368 PROPERTIES OF BORACIC ACID. the lapse of some time, hydrated boracic acid will be deposited from the solution in the form of glistening, colorless plates or scales. These crystals contain as much as 43.6 per cent. of water; their formula is HBO,, or, dualistic, 3H,O, BO. On being heated in a clean iron spoon, the crystals will first dissolve in the water which they contain, or, as the fact is usually stated, they will" melt in their water of crystallization;" if the heat be continued, the mass will become pasty, and will swell up as the water is expelled. After all the water has been driven off by strong heat, the anhydrous acid is left as a clear, viscous liquid, from which long threads of the solid acid may be drawn out by touching the surface of the liquid with the end of a stick or glass rod, and then gently pulling away the stick with the matter which has adhered to it. If the fused acid be allowed to cool, it will solidify to a hard, transparent glass, which soon cracks in every direction and splits up into fragments. Anhydrous boracic acid is of about 1.8 specific gravity; it is odorless and destitute of corrosive power; it has a slightly bitter, but not sour, taste. It is much more soluble in hot than in cold water, and more soluble in alcohol than in water. It imparts to the flame of burning alcohol a peculiar green tint, which is quite characteristic, and affords a valuable test by which the presence of the acid may be detected. Upon litmus and turmeric, boracic acid acts somewhat differently from other acids. Exp. 217.-Dissolve a little of the crystallized boracic acid of Exp. 216, in a teaspoonful of alcohol in a small porcelain capsule. Set fire to the alcohol and stir the burning solution with a rod, or agitate it by jarring the dish. Or moisten a tuft of cotton with alcohol, strew upon it some powdered boracic acid, and light the alchohol. In either case the flame of the alcohol will be of a fine green color. Exp. 218.-Pour into a test-glass 20 or 30 c. c. of a solution of blue litmus; in a small quantity of water, contained in another test-glass or tube, dissolve a little of the boracic acid of Exp. 216; add the solution of boracic acid to the litmus, and observe that the color of the latter changes to a brownish wine-red, decidedly different from the bright clear red which is obtained by the action of other acids upon litmus. If a large quantity of boracic acid, however, be added to a small portion of the litmus solution, the latter will be colored strongly, as if by a powerful acid. Exp. 219.-Dip into a solution of boracic acid a slip of yellow turmeric paper, and observe that the yellow color is changed to brown, as it would be by ammonia-water or by any other alkaline solution. None of the other acids produce a like effect. 448. When an aqueous solution of boracic acid is boiled, an appreciable quantity of the acid goes off with the vapor of water; but the dry acid, when heated by itself, is nevertheless one of the least volatile of all the acids. It does slowly sublime, however, at a white heat, and may be completely evaporated, if left for a long time in the hottest part of a porcelain furnace. As a consequence of this fixity, or lack of volatility, it follows that boracic acid is a comparatively powerful acid at temperatures high enough to volatilize the ordinary acids. On being heated with nitrates, or sulphates, for example, it quickly expels nitric or sulphuric acid, and unites with the other ingredients of the salt, though either of these acids would at once decompose the borate thus formed, if they were collected and added to it at the ordinary temperature. Even phosphoric acid is expelled by it from the phosphates. 449. Chloride of Boron (BC1,) is a colorless, mobile liquid, of 1.35 specific gravity, which boils at 17°. The specific gravity of its vapor has been found to be 58.78, a result which points directly to the formula BC1, as representing the true composition of the compound. From the weight of 2 vols. of chloride of boron (58.78×2) and the remainder will be. 117.56 106.50 11.06 a number almost precisely equal to the weight of one atom of boron as previously determined (§ 446). Upon being mixed with water, chloride of boron decomposes, with formation of boracic and chlorhydric acids: 2BC1, +3H,0 = B20, + 6HCI. 2 3 Chloride of boron may be prepared by slightly heating amorphous boron in an atmosphere of chlorine, or more readily by passing a current of chlorine over a mixture of anhydrous boracic acid and charcoal, heated to redness in a porcelain tube. In presence of the hot charcoal which stands ready to take oxygen from the boracic acid, chlorine can take boron away from boracic acid; and, conversely, in presence of the chlorine, ready to combine with the boron, carbon can take away oxygen: BO, 3C 6C1 = 2BC1, + 3C0. 2 3 The method here described, of converting an oxide into a chloride through the intervention of carbon, is a method of very general applicability, and is often employed for the preparation of chlorides of the metals. 450. Fluoride of Boron (BF1,) is a colorless gas of 34.19 specific gravity, as determined by experiment. Upon the assumption that an atom of boron weighs 11, its specific gravity would be (11+3x19)÷2=34. It fumes strongly in damp air, and, by pressure, may be readily condensed to a colorless and very mobile liquid. The gas is exceedingly caustic and corrosive; it carbonizes and destroys wood and other organic substances, in the same way as concentrated sulphuric acid. As with sulphuric acid (Exp. 104), so here, the fluoride of boron unites with the elements of water which are contained in the organic matter, and the integrity of the latter is destroyed. 