and then shaken up with mercury, and the gold thus extracted by amalgamation.

Gold possesses a brilliant yellow colour, and, in thin films, transmits green light; it is nearly as soft as lead; it can be drawn out into fine wire, and is the most malleable of all the metals. It does not tarnish at any temperature, in dry or moist air, nor is it affected by sulphur, like silver; it is not acted upon by any single acid (except selenic), but dissolves in presence of free chlorine and in nitro-hydrochloric acid. At high temperatures gold is slightly volatile. Pure gold is best prepared by dissolving the ordinary metal in aqua regia, and adding ferrous sulphate, which is oxidized to ferric salt and precipitates the gold as a brown powder. The standard gold of our country is an alloy of gold and copper in the proportion of 11 of gold to I of copper, or 8.33 per cent. of the latter metal: this alloy is harder and more fusible, but less ductile, than pure gold.

Gold unites with oxygen in two proportions, forming Gold suboxide, Au, O, and Gold trioxide, Au, O3. Neither of these oxides forms salts with acids; but the latter unites with bases to form compounds called aurates: thus potassium aurate is K Au O2. Gold trioxide is obtained by adding zinc oxide or magnesia to a solution of gold chloride : the oxide falls as a brown powder, from which the zinc can be separated by nitric acid. Gold trioxide decomposes, in direct sunlight, into metal and oxygen, and is also reduced when heated to a temperature of about 250°. The most important compound of gold trioxide is fulminating gold. This substance is obtained by acting on a solution of gold with excess of ammonia; a yellow-brown powder is precipitated, which, when dry, explodes very easily when heated to 100°, or when struck with a hammer. There are two gold chlorides known: (1) Gold monochloride, Aú Cl, obtained as an insoluble white mass when gold trichloride is heated to the melting-point of tin; (2) Gold trichloriae, Au Cl, obtained when gold is dissolved in aqua regia. This is the most important compound of gold. On eva

S

porating the solution, crystals of a compound of gold trichloride and hydrochloric acid are deposited. Gold trichloride also forms crystalline compounds with the alkaline chlorides. Gold salts can be easily recognised by the brown precipitate of metallic gold formed on addition of ferrous salts, which can be reduced to a globule before the blowpipe; and also by the formation of a purple colour (purple of cassius), when gold trichloride is added to a dilute solution of a mixture of the two tin chlorides.

PLATINUM.

Symbol Pt, Combining Weight 1974, Specific Gravity

21'5.

Platinum is a comparatively rare metal, which always occurs in the native state, and generally alloyed with five other metals, viz. palladium, rhodium, iridium, osmium, and ruthenium. This alloy occurs in small grains in detritus and gravel in Siberia and Brazil; it has not been found in situ in the original rock, which probably belongs to the old plutonic series.

The original mode of obtaining the metal was to dissolve the ore in aqua regia, and precipitate the platinum (together with several of the accompanying metals) with sal-ammoniac, as the insoluble double chloride of ammonium and platinum, 2 NH, Cl+Pt Cl. This precipitate, on heating, yields metallic platinum in a finely divided or spongy state; and this sponge, if forcibly pressed and hammered when hot, gradually assumes a coherent metallic mass, the particles of platinum welding together, when hot, like iron. A new mode of preparing the metal has recently been proposed, the ore being melted in a very powerful furnace heated with the oxyhydrogen blowpipe. In this way a pure alloy of platinum, iridium, and rhodium is formed, the other constituents and impurities of the ore either being volatilized by the intense heat, or absorbed by the lime of which the crucible is composed. This alloy

is in many respects more useful than pure platinum, being harder and less easily attacked by acids than the pure metal.

