almost all, excepting those of the first subdivision, are insoluble

in water.

THE POTASSIUM SALT (K, B0,0,+5aq) is soluble.

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THE SODIUM SALT (Na, Bo,O,+10aq), or borax, is soluble. THE BARIUM SALT is produced by the action of soluble barium salts on a solution of borax. It is a white precipitate. Its formula is Ba, Bo ̧0,+2aq. It is soluble in excess of chloride of barium, also in ammonium salts; it dissolves in 100 parts of cold water, and in a smaller quantity of boiling water; it is soluble in acids. Before the blowpipe it fuses to a grey glass.

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THE STRONTIUM SALT is produced by the action of a soluble strontium salt on a solution of borax: it is a white precipitate. It dissolves readily in cold solutions of chloride or nitrate of ammonium; 1 part is soluble in 130 parts of boiling water.

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The Calcium Salt is produced by the action of a soluble calcium salt on a solution of borax. Its formula is Ca, Bo, O,+aq after having been air-dried. It is scarcely soluble in water, but dissolves in acids. Before the blowpipe it fuses to a glass.

THE FERROUS, FERRIC, and MANY OTHER SALTS of the third and fourth subdivisions are yellowish or white precipitates, insoluble in water.

THE CUPRIC SALT is a pale green precipitate, slightly soluble in water, and dissolved easily by a solution of boracic acid. Before the blowpipe it fuses to a green opaque glass.

THE SILVER SALT is white: its formula, under all circumstances, is AgBoO,. It is sparingly soluble in water.

THE MERCUROUS and MERCURIC SALTS do not appear to exist; the biborates precipitate basic mercury compounds.

The Lead Salt is obtained by the action of soluble lead salts on a solution of borax: it is a white precipitate. Its formula is Pb, Bo107. It is insoluble in excess of its precipitant, but slightly soluble in pure water.

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This acid-radical is also detected by the following experiments. a. When sulphuric acid is added to the aqueous solution of a borate, boracic acid separates in characteristic crystalline scales (HBOO2+aq). If the boracic acid be then dissolved in alcohol,

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and the solution kindled, the flame produced will have a dark green edge, best seen against a black background. In this experiment, the presence of chlorides and of copper salts should be avoided.

B. The most characteristic reaction of boracic acid is the action of its aqueous solution on turmeric paper. All other acids exert no influence upon ordinary yellow turmeric paper, or, if it has been browned by an alkali, they simply restore its original colour; but boracic acid behaves like an alkali, turning the yellow of the paper to a reddish brown. The suspected borate is mixed with hydrochloric acid, and into the mixture a small piece of turmeric paper is placed. The test is applied in a white porcelain dish.

y. Sulphuretted hydrogen and ferrocyanide of potassium produce no characteristic actions upon boracic acid or soluble borates.

SALTS OF THE SILICIC RADICAL, OR SILICATES.

These salts are of universal distribution in nature: they form a very large proportion of the rocks constituting the earth's crust, and enter into the composition of many minerals.

Heated before the blowpipe, most silicates fuse to a colourless or coloured glass.

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THE HYDROGEN SALT (HSiO2), or silicic acid, is not very well known; it is believed that the body termed gelatinous silica has, when air-dried, this formula, but when dried at 100° the formula H, Si, O,, i. e. 2HSiO,+Si,O,: at a higher temperature it is resolved into water and silicic anhydride (Si,O,). This body (Si,O,), which bears the same relation to silicic acid (HSiO2) as boracic anhydride (Bo,O,) bears to boracic acid (HBOO,), is of very common occurrence, and is familiar to every one under the forms of quartz, amethyst, siliceous sand, flint, and chalcedony. Silicates are as various in their constitution as they are great in number; but, as we have seen in the tungstates, chromates, and especially in the borates, a number of acid-radicals may be built up by the assimilation of successive additions of the body which has been termed the anhydride,

into which and water all these complex acids are capable of splitting.

Silicates are best recognized, not by the formation of insoluble salts, but by certain characteristic products of decomposition which they yield.

Silicates are very generally insoluble in water, the neutral and basic silicates of the first subdivision being almost the only exceptions.

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THE POTASSIUM SALTS KSIO,, K, Si,O,, and K, Si, O,, are soluble in water; the salts K, Si, 025, K2 Si, О55+9aq, and K, Si4 07, +16aq are insoluble.

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THE SODIUM SALTS NaSiO2+3aq, Na, Si, O10, and Na, Si, O13 +12aq are soluble in water; those containing a larger addition of silicic anhydride are insoluble.

THE BARIUM, CALCIUM, and MAGNESIUM SALTS are white. They are insoluble in water, but soluble in acids.

THE FERROUS and OTHER SALTS of the third and fourth subdivisions are insoluble in water.

THE CUPRIC SALT is green; THE SILVER and MERCUROUS SALTS white.

THE MERCURIC SALT is tolerably soluble: it occurs in small dark crystals.

