ness and digesting them in a mixture of equal parts of alcohol and ether, which dissolves the chloride of lithium only.

Hydrate of lithia (LHO) fuses easily below redness, and corrodes platinum vessels powerfully: silver capsules should therefore always be used in preparing it. This action upon platinum is one of the best indications of the presence of lithium. It appears to be due to the formation of an unstable peroxide of lithium, which imparts its oxygen rapidly to the platinum.

H‚Ð

(605) Chloride of Lithium (LCI, 2 H2O = 42'5+36) is fusible at a dull red heat: it crystallizes at temperatures above 60° in anhydrous octohedra; but below 50° in square prisms with 2 H2O: it is one of the most deliquescent salts known. If its aqueous solution be evaporated at a high temperature, it loses a portion of its chlorine, whilst lithia is formed. Chloride of lithium is very soluble in alcohol, and in a mixture of equal parts of alcohol and ether; as this mixture does not dissolve the chlorides of sodium and potassium, it may be used to separate chloride of lithium from these salts.

ᎦᎾ, Ꮎ

Sulphate of Lithium (L2SO,, H2O = 110 + 18; Sp. Gr. 2.02) crystallizes in flat tables, which are very soluble in water. There appears to be no acid sulphate of lithium, though a double sulphate of potassium and lithium may be formed, consisting of ᏞᏦᎦᎾ .

4

Phosphate of Lithium characteristic salts of this taining phosphates of the very soluble in acids even when dilute. very caustic soda is added to the solution of a pure salt of lithium till it has an alkaline reaction; phosphate of sodium is added; the liquid is then boiled, and left for at least 12 hours. A heavy granular crystalline deposit of phosphate of lithium gradually occurs. This salt fuses with carbonate of sodium to a glass which is transparent while hot, but becomes opaque on cooling. The salt supposed by Berzelius to be a double phosphate of sodium and lithium, appears to have been a mixture and not a definite compound.

(LP→ = 116) is one of the most alkali: it is insoluble in water conalkalies, and in alkaline solutions, but In order to prepare it,

Carbonate of Lithium (LEO, = 74, or LO,CO2 = 37) is only sparingly soluble in water, but is rather more soluble in a solution of carbonic acid: it has an alkaline reaction upon turmeric. At a dull red heat it melts into a white enamel, and by prolonged ignition loses a large portion of its carbonic acid.

CHARACTERS OF THE SALTS OF LITHIUM.-Generally speaking the salts of lithium are remarkably fusible; many of them are

448

SALTS OF LITHIUM-RUBIDIUM.

;

very deliquescent. They have a burning saline taste, and are distinguished by yielding a white precipitate of carbonate of lithium in cold concentrated solutions with carbonate of potassium, but the precipitate disappears on adding water and applying heat this reaction is less delicate when salts of ammonium are present. On the addition of phosphate of sodium to solutions which are neutral or alkaline, the phosphate of lithium is formed; it is soluble in the acids and in solutions of salts of ammonium. Before the blowpipe the salts of lithium communicate a purplish red colour to the flame, which is masked by the presence of salts of sodium in very small proportion. By means of the spectroscope the occurrence of very minute traces of lithium may be discovered by a brilliant crimson band, which has a refrangibility between that of the lines в and c of the solar spectrum. At very high temperatures a faint band in the orange may sometimes be In these two lines the whole light of the lithium spectrum is contained when formed by the gas-flame of Bunsen's gas-burner, (fig. 82). When lithium salts are heated on platinum foil they corrode it rapidly.

seen.

B

§ IV. RUBIDIUM: Rb-85.36. Sp. Gr. 152; Fusing-pt. 101*3.

