the radical in question is obtained in solution, gelatinous silicic acid (HSiO,) being precipitated:

3SiF2+2H20=HSiO2+H ̧ Si2 F ̧.

Heated on charcoal, silicofluorides suffer various changes, according to the nature of the basic radical present.

THE HYDROGEN SALT (H, Si, F,) is known only in aqueous solution.

This radical may be recognized both by the formation of insoluble salts and by processes of decomposition.

The majority of the silicofluorides are soluble with greater or less facility in water; it is remarkable, however, that those of the first and second subdivisions of the basic radicals are the least soluble.

THE POTASSIUM SALT is produced by the action of a soluble potassium salt on hydrofluosilicic acid: it is a white gelatinous precipitate. Its formula is K, Si, F,. It is very slightly soluble in cold water, but dissolves somewhat more freely in hot.

The Barium Salt is produced by the action of a soluble barium salt upon hydrofluosilicic acid: it is a gelatinous precipitate, but appears crystalline under the microscope. It is precipitated more speedily on stirring the solution.

Its formula is Ba, Si, F.

It is but slightly soluble in cold water, 1 part requiring 3802 parts of pure water for its solution, and 733 parts of water acidulated with hydrochloric acid. In alcohol it is insoluble.

THE STRONTIUM and CALCIUM SALTS are comparatively soluble. THE MAGNESIUM SALT is exceedingly soluble.

THE FERROUS, FERRIC, CUPROUS, ARGENTIC, and MANY OTHER SALTS containing metals of the third and fourth subdivisions are soluble in water.

THE MERCUROUS SALT ([Hg,], Si, F,) is sparingly soluble.

THE MERCURIC and LEAD SALTS are soluble.

The salts of this acid-radical may be detected by the following processes of decomposition:

a. Hydrofluosilicic acid gradually decomposes when exposed to

the air in a glass vessel, terfluoride of silicon being given off, and the hydrofluoric acid thereby left attacking the glass and yielding another portion of the volatile terfluoride. In a glass bottle from which the air is always carefully excluded, and the volatilization of any part of the gas SiF, thus prevented, no decomposition

ensues.

B. Alkalies decompose hydrofluosilicic acid, yielding fluorides and silicates.

y. Strong sulphuric acid liberates gaseous terfluoride of silicon from hydrofluosilicic acid. From silicofluorides, acids separate a portion of silicic acid, chiefly on ebullition.

Of the acid-radicals which contain carbon and oxygen, two merit the attention of the student-the carbonic and the oxalic; both are bibasic. The salts of the former radical are of very common occurrence in nature, forming (in addition to their general distribution through the earth's crust) extensive deposits, as the strata of limestones, of which they are practically the sole constituents. The salts of the latter radical, the oxalic, are of less frequent occurrence, although they also are found in nature, existing in several members of the vegetable kingdom; nevertheless they are important as standing on what may be termed the neutral ground between the organic and inorganic domains of chemistry. The oxalates are also interesting, since they exhibit the simplification which complex organic molecules undergo in their gradual conversion into simpler forms. The oxalate is frequently the last stage of combination into which the carbon and oxygen of a complex body enter previously to their appearance as a carbonate, a salt which is very generally assumed to belong to inorganic nature.

SALTS OF THE CARBONIC RADICAL, OR CARBONATES.

These salts are not very numerous; for the radical is one of such weak combining power that it is only when in union with the most powerful basic constituents (those of the first or second subdivisions), or where the salt is insoluble, that stable compounds are formed by it. For this reason, and on account of the

facility with which carbonates, especially on the application of heat, divide into metallic oxides or hydrates and carbonic anhydride (CO), it is found that many of the precipitates which in double decompositions should be carbonates, are in reality mixed carbonates and hydrates. Take, for example, the precipitate produced by the action on carbonate of sodium of a zinc salt or any of the weaker bases, such as the triatomic molecules Al2, Cr2, Fe,, or Sb, some of which will not combine with this radical at all, or, if they do combine, yield salts which instantly or very rapidly decompose. All these precipitates of mixed hydrate and carbonate, even those which are insoluble in water, dissolve in the aqueous solution of carbonic acid gas; and from the solution of acid carbonate so formed, crystals of neutral carbonate are often deposited: this is the case with the zinc precipitate:

Zn, CO,, 3ZnHO+4H, CO,=5ZnHCO2+3H,0;

the acid carbonate of zinc thus formed decomposes in the following way :

The alkaline carbonates, and those of barium and strontium, may be fused on charcoal without decomposition; all others are decomposed.

