hydrogen given off. The formation of the brown or brownish yellow precipitate of sulphide of tin, and the evolution of hydrosulphuric acid, are due to the tendency which sulphites possess to pass into sulphates, by a change in which a portion of sulphur already combined with oxygen parts with that element, in order to increase the proportion of oxygen in the remaining compound. The sulphur thus liberated unites either with tin or hydrogen, according to the conditions of the experiment.

d. The action of sulphuretted hydrogen (H, S) upon sulphurous anhydride (SO) has been already described (p. 285, y).

e. Soluble sulphites treated with zinc and hydrochloric acid evolve sulphuretted hydrogen, which may be tested in the usual way with lead paper.

. A soluble sulphite acidified with hydrochloric acid, decolourizes permanganate of potassium (KMn, O,), reduces ferric to ferrous salts, and in general exerts a powerful reducing action.

The methods chiefly employed for the recognition of the sulphites are these:-the precipitation and metamorphoses of the barium and silver salts, and the tests a, ß, y, d, e, ; they are more particularly distinguished from the hyposulphites by the test a, in the case of the sulphites no precipitate occurring, while with the hyposulphites a deposit of sulphur takes place.

SALTS OF THE HYPOSULPHURIC RADICAL, OR HYPOSULPHATES.

The manganese salt of this radical is generally prepared; and from it the other compounds may be obtained. Binoxide of manganese suspended in water, and sulphurous acid gas (SO2) passed through it, yields hyposulphate of manganese tolerably free from sulphate if the temperature of the solution be low:

Mn, O2+280, Mn, S2O..

When heated, in some cases only to 100°, these salts evolve 1 eq. of sulphurous acid gas, and leave a neutral sulphate:

THE HYDROGEN SALT (H, S2O), or hyposulphuric acid, is a strongly acid liquid, which by exposure to the air is converted into sulphuric acid; by ebullition it also decomposes, just as the other salts decompose, viz.—

The hyposulphates may be identified by the decomposition of their acidradical:

a. The addition of a strong acid to a hyposulphate liberates hyposulphuric acid; and if the solution be then boiled, sulphurous acid gas escapes, sulphuric acid remaining in solution, which may be tested in the usual manner: no deposit of sulphur takes place.

B. Fused with carbonate of sodium on charcoal, hyposulphates yield sulphide of sodium.

y. Dilute hyposulphuric acid, or a soluble hyposulphate acidified with hydrochloric acid, dissolves zinc without forming hydrosulphuric acid; but after ebullition it behaves as a solution of sulphurous acid would.

d. Hyposulphuric acid does not decolourize permanganate of potassium, nor reduce ferric sulphate, nor precipitate sulphur from hydrosulphuric acid; but after boiling it behaves as sulphurous acid.

For the detection of this radical the tests just described may be used, while, for distinguishing it from the hyposulphurous and sulphurous radicals, the reactions described under y. and d. may be employed.

SALTS OF THE TRITHIONIC RADICAL, OR TRITHIONATES.

Of these salts, which are prepared with difficulty, that of potassium is the only one well known. The general formula of the salts is M2S, O; they are all soluble in water, and are easily destroyed.

SALTS OF THE TETRATHIONIC RADICAL, OR TETRATHIONATES.

Several tetrathionates are known, which are not very soluble in water, and much more stable than the trithionates. THE HYDROGEN COMPOUND (H2SO) may be boiled without decomposition, thus differing from trithionic acid.

SALTS OF THE PENTATHIONIC RADICAL, OR PENTATHIONATES.

The pentathionates are very prone to decomposition. Their general formula is MS, O; but one or two acid salts (MHS, O) are known. THE HYDROGEN COMPOUND is very soluble in water, has an intensely acid taste, and gives precipitates with copper, silver, mercury, and lead salts, of various colours at first, but which speedily become dark brown or black, the sulphide being formed.

SALTS OF THE SULPHURIC RADICAL, OR SULPHATES.

The salts of the sulphuric radical are more stable than those of any other acid-radical containing sulphur and oxygen; and it is indeed remarkable in this respect among compound acidradicals in general. It is in the form of combination which we are now considering that sulphur exists, to a considerable ex

tent, in nature, and supplies the necessary quantity of this element to the animal and vegetable economy. The sulphates are, with few exceptions, soluble salts, and are met with in all, or almost all, the different varieties of water found in the earth.

Many sulphates, when heated alone on charcoal, yield the metal; others, such as those of magnesium and zinc, are resolved into metallic oxides,-while some few give a residue of sulphide. In all cases, however, a mixture of sulphurous (SO) and carbonic (CO) anhydrides is evolved.

