in the presence of chloride of ammonium, soluble in excess of the hydrate, producing a blue solution, changing to red by exposure to the air. The composition of the hydrate is CrHO. It is insoluble in water or in dilute acids, but dissolves slowly in strong acids. The hydrate is generally precipitated as a more or less brown powder, hydrogen escaping and an oxide (Cr,O2) being formed.

The Sulphide is produced by the action of sulphide of potassium. It is a black precipitate, soluble in excess of the precipitant. It always contains some uncombined sulphur.

THE CARBONATE appears to be produced by the action of carbonate of potassium. If precipitated in the cold, it separates as a dense yellow powder, but from a hot solution as brilliant brownish yellow crystals. The substance becomes green by exposure to the air.

THE FERROCYANIDE is produced by ferrocyanide of potassium. Its formula is probably Cr,Cfy. It is a yellowish green precipitate.

THE PHOSPHATE is a blue precipitate, produced by the action of phosphate of sodium. It becomes green by exposure to the air, and is readily dissolved by acids.

The action of the other tests hitherto employed has not been ascertained upon this series of salts.

Chromous salts may be recognized by their peculiar hydrate and sulphide.

CHROMIC SALTS, OR SESQUI-SALTS OF CHROMIUM.

Solution for the reactions :-chromic sulphate ([Cr2],[SO1]) in

water.

These are the salts of chromium which are usually met with. Those insoluble salts which characterize the series are the chromate, the hydrate, the carbonate, the ferrocyanide, and the phosphate. THE CYANIDE is produced by the action of cyanide of potassium: it is a light bluish-grey precipitate.

Its formula is Cr2Cy..

It is insoluble in excess of the precipitant, but soluble in excess of chromium salt: it dissolves in dilute acid without decomposition, but is decomposed when dissolved in hydrate of potassium. THE CHROMATE is produced by the action of chromate of potassium. It is a yellowish-brown precipitate.

The formula is Cr,(CrO2)3.

It is a very unstable compound, and is perfectly decomposed, even by long washing with water, into chromic acid and sesquioxide of chromium, which remains upon the filter.

This salt is insoluble in excess of the precipitant, but soluble in acids, and also in hydrate of potassium, with decomposition into chromic acid and a sesqui-salt of chromium.

The Hydrate is produced by the action of the hydrates or sulphides of potassium or ammonium, and also by the addition of the insoluble carbonates of the alkaline earths to solutions of the sesqui-salts of chromium. It is a bulky precipitate, the colour of which varies with the precipitant: when produced by hydrate of potassium, it is green; when by hydrate of ammonium, bluishgreen inclining to bluish-grey. It is thought that these differences in colour are due to varying amounts of water in the precipitate.

The composition of the hydrate dried at 100° C. is Cr,H,O,; but if the precipitate has been dried over sulphuric acid, other compounds are obtained, as, for instance,

2(Cr,H,O,)+aq, and Cr,H,O,+aq;

if a current of dry air is the only desiccator, the formula of the precipitate is 2(Cr2H,O,)+7aq. It frequently retains traces of its precipitants.

This salt resembles the hydrate of aluminium in being perfectly soluble in excess of hydrate of potassium, from which solution it is reprecipitated either by neutralization or by the addition of chloride of ammonium; but it differs from hydrate of aluminium in being reprecipitated from its solution in hydrate of potassium, either by long standing or by boiling: the precipitate which thus separates has the formula Cr2H,O,+3aq. Hydrate of ammonium also dissolves this salt to a certain extent, the amount dissolved increasing with the strength of the reagent; with a very strong solution of hydrate of ammonium, a pink solution is formed, from which the hydrate of chromium is thrown down on boiling as a dull violet precipitate. Chloride of ammonium does not affect the solubility of this salt. It is readily soluble in the acids, except after its precipitation from its hydrate of potassium solution by boiling.

THE SULPHATE is soluble.

THE DOUBLE SULPHATE OF CHROMIUM AND POTASSIUM is a very

soluble salt, and crystallizes well in octohedra, if its solution has not been heated above 50° C.

Its formula is KCr,(SO), +12 aq.

It dissolves in 6 parts of water at the ordinary temperature.

THE CARBONATE is produced by the action of the carbonates of potassium or ammonium: it is a pale green or a bluish-green precipitate.

Its composition is that of a mixed carbonate and hydrate, and appears to be uncertain.

It dissolves slightly in excess of its precipitants, forming a greenish-blue solution with carbonate of potassium, and a pink solution with carbonate of ammonium: from both it is reprecipitated by boiling. It also dissolves readily in hydrate of potassium and in acids.

THE OXALATE is produced, by the action of oxalate of ammonium on chromic chloride, as a pale green precipitate.

Its composition is unknown; it is probably a basic salt, for the neutral salt (Cr),Ō, is very soluble.

It dissolves readily in many acids.

THE FERROCYANIDE AND THE FERRICYANIDE are not precipitated by their respective reagents.

THE PHOSPHATE is produced by the phosphate of sodium as a bluish-green precipitate.

Its composition is unknown.

Its behaviour with reagents is almost identical with that of the the hydrate.

The other special reagents of the first and second groups, as well as those of the present group, yield no precipitates with sesqui-salts of chromium.

