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in many mineral waters, in the mother-liquors from the evaporation of seawater, and in the ashes of plants. Although widely distributed, the quantities present are extremely minute, one of the richest lepidolites in which these metals occur containing only 0.24 per cent. of rubidium oxide.

The rare mineral pollux, a silicate of aluminium and caesium, containing also iron calcium and sodium, is the only known mineral in which either of these two elements occurs as an essential constituent. The analysis of Pisani (1864) gives 34.07 per cent, of caesium oxide in this substance.

Rubidium is obtained by heating the carbonate with carbon (the charred tartrate), as in the older method for the preparation of sodium and potassium,

Caesium cannot be isolated by this reaction, but is obtained by the electrolysis of the fused cyanide, Cs(CN) (mixed with barium cyanide in order to render it more readily fusible). Rubidium melts at 38.5°, caesium at 26.5o.

Rubidium gives a green vapour, and when sublimed in a vacuous tube yields a thin film of metal, which appears deep blue by transmitted light : when slowly sublimed in this way the metal forms small needle-shaped crystals. The compounds of these metals closely resemble those of potassium, from which they can only be distinguished by the different spectra they give.

AMMONIUM SALTS. The monovalent group or radical (NH) is capable of replacing one atom of hydrogen in acids, thereby giving rise to a series of salts which are closely analogous to, and are isomorphous with, those of potassium. The radical (NH), to which the name ammonium is given, has never been isolated. When an amalgam of sodium and mercury is thrown into a solution of ammonium chloride, the mercury swells up into a honeycombed or spongelike mass, which floats upon the surface of the liquid. This socalled ammonium amalgam was at one time thought to be a true amalgam of mercury with the metallic radical ammonium. It is now generally believed to consist of mercury which is simply inflated by the evolution of hydrogen and ammonia gas. When this sponge-like substance is subjected to changes of pressure, it is found to contract and expand in conformity to Boyle's law : its formation may be represented by the equation

Hg, Na, +yNH4Cl=yNaCl+xHg+yNH3 +yH.

In the course of a few minutes the inflated mass shrinks down, and ordinary mercury remains at the bottom of the solution, hydrogen and ammonia having been rapidly evolved.

The ammonium salts are obtained for the most part from the ammoniacal liquor of the gasworks. The material is treated with

lime, and distilled ; and the ammonia so driven off is absorbeds sulphuric or hydrochloric acid, giving rise to ammonium salpha or chloride.

Ammonium Chloride (sal ammoniac), NH Cl.— The produc obtained by absorbing ammonia from gas liquor in hydrochloris acid is purified by sublimation. The crude material is heated in large iron pots, covered with iron dome-shaped vessels, inte

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which the substance sublimes. Ammonium chloride crystallises in arborescent or fern-like crystals (Fig. 143), consisting of groups of small octahedra belonging to the regular system.

100 parts of water at 10° dissolve 32.8 parts, and at 100°, 77 parts of the salt. On boiling the aqueous solution, dissociation to a small extent takes place, and a portion of the ammonia escapes with the steam ; the solution at the same time becoming slightly acid.

Ammonium Sulphate (NH,),504:—The product obtained by

the absorption of ammonia obtained from gas liquors by sulphuric acid is purified by recrystallisation, when it forms colourless rhombic crystals, isomorphous with potassium sulphate. 100 parts of water at the ordinary temperature dissolve 50 parts of the salt. The chief use of ammonium sulphate is for agricultural purposes, as a manure ; and for this use the crude salt, as first obtained, which is usually more or less coloured with tarry matters, is employed. Ammonium sulphate is also used for the preparation of ammonia alum and other ammonium compounds, as well as in the ammonia-soda process.

