general, earthy substances are better examined when a single particle is at first dissolved, because when the substance is employed in the state of powder, the undecomposed part cannot be well distinguished from the separated and insoluble portions which it may contain. With borax this is not so often the case as with microcosmic salt, for with the latter silicic acid is separated. When the oxidating flame is playing upon the assay, the operator must observe if it dissolve quickly or slowly; whether any gas is generated; if, when entirely dissolved, the glass when held between the eye and the light is colored; and whether the color changes or remains the same on cooling. There are some bodies which, with borax in certain proportions, give a clear glass when hot or cold, but in the reduction, or more particularly in a quick intermittent flame, become opaque, milkwhite, or opalescent, and, in some cases, even colored. This reaction is generally observed in such bodies as form clear glasses while melting, but are enamelled on cooling. Such is the case with the alkaline earths, with yttria, glucina, zirconia, oxide of cerium, tantalic and titanic acids, and with some others, as silica, alumina, sesquioxide of iron, oxide of manganese, et cetera. The presence of silica hinders the phenomenon of the glass becoming enamelled on cooling. The same occurs with those silicates which give a transparent glass only when the assay is perfectly saturated,-oversaturated, the mass is, on cooling, opaque. When a bead of borax, containing a body in certain proportions, becomes opaque in a quick intermitting flame, it is said to become turbidly streaked. Where metallic oxides are present, and the glass may be colored, it is necessary to observe that too much of the sample be not taken; otherwise the color will be so deep as to be indistinguishable. When a glass is so dark that the hue cannot be decided upon, it must be pressed while hot between the points of a forceps, or drawn out into long thin threads; the color can then be more readily recognized. Many judge of the color best with the naked eye; others with a magnifying glass: but in the latter case, it must be particularly noticed that the glasses of the magnifier are not tinged, else false determinations will be the result. After the color which a substance gives with borax before the oxidating flame has been remarked, and all the phenomena carefully observed, the glass must be operated upon in the reducing flame, and in such a manner that no carbon be deposited on its surface. If there appear to be metallic oxides dissolved in the glass, which are not reduced by the borax, as for example, oxides of cerium, manganese, cobalt, iron, uranium, chromium, titanic and tungstic acids, et cetera, the operation must be effected on the platinum wire but if metallic oxides are present, which are readily reduced to the metallic state, as the oxides of zinc, nickel, cadmium, lead, bismuth, copper, silver, antimony, et cetera, the reduction must be produced on charcoal; otherwise, the metal would amalgamate with the platinum. When a trial is made upon the platinum wire, in the two flames, the change of color in the glass will be readily recognized, either when hot or cold; but if the assay be heated on charcoal, the glass must be removed from the wire as previously mentioned. The bead is placed in a small cavity formed in the charcoal, and then exposed to the reducing flame. After continuing the flame for a few minutes, the glass is quickly removed, and pressed between the broad ends of the forceps, in order that the color may be more accurately perceived. Pressing the glass in this manner also possesses this advantage, that the protoxides, if there be any present, are not so liable to become sesquioxides, as when left to cool gradually upon the charcoal. If the glass contains an easily reducible metallic oxide, it often occurs that the charcoal becomes coated with a sublimate at a certain distance from the assay; but this appearance takes place only when the oxides of selenium, tellurium, arsenic, antimony, bismuth, cadmium, zinc, tin, and lead are in excess. When no color is given with the reducing flame on the wire, an examination should be made with borax on charcoal, in the following manner. The hot glass is suddenly shook on the charcoal, and heated with a small quantity of pure metallic tin, and the mixture then strongly smelted in the reducing flame. The tin, having a great affinity for oxygen, abstracts part from the metallic oxide contained in the assay, and dissolves without tinging the glass, while the metallic oxide, which is concerted into a protoxide, gives a characteristic hue to the glass on cooling. This method is generally resorted to when examining for traces of copper or iron. When the behavior of a substance containing a sulphide or arsenide is to be ascertained by heating with borax, it is best to sublime, first, most of the sulphur or arsenic, in an open tube, and then to heat on charcoal, with the necessary proportion of borax, and tin if requisite, in both flames, and observe all the striking characteristics. When sublimation-tubes are employed, they should be free from lead, for arsenic, if heated in a tube containing this metal, decomposes the glass. § 2. TREATMENT OF THE SUBSTANCE WITH MICROCOSMIC SALT, - As with Borax, it can be treated partly on the platinum wire, and partly on charcoal. If the substance contains earthy