quently corked, and the whole apparatus placed in equilibrium upon the balance.

A few bubbles of air are sucked out of d by means of a small suction tube, or of a perforated cork. This serves to rarify the air in a likewise, and consequently causes the sulphuric acid in B to ascend in the tube c. This tube is watched for some time to ascertain whether the column of sulphuric acid in it remains perfectly stationary, since this will prove that the apparatus is airtight. Air is then again sucked out of d, to cause a portion of the sulphuric acid to flow over into a. The carbonate in the latter flask is decomposed by the sulphuric acid, and the liberated carbonic acid, completely dried in its passage through the concentrated sulphuric acid in B, escapes through d. As soon as the evolution of carbonic acid ceases, the same process is repeated until the carbonate is completely decomposed. A large amount of sulphuric acid is then made to pass over into A, by applying a greater degree of suction to d, than in the preceding part of the process; this influx of sulphuric acid into a causes the contents of this flask to become considerably heated; when all evolution of gas-bubbles has completely ceased the wax stopper on a is to be slighly opened and the suction applied to d until the air tastes no longer of carbonic acid. The apparatus, when cool, is replaced upon the balance; and the former equilibrium restored by adding weights to the apparatus. The sum of these weights expresses at once the amount of carbonic acid originally present in the analysed compound. The results are very accurate.

bb. Carbonates, the bases of which form insoluble salts with sulphuric acid.

In this case, the method aa. is not quite applicable, since the insoluble sulphate formed (gypsum, for instance,) will partially protect the not yet decomposed portion of the carbonate from decomposition. The construction of the apparatus is therefore modified as illustrated by Plate XXXV.

PLATE XXXV.

It will be seen from this engraving that the modification consists simply in the tube a being expanded at the upper end, into a tubulated bulb, and drawn out to a fine point at the lower end.

The process is conducted as follows:

The carbonate under examination is introduced into A, which

A certain

is filled to about one third or one half with water. amount of dilute nitric acid, more than sufficient for the decomposition of the carbonate, is introduced into the tube a, and prevented from flowing through the narrow aperture of this tube by a little wax stopper (b). The point of this tube must not dip into the water in A, but simply reach to its surface. The apparatus, when thus fully prepared, is placed in equilibrium upon the balance, and the tube a is then carefully and cautiously depressed, (by turning,) so that its point nearly touches the bottom of A. The wax-stopper bis then slightly and momentarily raised, so as to permit a small quantity of nitric acid to flow out of the tube a; and the same operation is repeated until the carbonate is completely decomposed. A is then placed in hot water, the little wax-stopper is partly removed, and suction applied to the tube d, until the air tastes no longer of carbonic acid. The diminution of the weight which the apparatus has undergone in this process, is ascertained when the apparatus has completely cooled.

§ 106.

B. SILICIC ACID.

1. DETERMINATION.

Silicic acid is invariably weighed in its insoluble state into

which the soluble modification is converted by evaporation and subsequent ignition. In cases where we have to operate upon free silicic acid in aqueous or acid solution, this is to be evaporated to d yness, and the residue ignited and weighed.

II. SEPARATION OF SILICIC ACID FROM THE BASES.

a. Compounds which are soluble in water.

The solution is mixed with hydrochloric acid in excess, and the mixture evaporated to dryness in the water-bath; the residue is heated in the air-bath, or sand-bath, with stirring, until all moisture is completely and absolutely expelled; the dry residue is digested with hydrochloric acid, water added, a gentle heat applied, and the fluid filtered off from the silicic acid. The acid is washed with hot water, carefully and thoroughly dried and ignited. Properties of the residue, vide § 67, 9. This method yields accurate results. For the methods of determining the bases which we have as chlorides in the filtrate, I refer to the first part of this section.

b. Compounds which are decomposed by hydrochloric acid or nitric acid.

The compound under examination is very finely levigated, and dried at 212° (taking care not to raise the temperature higher). The powder is introduced into a platinum dish, or of genuine porcelain, and drenched with strong hydrochloric acid, or (in cases where the substance under examination contains lead or silver) with strong nitric acid. This mixture is digested at a very gentle heat, until the substance under examination is completely decomposed. To ascertain whether this point has been attained, the mixture is to be stirred from time to time. with a glass rod, rounded and smooth at the lower end the process may be considered at an end, when the stirring ceases to cause the slightest grating or crepitating noise, the glass rod moving smoothly along the sides, and over the bottom of the dish.

The various compounds of silica manifest some difference of

deportment in this process; some of them, for instance, form a bulky gelatinous mass, whilst the silica of others separates as a light, pulverulent precipitate. Again, many of them are decomposed very readily, whilst others require long-protracted digestion at a high temperature.

When the decomposition is completed, the whole mixture is to be evaporated to dryness in the water bath, and the residue heated until all moisture is completely and absolutely expelled; the dry residue is then again digested with hydrochloric acid, or nitric acid, until the base, or bases, which it contains are completely dissolved out. The further process is conducted exactly as directed sub. II. a.

c. Compounds which are not decomposed by hydrochloric acid or nitric acid.

a. Compounds of this kind are fluxed with carbonate of soda, hydrate of barytes, or carbonate of barytes, according to the nature of the bases with which the silicic acid may happen to be combined, (vide § 20, b.) They are then dissolved in hydrochloric acid or in nitric acid, and the solution is treated as directed sub. II. b.

B. The compound to be analysed is very finely levigated and elutriated, and one part of the powder obtained by these operations is mixed with about five parts of pure, finely powdered, fluor spar, in a platinum dish or in a capacious platinum crucible. The mixture is moistened with pure concentrated sulphuric acid, and stirred into an uniform paste with a small piece of platinum wire, which is then wiped clean against some powdered fluor spar, and the latter likewise projected into the dish or crucible. A gentle heat is then applied; fluoride of silica and hydrochloric acid escape. The heat is finally increased to gentle redness, so as to expel the sulphuric acid completely. The residue, which is now perfectly free from silicic acid, is digested for some time with concentrated hydrochloric acid; the mixture is then diluted with water, and the fluid filtered off from the gypsum, the greatest part of which remains undissolved; the bases are finally determined

according to the methods which will be found in the fifth section. This method should be pursued only in cases where we wish to determine the alkalies in compounds of which the other constituents have been determined already according to II. c. a. Pure fuming hydrofluoric acid is also applied very advantageously for the resolution of insoluble silicates, but as this method requires the possession of a small distilling apparatus of platinum, I will content myself with simply alluding to it. Brunner's lead apparatus for resolving insoluble silicates by means of hydrofluoric gas, is less expensive and greatly to be recommended to analytical chemists. (Vide POGGENDORFF'S Annalen, xliv. p. 134.)

SECOND GROUP.

HYDROCHLORIC ACID-HYDROBROMIC ACID-HYDRIODIC ACIDHYDROCYANIC ACID-HYDROSULPHURIC ACID (SULPHURETTED HYDROGEN.)

$107.

1. HYDROCHLORIC ACID.

I. DETERMINATION.

a. Hydrochloric acid is almost invariably determined as chloride of silver. Solution of nitrate of silver containing a small amount of free nitric acid in admixture, is added in excess to the solution of the chloride under examination; the supernatant fluid is carefully decanted from off the precipitated chloride of silver, which is then washed, dried, and ignited. For the details of this method, we refer to §86. The solution of the chloride should not be heated until the solution of nitrate of silver has been added to it. The process of determining hydrochloric acid by means of silver is more readily executed than that of determining silver by means of hydrochloric acid, since, in the former, the chloride of silver will immediately and completely subside, upon agitating the vessel, as soon as the nitrate of silver begins to be present in excess, and