451. Fluoride of boron is absorbed by water rapidly and in large quantity, 1 volume of water being capable of dissolving 700 or 800 volumes of the gas; but in the act of solution decomposition occurs as well, and there is obtained, not a simple solution of fluoride of boron in water, but a mixture, or rather a compound, of fluorhydric and boracic acids: The reaction is interesting in all its stages, inasmuch as it well illustrates the vagueness and indefiniteness of a considerable class of chemical reactions. When water dissolves fluoride of boron, it increases in bulk to a considerable extent, and in density also, its specific gravity rising as high as 1.77. Upon warming the saturated solution, some fluoride of boron is again disengaged, perhaps as much as one-fifth of all that had been absorbed; but on continuing to heat the solution, it distils over unchanged, and the condensed liquid presents the appearance of oil of vitriol. In it the elements of boracic and fluorhydric acids are undoubtedly held together in a loose condition of chemical combination. By many chemists the compound is called fluoboric acid, though, in FLUOBORIC ACID. 371 order to avoid confusion, it would, perhaps, be better if the name fluorhydrate of boracic acid were allotted to it; for when this compound is largely diluted with water, boracic acid is deposited, and another acid compound is left in solution, the composition of which may be represented by the formula HF1,BF1 ̧. This new acid is called indifferently fluoboric acid or fluorborhydric acid, and the salts formed by its union with metals are called fluoborates. It is remarkable that the first-named compound, the fluorhydrate of boracic acid, upon being neutralized with alkalies, yields, not mixtures of a borate and a fluoride of the alkali employed, but true chemical compounds, double salts, of the general formula M,O,B,O,; 6MF1, whence the name fluoboric acid has arisen. The best way of preparing the fluorhydrate of boracic acid is to dissolve boracic acid by small portions in fluorhydric acid. 3 Fluoride of boron may be itself prepared by heating in a glass flask a mixture of 1 part of fused boracic acid, 2 parts of powdered fluorspar, and 10 or 12 parts of concentrated sulphuric acid:3CaF1, + BO, + 3H2SO1 = 3CaSO, + 3H20 + 2BF1 ̧. On account of its easy solubility in water, the gas must be collected over mercury. Fluoride of boron may be employed as a test to determine whether a given sample of any gas is completely dry; if a few bubbles of it are added to the gas to be tested, the slightest trace of moisture in the gas will be made manifest by the appearance of white fumes of the fluorhydrate of boracic acid above described. 452. Sulphide of Boron (BS) is a white, crystalline solid, decomposable by water, in accordance with the following formula: It may be prepared by passing a current of sulphide-of-carbon vapor over a mixture of boracic acid and charcoal strongly heated in a porcelain tube. 453. Nitride of Boron (BN) is a soft, white, amorphous solid, tasteless, odorless, infusible, and non-volatile. It is, in general, but little acted upon by chemical agents. 454. It will be remarked that while boron is closely analogous to carbon in many respects, it differs from it decidedly in others. Thus, while in their allotropic modifications the two elements are almost precisely alike in appearance and properties, and while many of the salts of boracic acid are strikingly similar to the corresponding carbonates, the compounds of boron are not comparable as regards their composition with the compounds of carbon. While one atom of carbon unites by preference with four atoms of any member of the chlorine group (as in CCI), or with two atoms of any member of the sulphur group (as in CO2), an atom of boron unites with only three atoms of chlorine (BC1), or two atoms of it unite with three atoms of oxygen or sulphur (as in BO,). The exceptional character of the composition of Doron compounds will appear still more clearly in the next chapter, where it will be shown that silicon, the third member of the carbon group, resembles carbon as regards the atomic composition of its compounds, as well as in other respects. The atomic weight of boron above given (§ 446) cannot be accepted as established beyond a doubt, and it may happen that further investigation will show that the boron compounds are really formed upon the same type or pattern as those of carbon and silicon; but in face of the experimental evidence now at hand, this view cannot be maintained. In the meantime the student will better understand the physical and chemical properties of boron and its compounds, if this element is studied in company with carbon and silicon, which it so closely resembles, than if it were described in connexion with arsenic, antimony, and the other elements of the nitrogen group, which form teroxides and terchlorides indeed, but which present not the least other analogy to boron, either in the simple or the compounded condition. CHAPTER XXII. SILICON. 455. Like carbon, silicon may be obtained in three distinct allotropic conditions, which have been designated as amorphous, diamond-like, and graphitoidal. After oxygen, it is the most abundant and widely diffused of all the chemical elements; at |