(606) RUBIDIUM derives its name from rubidus (dark red), because the spectra of its salts, when volatilized in the colourless flame of a Bunsen gas-burner, exhibit a remarkable pair of red lines, less refrangible than Fraunhofer's line a (Part I. Fig. 82, Rb, p. 172.) Rubidium was discovered in 1860 by Bunsen and Kirchhoff during their investigations on the spectra of artificial flames (Liebig's Annal. cxix. 107, and exxii. 347). It is usually present in small quantity in lepidolite, and traces of it occur in many of the mineral springs of Germany. It has also been found by Grandeau (Ann. de Chimie, III. lxvii. 155) in minute quantity in beet-root, in tobacco, and in the ashes of a great variety of plants, being, though sparingly, yet very widely distributed. The separation of rubidium from other metals is founded upon the sparing solubility of the chloride of rubidium and platinum, by a method which will be described when speaking of the extraction of cœsium. Metallic rubidium was extracted by Bunsen from the charred acid tartrate, 1100 grains of which when distilled furnished about 80 grains of a brilliant silver-white metallic mass. It tarnishes rapidly on exposure to the air, becoming coated with a blue suboxide; in a few moments it takes fire spontaneously. At 14° F. it is soft, like wax; at 101°3 it melts; and at a heat

SALTS OF RUBIDIUM.

449

below redness it furnishes a blue vapour with a shade of green. Rubidium is decidedly more electropositive than potassium: when thrown upon water it takes fire and burns with a flame in appearance exactly resembling that of potassium.

(607) Rubidia, or protoxide of rubidium (Rb20=186·7) is a powerful alkaline base which may be obtained from the carbonate or sulphate in the form of hydrate (RbH=102 3) by processes resembling those adopted for hydrate of potash. It is very deliquescent, is soluble in alcohol, and absorbs carbonic acid with avidity.

Chloride of rubidium (RbCl=120'9) crystallizes with difficulty in cubes; it is easily fusible, and is more soluble than chloride of potassium, but is permanent in the air. With chloride of platinum it forms a sparingly soluble double chloride (2 RbCl,PtCl) which requires 185 times its weight of boiling water for its solution. If fused chloride of rubidium is submitted to electrolysis, the reduced metal is dissolved by the chloride and forms a smalt-blue subchloride.

Sulphate of rubidium (Rb2SO=266.7) crystallizes in hard brilliant anhydrous prisms, isomorphous with those of sulphate of potassium, but it is much more soluble than this salt. A true rubidium alum may be obtained in octohedral crystals by allowing a mixture of the sulphate with sulphate of aluminum to evaporate spontaneously. An acid sulphate of rubidium (RHS) is also known.

Nitrate of rubidium (RbN✪=147°3) is a very soluble salt requiring 2'3 parts of water at 51° for its solution. It crystallizes in dihexagonal prisms, terminated by dihexagonal pyramids.

Carbonate of rubidium (Rb2€Ð ̧=2307) is a deliquescent salt which may be obtained with difficulty in crystals with H2O. It absorbs carbonic acid with avidity and furnishes an acid carbonate (RbHЄ0g) which crystallizes in brilliant prisms that are permanent in the air and insoluble in alcohol. When heated they are converted into the normal carbonate, which by further elevation of temperature fuses easily.

CHARACTERS OF THE SALTS OF RUBIDIUM.-The salts of rubidium are distinguished from those of potassium with difficulty. The chloride of rubidium and platinum is the most characteristic: by its sparing solubility in boiling water, it may be separated from the potassium salt, which is soluble in one-eighth of the quantity of water required for solution of an equal weight of the rubidium salt. The most certain test is the appearance of the flame in the spectroscope, which exhibits two characteristic lines

II.

G G

in the red, less refrangible than that of potassium, and two lines in the blue, intermediate between those of cœsium and potassium.

§ V. CESIUM: Cs=133.

(608) This metal derives its name from cœsius, sky blue, in allusion to the two brilliant blue bands which it produces in the spectrum of a gas-flame in which its compounds are undergoing volatilization. It was discovered by Bunsen and Kirchhoff at the same time as rubidium (Pogg. Annal. cxiii. 337), which indeed it usually accompanies in very small quantity. Cosium was originally discovered amongst the saline constituents of the Durkheim spring, the water of which contains about one five-millionth of its weight of a salt of cœesium, or about 1 grain in 140 gallons. Ordinary lepidolite contains only traces; but a variety of this mineral from Hebron, in the State of Maine, N.A., was found, by Johnson and Allen, to yield o'24 per cent. of the metal. Still more recently, Pisani has found cosium to the extent of 32 per cent. in a rare mineral named pollux, analogous to analcime, obtained from the island of Elba (Comptes Rendus, 21 April, 1864).