Platinum possesses a bright white colour, and does not tarnish under any circumstances in the air; it is extremely infusible, and can only be melted by the heat of the oxyhydrogen blowpipe. It is unacted upon by the ordinary acids, but dissolves in aqua regia; and hence platinum vessels are much used in the laboratory. Caustic alkalies, however, act upon the metal at high temperatures. When finely divided, metallic platinum has the power of condensing gases on to its surface in a remarkable degree: the effect of bringing spongy platinum in contact with a mixture of oxygen and hydrogen has already been mentioned. Platinum and oxygen unite in two proportions to form―(1) Platinum monoxide, PtO; and (2) Platinum dioxide, Pt O2. The first of these oxides is a black powder, easily decomposed on heating, and yielding a series of unstable salts; the second is obtained as a brown hydrate, by adding to a solution of platinic nitrate half its equivalent of caustic potash: the hydrate, when heated, first loses its water, forming the anhydrous oxide, and then parts with its oxygen, leaving the metal. Platinum dichloride, Pt Cl2, is a green insoluble powder, obtained by heating the higher chloride to 200°. Platinum tetrachlo ride, Pt Cl, is the most important platinum compound. It is obtained as a yellowish-red solution by dissolving the metal in aqua regia; on evaporation, crystals of a compound of platinum tetrachloride with hydrochloric acid separate out. Platinum tetrachloride combines with many alkaline chlorides to form double salts: these compounds with potassium, rubidium, cæsium, and ammonium are insoluble in water, and are isomorphous, crystallizing in cubes ; whilst the sodium salt is soluble, and crystallizes in large prisms.

Platinum dichloride, when acted upon by ammonia, gives rise to several very remarkable compounds, containing platinum, nitrogen, and hydrogen: these substances

act as bases, and form a well-defined series of salts. These salts may be considered as molecules of ammonium, in which the hydrogen has been partly replaced by either a diatomic or tetratomic platinum.

[For the properties of the rare metals palladium, rhodium, ruthenium, iridium, and osmium, the larger manuals must be consulted.]

LESSON XXVI.

SPECTRUM ANALYSIS.

AN entirely new branch of chemical analysis, of great delicacy and importance, has recently been developed, chiefly by the researches of Bunsen and Kirchhoff, the principles of which may here be shortly stated.

It has long been known that certain chemical substances, especially the salts of the alkalies and alkaline earths, when strongly heated in the blowpipe, or other nearly colourless flame, impart to that flame a peculiar colour, by the occurrence of which the presence of the substance may be detected. If many of these substances are present together, the detection of each by the naked eye becomes impossible, owing to the colours being blended, and thus interfering with each other. Thus, for instance, the sodium compounds colour the flame an intense yellow, whilst the potassium salts tinge the flame violet: the yellow soda colour is, however, so much more intense than the purple potash tint, that a small trace of soda prevents the eye from detecting the purple, even if large quantities of potash salts are present. This difficulty is altogether overcome, and this method of observation rendered extremely sensitive, if, instead of regarding the flame with the naked eye, it is examined through a prism. This consists of a triangular piece of glass, in passing through which the light is refracted, or bent out of its course; each differently coloured ray being differently refracted:

so that if a source of white light, such as the flame of a candle, is thus regarded, a continuous band of differently coloured rays is observed; the compound white light being resolved into all its variously coloured constituents. This coloured band is termed a spectrum; and each source of pure white light gives the same continuous spectrum, stretching from red (the least refrangible) to violet (the most refrangible) colour, identical in fact with the colours of the rainbow. (See No. 1 of the chromolith. plate at beginning of volume.)

If these coloured flames are examined by means of a prism, the light being allowed to fall through a narrow slit upon the prism, it is at once seen that the light thus refracted differs essentially from white light, inasmuch as it consists of only a particular set of rays, each flame giving a spectrum containing a few bright bands. Thus the spectrum of the yellow soda flame contains only one fine bright yellow line, whilst the purple potash flame exhibits a spectrum in which there are two bright lines, one lying at the extreme red, and the other at the extreme violet end. (See Nos. 6 and 2 on the above plate.) These peculiar lines are always produced by the same chemical element, and by no other known substance; and the position of these lines always remains unaltered. When the spectrum of a flame tinted by a mixture of sodium and potassium salts is examined, the yellow ray of sodium is found to be confined to its own position, whilst the potassium red and purple lines are as plainly seen as they would have been had no sodium been present.

The coloured flames which are exhibited by the salts of lithium, barium, strontium, and calcium, likewise each give rise to a peculiar spectrum, by means of which the presence or absence of very small quantities of these substances can be ascertained with certainty when mixed together, simply by observing the presence or absence of the peculiar bright bands characteristic of the particular body. (See chromolith.)