Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/856

838 by charcoal). Most commonly it is used in the form of wire, with a small bend or loop at the end. In flux experiments this loop is dipped when ignited in the powdered flux (e.g., borax), then held in a lamp flame till the powder is fused ; and the process is repeated, if neces sary, till the loop is quite filled with a bead of the flux ; to this is now added a little of the substance to be examined. Platinum is also used in the form of foil and of spoons, and for the points of forceps. Metals and easily reducible oxides, sulphides, or chlorides should not be treated upon platinum, as these substances may combine with and damage it. Tubes of hard German glass, 5 to G inches long, about ~th inch diameter, and open at both ends, are useful in the examination of substances containing sulphur, selenium, arsenic, antimony, and tellurium ; these, when heated with access of air, evolve characteristic fumes. They are put in the tube near one end (which is held slightly depressed), and subjected to the blowpipe flame. The sublimates often condense on the cooler parts of the tube. Small tubes, closed at one end, are used, where it is required to detect the presence of water, mercury, or other bodies which are volatilized by heat without access of air. The most important fluxes used in blowpipe analysis are carbonate of sodium, borax, and microcosmic salt. The first (which must be anhydrous and quite free from sulphates) serves chiefly in reducing metallic oxides and sulphides on charcoal, decomposing silicates, determining the presence of sulphur, and discriminating between lime and other earthy bases in minerals. Pure borax, or acid borate of sodium deprived of its water of crystallization by heating, is used for the purpose of dissolving up metallic oxides, when in a state of fusion at a red heat, such fused masses usually having characteristic colours when cold. In some cases the colour and transparency change on cooling. Microcosmic sails, or ammonio-phosphate of sodium, is used on platinum wire in the same way as borax j on heating, water and ammonia are given off. The following are some other reagents for certain cases nitrate of potash, bisulphate of potash, nitrate of cobalt, silica, fluoride of calcium, oxide or oxalate of nickel, protoxide of copper, tinfoil, fine silver, dry chloride of silver, bone ash, and litmus and Brazil-wood paper. It may be useful here to pass briefly under review a few of the effects obtained in qualitative examinations with the blowpipe. Beginning with the closed tube, organic sub stances may be revealed by the etnpyreumatic odour given off, and by charring. Mercury condenses on the tube in minute globules. Selenium gives a reddish-brown, tellurium i grey, arsenic a black sublimate. Oxygen is sometimes given off. and will inflame an incandescent splinter of wood when introduced; while ammonia may be detected by red litmus paper, as also the acid or alkaline reaction of any liquid product. In the open tube, sulphur and sulphides give off pungent-smelling sulphurous acid gas. Selenium gives a steel-grey deposit, and an odour resembling that of horse radish. Arsenic, antimony, tellurium, yield their respective acids, forming white sublimates. The deposit from arsenic is crystalline, that from the others amorphous. In examination on charcoal, it is useful, in practice, to commence with pure materials and familiarize one s self with their phenomena. Most of the metals fuse in the heat of the blowpipe flame ; and in the outer flame they oxidize. The noble metals do not oxidize, but they fuse. The metals platinum, iridium, rhodium, and palladium do not fuse. The incrustations (when such occur) are in each case characteristic, both in aspect and in the effects they give before the blowpipe flame. Among the most com mon oxides capable of reduction on charcoal alone, in the inner flame, are those of zinc, silver, lead, copper, bismuth, and antimony. The principal minerals that cannot be so reduced are those containing alkalies and alkaline earths, and the oxides of iron, manganese, and chromium. Many substances give a characteristic colour, when held by platinum forceps in the oxidizing flame. For example, arsenic, antimony, lead, colour the flame Hue; copper, baryta, zinc, green ; lime, lithia, strontia, red; potash, violet. Heated with borax, some bodies give a clear bead, both while hot and cold, except when heated by the inter mittent oxidizing flame, or the flame of reduction, when the bead becomes opalescent, opaque, or milky white. The alkaline earths, tantalic and titanic acids, yttria and zir- conia are examples of this. The oxides of most of the heavy metals give coloured glasses with borax, similar to those obtained by their use in glass or enamel painting. Thus oxide of cobalt gives a showy blue, and oxide of nickel a reddish-brownish colour, both being very characteristic and delicate tests of the presence of these metals. Ferric oxide gives a feeble yellow colour, which is darker while hot; but when the bead so coloured is treated in the reduc ing flame the iron passes into the state of ferrous oxide,giving an intensely green or nearly black colour. This reaction may be more certainly brought about by touching the bead while melted with a fragment of tin, when the ferric oxide is probably reduced at the expense of the metal. With manganese the reverse effect is produced. A bead containing a considerable quantity of manganous oxide, such as is produced by a clean reducing flame, is colourless, but when treated in the oxidizing flame the showy violet colour of the higher oxide is brought out. This reaction is a very. delicate one, and is to be recommended to begin ners as a test exercise in blowing a clean flame, the bead being rendered alternately coloured and colourless accord ing as the oxidizing or reducing flame is used. Molybdic acid, which gives a black bead in the reducing, and a clear bead in the oxidizing flame, but requires more careful management, was usually recommended by Plattner to his students for this kind of exercise. Copper salts give a green bead in the oxidizing and a deep sealing-wax red in the reducing flame. This latter indication is of value in detecting a trace of copper in the presence of iron, which is done by reducing with tin as already described for iron. The effects obtained with beads of microcosmic salt, or as it is more generally called salt of phos2)lioms, are generally similar to those described for borax, but in certain cases it is to be preferred, especially in the detection of silica, which remains undissolved, and titanic acid, which can be made to assume the form of crystals similar to the natural mineral anataseby particular treatment and microscopic examination. Several new phenomena, due to the crystallization of titanic acid and similar bodies, have been described by Gustav Rose. With carbonate of sodium as flux (a paste of which and the substance to be examined is made with water, and held on charcoal to the flame), three reactions may occur. The substance may fuse with effervescence, or it may be re duced, or the soda may sink into the charcoal, leaving the substance intact on the surface. The first takes place with silica, and with titanic and tungstic acids. The oxides of tungsten, antimony, arsenic, copper, mercury, bismuth, tin, lead, zinc, iron, nickel, and cobalt are reduced. Lead, zinc, antimony, bismuth, cadmium, and tellurium are volatilized partially, and form sublimates on the charcoal. Mercury and arsenic are dissipated as soon as reduced. Silica and titanic acid are the only two substances that produce a clear bead. The bead in which silica is fused is sometimes rendered yellow by the presence of sulphur. Carbonate of soda, with addition of a little nitrate of potassa, is very useful for detecting minute quantities of manganese. The fused mass, when clear, lias, from the production of mail- 