ingredients, or only difficultly reducible metallic oxides, the operation is to be conducted on platinum wire, in exactly the same manner as with borax; but if it likewise comprises sulphides or arsenides, it must, as with borax, be treated on charcoal. As microcosmic salt gives a very fluid bead when heated, a few additional bends should be made on the platinum wire, when this salt is used, to prevent the melted drop from falling; on charcoal, however, enough of the salt can be taken at once, as there are no obstacles in the way. This flux is better for many metallic oxides than borax, as the colors are often better ascertained when a sufficient quantity of the substance is dissolved. It also separates the acids; the volatile are driven off, and the fixed remain, and divide the bases either with the phosphoric acid, or remain uncombined, and float about in the molten mass. With the silicates, this is particularly the case: the silicic acid is separated, and flows through the glass in a gelatinous state. Silicates, when containing bases that, by themselves, produce in vitreous fluxes an opaque glass on cooling, give, in combination with borax, a clear glass; but an opalescent, with microcosmic salt. § 3. TREATMENT OF THE SUBSTANCE WITH CARBONATE OF SODA. When assaying a substance with soda, it is necessary to learn if it consists of earthy ingredients only, or of acids, free or in combination; whether it is soluble; whether it contains metallic oxides which give a reduced metal; or if sulphides or arsenides are present, et cetera. The numerous examinations will be fully treated of, together with the pyrognostic effects produced upon metallic oxides, when heated in the Blowpipe-flame with Carbonate of Soda. a. Fusibility of a Substance with Carbonate of Soda.* A great number of bodies possess the property of uniting with soda, and forming combinations partly fusible and partly infusible to the former, but few belong, such as silica, and the metallic oxides of titanic, tungstic, and molybdic acids, et cetera; they are, however, more or less absorbed by the charcoal, if not in combination with silica. In testing the fusibility of a substance with Carbonate of Soda, the following is the mode of procedure:- The substance, when in powder, is kneaded into a paste with soda, a moistened spatula being used for the purpose; but when in a granular state, one of the grains is taken, and covered with the soda paste; in both cases a portion is placed on charcoal, and heated till all water is expelled; the mass is then strongly ignited in the oxidating flame. The carbonate of soda will first be absorbed by the charcoal; but when a sufficient quantity of the assay dissolves, it exudes, effervescence takes place, and a bead is formed. The treated substance, if insoluble in carbonate of soda, will, if decomposed, tumefy gradually, and change its appearance, without forming a bead. The operator should add fresh supplies of the flux, and renew the blast until he arrives at a satisfactory conclusion. When, to a substance, soluble in this flux, too little of it has been added, a part will remain opaque, surrounded by a clear glass; if too much carbonate of soda has been supplied, the glass will be quite opaque; it is therefore always advisable that the carbonate of soda be supplied in small doses at first, and more added as the different changes ensue. If the glass assumes a yellow, red, or brown color, sulphur or sulphuric acid has been present; the smallest trace of these substances gives a decided reaction. When the substance has been examined on charcoal, it should be treated on the platinum wire in the oxidating flame; if it expands and gives a bluish-green glass in cooling, manganate of soda has been formed, and, therefore, some oxide of manganese has been present. Should the substance contain silica and oxide of cobalt, silicate of soda is formed, and the glass assumes the characteristic cobalt-blue color. When the previous reaction has been decided upon, and an operator wishes to ascertain the presence of an alkali or earth, the substance must be treated upon platinum foil, with carbonate of soda. If the glass during the fusion is clear and limpid, only an alkali is contained therein, but if something is deposited, an earth is present. The alkali will also tinge the flame, which must be remarked. b. Reduction of Metallic Oxides by means of Carbonate of Soda. In this process, metallic oxides, existing in only very minute quantities in minerals, et cetera, may be more readily found than by the moist way. There are some metals which, without the addition of carbonate of soda, are reduced on charcoal in the reducing flame, and thus determined; but if they are mixed or in combination with other substances irreducible, it is not only difficult, but, in many cases, altogether impossible, to accomplish the reduction sufficiently, in order to arrive at satisfactory results. For instance, when a quantity of native or artificial oxide of tin is treated in the reducing flame upon charcoal, it is possible, with great exertion and fatigue, to obtain a metallic globule; but when carbonate of soda is used as an auxiliary, the reduction takes place readily. When a metallic oxide is in combination with a non-reducible body, which prevents its giving a characteristic reaction with carbonate of soda, it must be treated as the annexed process directs, Knead the powdered sample, with moist carbonate of soda, into |