Böttger, in examining the salt obtained by evaporating down the mother-liquor of the Nauheim spring, discovered in it cœsium, rubidium, and thallium. He, indeed, recommends it as the cheapest source of the two new alkaline metals; they exist in the spring in the form of chlorides.

In order to obtain the compounds of cœesium, advantage is taken of the insolubility of its double chloride with platinum, which is little more than half as soluble in boiling water as the corresponding salt of rubidium. The mixture which contains the rubidium and cosium is freed from compounds of the earths and other metals by the ordinary methods, and the residue, which contains salts of the alkalies only, is mixed with a solution of perchloride of platinum, which if added in excess precipitates nearly the whole of the rubidium and cœsium, together with a large proportion of potassium. By continued boilings of the precipitate with small quantities of water, repeated eighteen or twenty times in succession, so long as the washings have a yellow colour, the potassium salt is removed. The platinum salt is reduced by heating it in a current of hydrogen. The mixed chlorides of rubidium and cœesium are dissolved out by water, and converted into sulphates by heating them with an excess of sulphuric acid, which is expelled by ignition. On adding pure baryta water to the solution of the sulphates, the alkalies are obtained in the

caustic state, and may then easily be converted into carbonates, either by carbonic acid or carbonate of ammonium. Once more

the solution of the mixed carbonates is evaporated to complete dryness, and treated with boiling absolute alcohol, which dissolves out the carbonate of cœesium, and leaves the carbonate of rubidium.* On evaporation of the alcoholic solution a tolerably pure carbonate of cœesium is obtained.

An amalgam of cœesium may be procured by submitting a solution of the chloride to electrolysis, employing a globule of mercury for the negative electrode. This amalgam is even more electropositive than that of rubidium, so that cœsium is the most electropositive element as yet discovered.

(609) Cœsia (Cs,).-Cœsium appears to form two oxides: a blue suboxide, and a powerfully basic oxide corresponding to potash and soda, termed cœsia. The hydrate of casia (CsHO=150) is very deliquescent, and powerfully caustic: it is readily soluble in alcohol. When heated, it fuses readily, and attacks platinum. Chloride of caesium (CsCl=168·5) crystallizes in cubes, and is deliquescent: it melts at a low red heat. 100 parts of chloride of cœsium contain 2107 parts of chlorine, while an equal weight of chloride of rubidium contains 29'7, and of chloride of potassium 47'5 of chlorine.

The chloride of platinum and cœsium (2 CsCl, PtCl1=676) crystallizes in yellow transparent octohedra: 100 parts of boiling water dissolve o'377 of the salt (Bunsen).

Sulphate of cosium (Cs2SO4) is anhydrous, permanent in the air, but very soluble in water. It forms double salts with sulphate of magnesium, and other sulphates of that class of the form (Cs,ᎦᎾ, MgᎦᎾ . 6 Ꮋ Ꮎ) . It also yields a crystallizable alum. An acid sulphate (CSHSO) may be obtained in short rhombic prisms.

Nitrate of cosium (CsN→ ̧=195) is anhydrous and isomorphous with nitrates of rubidium and potassium. It is permanent in the

*The best plan of separating cosium from rubidium, according to Allen and Johnson, consists in taking advantage of the inferior solubility of the acid tartrate of rubidium. The mixed carbonates are to be neutralized with tartaric acid, and then a quantity of the acid is added equal to that required for converting the rubidium into the acid tartrate, leaving the cœsium in the solution as normal tartrate, which is deliquescent. The solution is concentrated by evaporation until it is nearly saturated at the boiling-point. The rubidium salt crystallizes out on cooling, and may be purified by recrystallization. This acid tartrate (RbH,H) requires 85 parts of boiling and 94 parts of water at 77° for its solution. The acid cosium salt is soluble in its own weight of boiling water, and in 10 parts of water at 77°, and the normal salt is deliquescent.-Bunsen, Pogg. Annal. cxix. i.