THE HYDROGEN SALT (H, SO), or sulphuric acid, is a dense liquid, to which the common term "oil of vitriol" is by no means inappropriate. The density of the acid H, SO, is 1.848; it evolves great heat when mixing with water, and combines with it to form a hydrated sulphuric acid, which is strictly analogous to a sulphate with 1 equivalent of water of crystallization, has the formula H, SO1+aq, and crystallizes at 9° C. Other such hydrates are known, such as H, SO1+2aq. There is an acid obtained by distilling ignited and oxidized ferrous sulphate and receiving the products in ordinary oil of vitriol, which is termed Nordhausen sulphuric acid, or fuming oil of vitriol: this liquid has a specific gravity of 1.896, and contains dissolved in it a substance which is known as anhydrous sulphuric acid, or sulphuric anhydride (SO,), and which may be separated from the fuming acid by distillation. Thus obtained, it occurs in long silky needles; it bears the same relation to sulphuric acid (H, SO) as sulphurous anhydride (SO) does to sulphurous acid (H, SO1).

The great majority of the normal sulphates (M, SO1), all the acid sulphates (MHSO), and all the so-called tersulphates are soluble in water. The few sulphates that are insoluble or sparingly soluble in water are dissolved to some extent by strong sulphuric acid.

Sulphates may be recognized by the formation of insoluble salts, and by the decomposition of their acid-radical.

The barium, mercuric, and lead salts are among the more important insoluble sulphates.

THE POTASSIUM and SODIUM SALTS are soluble.

The Barium Salt is produced by the addition of a soluble barium salt to sulphuric acid, or to a neutral or acid solution of any sulphate: it is a heavy white precipitate.

Its formula is Ba, SO.

It requires 43,000 parts of cold water for its solution, and is but slightly more soluble in hot water, acids, or saline solutions, although there are circumstances in which certain organic salts, those of citric acid especially, prevent the precipitation of sulphate of barium. It dissolves sparingly in concentrated sulphuric acid.

THE STRONTIUM SALT is produced by the addition of a soluble strontium salt to sulphuric acid or to solutions of soluble sulphates (even of sulphate of calcium): it is a white precipitate. Its formula is Sr, SO1. It is soluble in 6895 parts of water at 14°, and in 9638 parts of boiling water.

THE CALCIUM SALT is produced by the addition of a soluble calcium salt to sulphuric acid or to solutions of soluble sulphates. It is a white precipitate, to which the formula Ca2, SO1+2aq has been assigned. It is far more soluble than the corresponding strontium salt, 1 part requiring only 460 parts of boiling or cold water for its solution.

THE MAGNESIUM, FERROUS, FERRIC, MANGANOUS, COBALT, NICKEL, ZINC, CADMIUM, CUPROUS, and CUPRIC SALTS are soluble.

THE SILVER SALT (Ag2SO) is sparingly soluble, 1 part requiring 87 of water for solution; it is more soluble in nitric acid, and very much more soluble in sulphuric acid.

THE MERCUROUS SALT is produced by the addition of a soluble mercurous salt to sulphuric acid or a soluble sulphate. Its formula is (Hg2), SO. It is somewhat soluble in water; but a basic salt is thereby produced. It dissolves in dilute nitric acid, but is reprecipitated on the addition of dilute sulphuric acid. It dissolves abundantly in strong sulphuric

acid.

THE MERCURIC SALT is obtained by precipitating a mercuric salt with a soluble sulphate at a high temperature. It is a basic

salt (Hg, SO, 2Hg, O). It dissolves in 2000 parts of cold, or 600 of boiling water.

THE LEAD SALT is obtained by the addition of a soluble lead salt to sulphuric acid or a solution of a soluble sulphate. It is a dense white precipitate. Its formula is Pb, SO. It dissolves in 22,816 parts of water at 10°, and in 36,505 of dilute sulphuric acid; it is more soluble in concentrated sulphuric acid, and also in nitric acid. Certain ammonium salts also dissolve it sparingly. The sulphates may also be recognized by the following tests :a. When sulphates are acted on by strong mineral acids, no apparent liberation of the acid (H2SO1) takes place, nor is any characteristic reaction produced.

2

B. Sulphates, when fused on charcoal with carbonate of sodium, are reduced, in common with other salts of radicals containing sulphur and oxygen, to sulphides.

7. Sulphuric acid and sulphates do not yield any trace of hydrosulphuric acid on the addition of hydrochloric acid and stannous chloride.

8. Sulphuric acid and sulphates give no characteristic reaction with sulphuretted hydrogen.

e. Sulphuric acid and sulphates are not reduced to a lower stage of oxidation by hydrochloric acid and zinc.

4. Sulphuric acid and sulphates exert no reducing action upon ferric salts, &c.; nor do they decolourize permanganate of potassium.

This radical is generally recognized by the formation of the insoluble barium salt, and by the test 6,-while it is distinguished from the other radicals containing sulphur and oxygen, by the absence of reaction on the application of the tests a, y, d, e, L.

Of the acid-radicals containing selenium combined with oxygen, only two are known-the selenious and selenic; for the combination of selenium with oxygen, the gas (SeO), to which is attributed the odour of horseradish produced whenever selenium or a selenium compound is heated strongly in the air, appears itself to possess no acid characters.