2

THE HYDROGEN SALT, or carbonic acid (H,CO,), is not known in the separate state. It is believed to be formed when the gas CO, is passed into water; and the occurrence of acid carbonates leaves little doubt of the existence of the acid, although, from its extreme instability and tendency to be resolved into water and carbonic anhydride, attempts to isolate it have not been successful. The gaseous anhydride has been condensed to a liquid, and even to a solid. The following formulæ represent the composition of carbonic acid and of the two varieties of carbonates:

The carbonic radical may be recognized both by the formation of insoluble carbonates, and by a process of decomposition.

The carbonates that have been obtained are almost all insoluble, the salts of potassium, sodium, and ammonium being the only exceptions.

THE POTASSIUM, SODIUM, and AMMONIUM SALTS are soluble.
The Barium Salt is a white precipitate.

Its formula is Ba, CO,.

It is soluble in 141,000 parts of a solution of ammonia, or carbonate of ammonium; in 14,130 parts of pure water at 16° to 20°, or in 15,421 parts of boiling water. It is soluble in carbonic and other acids.

THE STRONTIUM SALT is a white precipitate, soluble in 18,045 parts of cold water, and readily dissolved by most acids. (See p. 88.)

THE CALCIUM SALT is a white precipitate, soluble in 10,600 parts of cold water, and readily dissolved by most acids. (See p. 91.)

THE MAGNESIUM SALT is a white precipitate, which requires 2493 parts of cold water for its solution, and is readily dissolved by nearly all acids. (See p. 94.)

THE FERROUS and MANY OTHER SALTS of the third subdivision, if existing at all, decompose soon after formation as already described, or when exposed to the air or to a slight increase of temperature.

THE CUPRIC SALT is produced by the action of cupric sulphate upon soluble carbonates: it is a bluish green precipitate. Its formula is Cu, CO,,2CuHO. It dissolves in ammonium salts and in acids, but is insoluble in water.

THE SILVER SALT is produced by the action of nitrate of silver on soluble carbonates: it is a white precipitate. Its formula is Ag, CO,. It is soluble in ammonium salts and in acids, but insoluble in water.

THE MERCUROUS SALT is produced by the action of mercurous nitrate on a solution of a carbonate: it is a yellow precipitate. Its formula is (Hg)2CO3.

THE MERCURIC SALT is brownish red. Its formula is Hg, CO,, Hg,O. It is slightly soluble in carbonate of potassium, soluble in chloride of ammonium and in acids; in water it is insoluble.

THE LEAD SALT is white. Its formula is Pb, CO,,2PbHO. It is soluble in 23,450 parts of water containing acetate, hydrate, or carbonate of ammonium, more soluble in water containing nitrate of ammonium; it is readily soluble in many acids. 1 part dissolves in 50,551 parts of pure water at the ordinary temperature.

This acid-radical may be readily recognized by the products of its decomposition.

a. Almost any acid, however weak or dilute, when added to a soluble or insoluble carbonate, causes its decomposition; and the resulting carbonic acid, by its resolution into water and the gaseous anhydride CO2, gives rise to the phænomenon of effervescence. Effervescence is caused by the escape of several other gases, such as hydrosulphuric acid or sulphurous anhydride, when certain sulphides or sulphites are similarly treated: but a peculiarity attends the formation of carbonic anhydride from carbonates; for all these salts, whether insoluble or not, and whatever basic radicals they may contain, are decomposed by acids, even the weakest, with evolution of the gas CO,. Now, although with the sulphides and cyanides of the first and second subdivisions, and with some few others, a somewhat similar evolution of gas takes place, yet it does not occur with all, and in many cases requires the employment of certain powerful acids. Chlorides too, especially alkaline chlorides, effervesce, evolving hydrochloric acid (HCI) when treated with concentrated sulphuric acid; but weak sulphuric acid has no action upon chlorides unless heat also be applied.

B. The carbonic anhydride (CO2), usually called carbonic acid gas, which is obtained in the reaction of acids upon carbonates, may be recognized by its peculiar smell, but especially by its immediately forming a white precipitate of carbonate of barium when passed into baryta-water-a precipitate which, by the continued passage of the gas, redissolves from formation of the soluble acid carbonate (BaHCO,). The experiment may best be