Bismuth, in Bismuth Blende; and with Silver, Iron, and Antimony, in Arsenical Antimonial Silver.

b. With Sulphur, both per se and combined with other Sulphides; namely, per se, as Sulphide of Arsenic, red [As S2]— Realgar; yellow [As S3]-Orpiment ;—with Iron, in Arsenical Pyrites; with Cobalt and a little Iron, in Bright White Cobalt ; with Nickel and Iron, in Nickel Glanz; with Copper, Antimony, Iron, and Silver, in Grey Copper-Fahlerz ;-with Silver and a little Antimony, in the light red variety of Ruby SilverArgent Rouge Arsenié, Necker ;-with Nickel and Antimony, in Antimonial Nickel-Nickelspeiss-glanzerz ;—and with Copper and Iron, in Tennantite.

c. As an Acid; thus,-as Arsenious Acid per se in Oxide of Arsenic-Arsenikblüthe-[As O3]; and as Arsenic Acid and Arsenious Acid, combined with other bodies; namely, as Arsenic Acid, with Lime and Water, in Pharmacolite; with Oxide of Cobalt and Water, in Cobalt Bloom; with oxide of Copper and Water, in Condurrite, Euchroite, Erinite, Rhomboidal Arseniate, and, accompanied by Alumina, in Liroconite; with Oxide of Nickel and Water, in Nickel Ochre, which often contains traces of Oxides of Cobalt and Iron, and Sulphuric Acid; with Protoxide of Iron and Water, in Scorodite; with Protoxide and Sesquioxide of Iron, and Water, in Cube Ore, or Arseniate of Iron, from Cornwall; with Oxide and Chloride of Lead, in Arseniate of Lead, from Johann Georgenstadt and Cornwall, which also contains traces of Iron and Phosphoric Acid; and with Oxide and Basic Sulphate of the Oxide of Iron, together with Constitutional Water, in Pitchy Iron Ore, or Eisensinter; lastly, as Arsenious Acid, combined with Oxide of Cobalt, in a pulverulent substance from Schneeberg.

As many of the above mentioned Arsenic combinations, partly per se, and partly in combination with other substances, are subjected, on a great scale, to operations for the reduction of the metals which they contain, and as Arsenic is a metal which is very difficultly separated in the dry way from a number of other metals, it must form more or less a constituent of the Dressed Ores, and of the various intermediate products of the smelting operation. To the latter belong especially the products which are obtained in Arsenic Smelting Works, as well as many of those which are obtained in Silver Works, as Rohstein, Bleistein, Tutty, Abstrich, et cetera, and also the Speiss produced in Cobalt Smelting Works, and often even the Smalt.

Examination for Arsenic.

The examination for arsenic is in most cases very simple. It should, however, be ascertained whether the arsenic exists in the compound in a metallic state, or as an acid. Native Arsenic is instantly recognized by the alliaceous odor which it exhales in volatilizing, when treated on charcoal before the Blowpipe. In a glass matrass, it sublimes in a metallic state, leaving a nonvolatile metallic mass behind, which, by fusion with lead and cupellation on bone ashes, sometimes yields a button of silver. The combinations of arsenic with sulphur, as Realgar and Orpiment, burn with a whitish-yellow flame, when ignited by the aid of the Blowpipe on charcoal, and give off a dense greyish-white vapor, which smells like garlic, and deposits in part on the charcoal. In a flask it fuses, boils, and sublimes. The sublimate is transparent, and of a dark-yellow or red color. Heated in an open glass tube, it burns and deposits white arsenic on the upper side of the tube; at the end of the tube, held upwards, the smell of sulphurous acid is readily recognized. Metallic arsenic is best obtained from sulphide of arsenic by the following method:-The compound is first decomposed in an open glass tube into arsenious and sulphurous acids, the former deposits in a crystalline form in the tube, and the latter escapes. For this purpose, the tube must be held obliquely, and heated a very little above the