Ammonium Carbonates.-Commercial ammonium carbonate (sal volatile) is obtained by heating a mixture of ammonium sulphate and ground chalk to redness in horizontal iron retorts or cylinders, and conducting the vapours into leaden receivers or chambers, where the carbonate condenses as a solid crust. It is afterwards purified by resublimation, when it is obtained as a white fibrous mass. This substance is a mixture of hydrogen ammonium carbonate, H(NH)CO2, and ammonium carbamate, (NH)CO,(NH), and smells strongly ammoniacal. When treated with alcohol the ammonium carbamate is dissolved, leaving the carbonate behind.

Normal Ammonium Carbonate, (NH),CO3, is obtained from the commercial compound, by passing ammonia gas into a strong aqueous solution, or by digesting the compound in strong aqueous ammonia. The carbamate present is converted into normal carbonate by the action of the water, thus

(NH)CO,(NH2)+H,=(NH)CO(NH,)=(NH, CO, ; and the ammonia converts the bicarbonate into the normal salt, thus

H(NH4)CO2 + NH3=(NH4)2CO3. Normal ammonium carbonate on exposure to the air gives off ammonia, and passes back into hydrogen ammonium carbonate. When heated to 60° the salt breaks up into carbon dioxide, ammonia, and water.

Hydrogen Ammonium Carbonate, H(NH,)C03, may also be obtained by passing carbon dioxide into a solution of the normal salt

(NH) Co,+CO,+H2O=2H(NH,)COg. It forms large lustrous crystals belonging to the rhombic system,

which, when dry, do not smell of ammonia. 100 parts of water at 15° dissolve 12.5 parts of this salt. At ordinary temperatures this solution on exposure to the air slowly gives off carbon dioxide, and becomes alkaline ; and when heated above 36° the liquid begins to effervesce, owing to the rapid evolution of carbon dioxide. This salt forms with the normal carbonate a double salt analogous to sodium sesquicarbonate, and having the composition (NH4)2CO3 2H(NH,CO,H,O.

Ammonium Thiocyanate, NH,S(CN), is prepared by adding aqueous ammonia to an alcoholic solution of carbon disulphide, and allowing the mixture to stand, when ammonium thiocarbonate is formed, thus

6NH3+3H,0+3CS,=2(NH4),CS3+(NH.),CO.. On heating this solution, the ammonium thiocarbonate is decomposed with evolution of sulphuretted hydrogen

(NH2),CS7=2H„S+NH,S(CN). Ammonium thiocyanate (known also as ammonium sulphocyanate) forms colourless crystals, which are extremely soluble both in water and alcohol. The solution in water is attended with considerable absorption of heat : thus, if 20 grammes of the salt be dissolved in 25 cubic centimetres of water at 18, the temperature of the liquid falls to – 13°.

CHAPTER V

THE ELEMENTS OF GROUP 1. (FAMILY B.)

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The elements of this family present many striking contrasts to those of the other family belonging to the first group. These three metals are not acted upon by oxygen, or by water, at ordinary temperatures ; they are all found native in the uncombined state, and on this account are amongst the earliest metals known to man. The alkali metals, on the other hand, are instantly oxidised on exposure to air, they decompose water at the ordinary temperature, are never found native, and are amongst the most recently discovered metals. With the exception of sodium and potassium, which are used in a few manufacturing processes, the alkali metals, as such, are of little practical service to mankind, whilst the metals of this family are amongst the most useful of all the metals, and are the three universally adopted for coinage. Many of the compounds of the elements of this family are similarly constituted to those of the alkali metals : thus, with oxygen and with sulphur we have Cu.,O, Ag,0, Au,O, and Cu, S, Ag,S, Au,S, corresponding to Li,O and K,S.

With the halogens they all form compounds of the type RX. Although the three elements, copper, silver, and gold, fall into the same family upon the basis of the periodic classification of the elements, they are in many respects widely dissimilar. Thus, silver is consistently monovalent, while copper is divalent, forming compounds of the type CuX.,, and gold is trivalent, giving compounds AuXy. The chlorides, AgCl and Cu.Cl,, on the other hand, are both insoluble in water, are both soluble in ammonia, and both absorb ammonia.

In many of their physical attributes, these metals show a regular

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