The chief means for the recognition of the metal chromium are these,—the production of the sesquioxide by the ignition of most chromous and chromic salts, and the formation of the chromate of potassium described above from the salts of either series; the borax bead, and the precipitation and behaviour of the chromic hydrate.

SALTS OF URANIUM.

This metal forms a great number of combinations with oxygen: many of these, however, are probably combinations of oxides one with another; for corresponding series of salts are not found. Uranium forms two series of salts, the uranous corresponding to the chloride UCl, and the uranic represented by the chloride U2Cl,: the salts of the first series are generally of a greenish, while those of the latter are for the most part of a yellowish colour. Most salts of uranium when heated before the blowpipe are converted into uranous oxide U2O, which remains as a black infusible residue: they impart no colour to the blowpipe flame, nor do they give any characteristic reactions with nitrate of cobalt; but when heated with borax, the glass in the outer flame becomes red or yellow while hot, and yellow or colourless when cold, the depth of colour depending on the amount of salt present, while in the reducing flame the colour changes to a bottle-green.

URANOUS SALTS, OR PROTO-SALTS OF URANIUM.

Solution for the reactions:-protochloride of uranium (U Cl) in water. The principal insoluble salts of this series are these,—the hydrate, the sulphide, the carbonate, the oxalate, the ferrocyanide and the phosphate.

THE CYANIDE AND CHROMATE do not appear to exist. Cyanide of potassium produces a black precipitate of U2O with evolution of hydrocyanic acid; and chromate of potassium produces a yellowish-brown precipitate, which appears to be a mixture of uranic chromate and uranous chromite.

The Hydrate is produced by the action of the hydrates of potassium or ammonium, also by carbonates of calcium on solutions of uranous salts; it separates in reddish-brown gelatinous flakes, which turn black by boiling, the change being due, it is thought, to loss of water. If not washed free from its precipitant, it rapidly absorbs oxygen from the air, and is converted into the yellow compound known as the uranate of potassium or ammonium (p. 118). Its composition is not known.

It dissolves easily in acids, but, after ignition, is soluble only in concentrated sulphuric acid.

The Sulphide is produced by the action of sulphide of ammonium. It is a black precipitate, which becomes grey by washing with water, and appears to be thus converted into a mixture of uranous hydrate and sulphur. Its composition is not known.

It dissolves in acids, especially in nitric acid.

THE SULPHATE is soluble, and it forms no insoluble DOUBLE SULPHATE. THE CARBONATE is produced by the action of carbonate of potassium or ammonium. It is a green precipitate.

Its composition is probably that of a mixed carbonate and hydrate.

It dissolves in a large excess of its precipitants, especially in carbonate of ammonium, forming a green solution.

THE OXALATE is produced by the action of oxalic acid; it is a greenishwhite precipitate, which does not oxidize by exposure to the air.

The formula of the air-dried salt is U2 C2O4+3 aq.

It is insoluble in water, and but slightly soluble in hydrochloric acid. The Ferrocyanide is produced by the action of ferrocyanide of potassium; it is a light brown precipitate.

Its formula is U, Cfy.

It dissolves slightly in hydrochloric or nitric acid.

THE FERRICYANIDE is produced by the action of ferricyanide of potassium : it is a brownish-red precipitate, which appears only after the lapse of some time.

THE PHOSPHATE is produced by the action of phosphate of sodium; it is green and gelatinous.

Its formula is U2H PO1+aq.

It dissolves in concentrated hydrochloric acid, and is reprecipitated by the addition of water.

The other special reagents of the two former subdivisions are not known to yield any characteristic precipitates with uranous salts.

The chief methods employed for the detection of uranous salts are these,the formation of the hydrate, the carbonate, and the phosphate.

URANIC OR URANYLIC SALTS.

2

Solution for the reactions:—the nitrate of uranyle (U2O, NO3) in water. This series of salts is one of the most remarkable that have yet been mentioned. The oxide, which is taken as its representative, has the formula (U2)2O3; but when the other salts of the series are examined, it is found that they do not correspond to the typical formula, but are all at total variance with it: thus the nitrate is U2ONO,, not U2 (NO3)3, the ordinary composition of a sesquinitrate; and the sulphate is (U2 O), SO4, instead of Ụ(SO4)3 ; and these are not imaginary formulæ, but the simplest expressions for the salts of this series, legitimately derived from uranic oxide or uranic chloride by double decomposition, assuming of course that in the nitrate and sulphate the same acid-radicals exist as are found in other nitrates and sulphates. The idea was then conceived, that uranic oxide was not a sesquioxide, but a protoxide of the compound basic radical uranyle U2O; uranic oxide then becomes (U2O), O, the chloride U2OCI, and the sulphate (U2O)2SO4: this view harmonizes with the composition of uranic salts in general, and is commonly adopted; its probability is confirmed by the composition of certain compounds of antimony and other metals.

2

The chief insoluble salts of this radical are these,-the cyanide, the chromate, the hydrate, the sulphide, the carbonate, the oxalate, the ferrocyanide, the ferricyanide, and the phosphate.

THE CYANIDE is produced by the action of cyanide of potassium. It is of a fine yellow colour. It dissolves in excess of the precipitant and in nitric acid. THE CHROMATE is produced by chromate of potassium. It is a yellow precipitate.

The Hydrate is obtained by the decomposition of the oxalate in sunlight,