of potash, while the other precipitates are insoluble. Should the excess of potash effect an obvious solution of the precipitate at first thrown down, but the liquid at the same time not become perfectly bright, it may be filtered off from the oxide of iron, or other insoluble impurity, and tested for chromium, aluminum, and zinc (8).

(39.) y. Ammonia throws down from solutions of nickel, cobalt, manganese, iron, and earthy salts, precipitates which have respectively the same composition as those thrown down by potash. The precipitates produced in solutions of the earthy salts and in sesquisalts of iron are equally insoluble in excess of ammonia as in excess of potash; but the prothydrates of nickel, cobalt, and manganese are readily soluble, and that of iron sparingly soluble, in excess of ammonia, when chloride of ammonium or some other neutral ammoniacal salt is also present. Hence when the previous experiment with potash has shown the probable presence of a protosalt of iron, it is necessary, before adding ammonia, to boil the solution with a little nitric acid for a few minutes, so as to convert the protosalt into a sesquisalt, whereby all the iron may be precipitated as a red-brown sesquihydrate, quite insoluble in excess of ammonia.

But when the colour of the precipitate produced by potash, or some other reaction, has indicated the absence of a protosalt of iron, the boiling with nitric acid may be dispensed with. The presence of a protosalt of iron in the original solution is best shown by the addition of ferridcyanide of potassium, which produces therewith an abundant dark blue precipitate of ferridcyanide of iron, or Turnbull's blue :

3FeCl + 2K,FeCy

= 6KCI + Fe Fe, Cy12

The presence of a sesquisalt of iron is best shown by sulphocyanate of potassium, which produces therewith a blood-red liquid, due to the formation of sesqui-sulphocyanate of iron:

+

FeCl 3KCyS

=

3KCl + Fe(CyS)3.

The mode of examining the earthy salts is described in

par. 59.

When excess of ammonia is added to a strongly acid solution, or when chloride of ammonium and excess of ammonia are added to a nearly neutral solution of nickel, cobalt, or manganese, the precipitate at first produced is immediately redissolved.

The ammoniacal solution of nickel has a deep purple-blue colour, closely resembling that of the similarly constituted solution of copper. Potash added to the ammoniacal nickel solution throws down an apple-green precipitate of hydrate of nickel; but a very large quantity of potash may be added to the ammoniacal copper solution without disturbing its transparency, and the precipitate finally produced is for the most part blue. Moreover, ferrocyanide of potassium produces with ordinary salts of nickel a pale green, and with ordinary salts of copper a chocolate-red, precipitate of the respective ferrocyanides; and the two metals are otherwise well characterised.

The ammoniacal solution of cobalt has a brownish-pink colour, which gradually becomes darker by exposure to air, until eventually a brownish precipitate of hydrated sesquioxide of cobalt is produced. But the cobalt solution, when made with ammonia containing carbonate of ammonium, or even with carbonate of ammonium itself, has a fine pink colour, which is tolerably permanent. Cobalt compounds are best recognised by fusion with the borax bead, to which they impart a deep purpleblue colour.

The ammoniacal solution of manganese is at first colourless, but by exposure to air it speedily becomes brown and opaque, from the conversion of the white soluble prothydrate into the brown insoluble sesquihydrate. Manganese compounds are very frequently contaminated with iron; hence, when excess of ammonia produces a brown precipitate, it is often worth while to pour the mixture on to a filter, and to test the ammoniacal filtrate with sulphide of ammonium; whereupon manganese, if present, is precipitated as a flesh-coloured sulphide. Manganese is best recognised before the blowpipe by fusing some carbonate of sodium, to which a little nitre may be added, upon platinum foil,

and then introducing a minute portion of the manganese compound, whereby a bright green fusible mass of manganate of sodium Na MnO4, is produced.

(40.) d. The potash solution of chromium is of a green colour, and when boiled for a longer or shorter time, according to the relative proportions of potash and chrome present, deposits the whole of its chrome in the form of a green precipitate. Chrome compounds are best recognised by the emerald green colour they impart to the borax bead; and by the bright yellow mass of chromate of alkali metal which they yield when ignited on platinum foil, with carbonate of sodium and nitre, before the blowpipe.

ε. The potash solution of zinc, when containing a considerable excess of potash, is not affected even by prolonged ebullition ; but when containing only a slight excess, it soon deposits an opaque white precipitate. On adding a few drops of sulphide of ammonium to potash solution of zinc, sulphide of zinc is formed, which being, unlike the hydrate of zinc, insoluble in potash, is thrown down as a white precipitate, frequently having a greyish tinge from the accidental presence of a trace of lead in the potash used as the reagent.

Zinc compounds when strongly heated leave a fixed infusible residue, which when cold is quite white, but when hot has a deep greenish-yellow tint. Moistened with nitrate of cobalt solution and re-ignited, it acquires a pure permanent chrome-green colour.

2. The potash solution of alumina is unaffected by boiling, and also by the addition of a few drops of sulphide of ammonium, unless indeed the potash contains an appreciable quantity of lead, when a black precipitate of sulphide of lead is formed.

Ammonia or carbonate of ammonium gives a similar white precipitate with zincous and aluminous salts, but the precipitate produced with the former is freely soluble, while that produced with the latter is insoluble in excess of the reagent. Hence the addition of chloride of ammonium to potash solution of zinc

G

produces no precipitate; but when added in sufficient quantity to potash solution of alumina, it throws down a white gelatinous precipitate of hydrate of aluminum, owing to a substitution of free ammonia for free potash, thus:

NHCI + KHO

=

KCI + NH HO, or NH.H,O.

Compounds of aluminum, when strongly heated in the blowpipe flame, leave a white, infusible, highly incandescent residue, which, when moistened with solution of nitrate of cobalt, and reignited, assumes a fine permanent blue colour.

§ V.-EXAMINATION FOR BASES OF GROUP III.

(41.) This group includes the bases which are not precipitated either by sulphuretted hydrogen, or by sulphide of ammonium. A solution of the salt in water or acid may be at once examined, according to the directions of Table III. Or the solution, which has been successively treated with sulphuretted hydrogen and sulphide of ammonium without the production of a precipitate, may be tested for any of the metals of this group, except potassium, sodium, and ammonium. If the original substance required an acid to effect its solution, its base cannot be any one of the last three, inasmuch as all simple salts of potassium, sodium, and ammonium are soluble in hot water.

TABLE III.

(42.) Examination of a solution containing some one member of the third group of bases; namely, BARIUM, STRONTIUM, CALCIUM, MAGNESIUM, POTASSIUM, SODIUM, or AMMONIUM. The sulphides or sulphydrates of these metals being soluble in water, the solutions of their salts are not precipitated either by sulphuretted hydrogen, or by sulphide or sulphydrate of ommonium.

a. Add Chloride and Carbonate of Ammonium to the original solution, and warm gently.