THE LEAD SALT is precipitated from silicofluoride of lead by hydrate of ammonium: it is a white precipitate, insoluble in water.

This acid-radical is invariably recognized by the decomposition and behaviour of its salts.

a. Many silicates, insoluble in water, are readily dissolved by acids; others, again, are insoluble both in water and in all acids, the hydrofluoric excepted. The latter class of silicates may be converted into compounds quite soluble in acids and even in water, by the following process:-The insoluble silicate is mixed with four times its weight of hydrate or carbonate of potassium, or of the mixed carbonates of sodium and potassium, and the mixture fused for 15 minutes; the mass should remain quite liquid for 10 minutes; this fusion of course yields the alkaline silicate

by ordinary double decomposition. By the preceding process, a mass will be obtained which, in most cases, will dissolve slowly in water, or at least the alkali and alkaline silicate will dissolve, leaving a residue of metallic carbonate or oxide. In acids the fused mass will dissolve readily; and then, if to the solution we proceed to add excess of acid (e. g. hydrochloric acid), a precipitate of silicic acid will be formed, which, on the addition of a further portion of acid, again dissolves. If now the clear solution be evaporated to perfect dryness on a water-bath (at 100°), then moistened with water, and again evaporated to ensure the expulsion of all free acid and moisture, the decomposition of the silicic acid into water and silicic anhydride will have taken place, and the resulting white powder will be found perfectly insoluble in water and in all acids except hydrofluoric. So perfect is this decomposition, that the most minute trace of silicic acid may be thus detected, and it is employed in the quantitative estimation of this substance.

B. An experiment already given on page 66, and also described as one of the tests for the presence of fluorine, serves for the detection of silicon. A small tube of lead closed at one end, perfectly dry, and fitted with a dry cork and leaden deliverytube, is employed; and into this vessel a mixture of equal weights of the substance to be tested for silicon and of fluoride of calcium is introduced, together with about twice the bulk of oil of vitriol. The delivery-tube should then be dipped under the surface of a small quantity of water contained in a cup of lead, and heat applied to the mixture: if silicon be present, the gaseous terfluoride of silicon (SiF) will be disengaged, which, coming in contact with water, will yield a gelatinous precipitate of silicic acid, and the solution will contain hydrofluosilicic acid, recognizable by the usual tests.

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y. When gelatinous silicic acid is dried, it yields, as has been stated, the anhydride (Si ̧ O ̧), a fine white scaly powder of extreme lightness. If now a bead of carbonate of sodium be made on a platinum wire, and, when it is red-hot, silicic anhydride be added little by little, fusing the bead between each addition of

substance, a period will arrive at which the bead no longer presents the opacity of fused carbonate of sodium when cold, but remains perfectly pellucid.

d. If a bead of microcosmic salt be made, and some silica (Si,O,) fused with it, the anhydride will remain undissolved, and float about the bead as a network or skeleton.

e. Hydrosulphuric acid and ferrocyanide of potassium give no characteristic reactions with silicic acid.

This radical is usually detected by the tests a, 6, and y.

Of compound acid-radicals containing tantalum and oxygen, two are known: they occur in a mineral termed "Tantalite," one variety of which consists of ferrous tantalite, Fe,Ta,O,; other varieties are composed of ferrous tantalate, FeTaO2.

SALTS OF THE TANTALIC RADICAL, OR TANTALATES.

Tantalic acid, and many tantalates, when heated, yield tantalic anhydride (Ta203).

THE HYDROGEN SALT (HTаO2), or tantalic acid, is known: it is obtained as a snow-white bulky precipitate, quite insoluble in water, and, when washed, but slightly soluble in many acids, except sulphuric acid, from its solution in which, water reprecipitates it, and hydrochloric acid, which dissolves it somewhat more abundantly. It dissolves readily in the hydrate or the acid oxalate of potassium, in boiling solutions of alkaline carbonates, and in hydrofluoric acid. The formula of the precipitate is HTaO2+aq.

The tantalic radical is detected by the formation of insoluble salts, but especially by processes of decomposition.

The alkaline tantalates are soluble in water, but are precipitated by ordinary salts of ammonium. The sodium salt is also precipitated by the addition of excess of hydrate or carbonate of sodium,—a behaviour whereby tantalic acid is distinguished from all other acids (except niobic and pelopic), particularly from tungstic acid, to which, in many points, it bears a great resemblance.

THE POTASSIUM SALT is soluble in pure water, nearly insoluble in cold carbonate of potassium solution.

THE SODIUM SALT is sparingly soluble in cold water; when the solution is boiled, it deposits a bitantalate.

THE BARIUM and CALCIUM SALTS are white and insoluble. THE CALCIUM, MAGNESIUM, and COPPER SALTS are unknown.

THE SILVER SALT is white and insoluble.

THE MERCURY and LEAD SALTS are unknown.

This acid-radical is detected by the following tests :

a. From soluble tantalates, hydrochloric acid precipitates tantalic acid