450

451

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of sulphuric acid with calcium fluoride CaF; thus
CaF2+ H2SO2Aq = CaSO ̧ + 2HFAq (compare preparation of
HCl, HBr, and HI, Chap. XI. par. 153). Hydrogen fluoride
is a colourless strongly smelling and irritating gas at temps.
above 20o, and a light mobile liquid at temps. under 20°.
The vapour-density points to the existence of the gaseous
molecule HF only at fairly high temperatures, and to the
existence of the gaseous molecule HF, at temps. not very
far above 20o. Liquid hydrogen fluoride chars organic matter
rapidly, and dissolves many bodies which are insoluble in all
other acids, e.g. strongly heated silica, titanium oxide, boron,
silicon, &c. An aqueous solution of this compound interacts
with metals and basic oxides similarly to aqueous solutions of
hydrochloric, hydrobromic, and hydriodic, acids; fluorides,
Ba Bi
salts of the form MF (M = K, Na,,, &c.), are produced.
The metallic fluorides shew great readiness to combine with
hydrogen fluoride and produce double compounds; e.g. KF. HF,
BiF. 3HF, &c. Some of the fluorides of non-metallic elements
also combine with hydrogen fluoride; the products in some
cases react as acids; thus SiF,.2HF is a dibasic acid
(H,SiF, Auosilicic acid), and BF. HF is a monobasic acid
(HBF, fuoboric acid). A few similar compounds of hydrogen
chloride and bromide are known, e.g. HAuCl, and HAuBr,
both of which react as monobasic acids (s. par. 441.)

Hydrofluoric acid, HFAq, is an extremely weak acid; its affinity for bases is less than 5 when that of hydrochloric acid is taken as 100 (s. Chap. XIII. pars. 251, 255).

Whether fluorine does or does not interact with water and solutions of alkalis similarly to chlorine, bromine, and iodine, cannot be determined until the properties of fluorine have been more fully investigated.

The chemical properties of fluorine, so far as they have been investigated, shew that this element is very similar to the elements chlorine, bromine, and iodine; but, at the same time, there are fairly marked differences between fluorine and these three elements. No one of the four elements shews any tendencies to react as a metal.

MANGANESE is the second member of the even-series of Group VII. The sketch of the chemical properties of manganese given in pars. 195-199 of Chap. XI. shews that manganese is at once metallic and non-metallic in its chemical functions. The oxides MnO, Mn,O,, and Mn,O, are basic; MnO,

is feebly acidic. A series of manganous salts MnX (X=SO,, 2NO,, PO,, &c.) exists; a few manganic salts Mn,3X are also known. Permanganic acid, H.Mn,O,, has been isolated and a number of permanganates have been obtained as definite stable salts, generally isomorphous with perchlorates MCI,O, (M = K,, Ba, &c.). Many manganates, MMnO,, are also known; these salts do not correspond in composition with any salts derived from acids of chlorine, bromine, iodine, or fluorine; they are similar to the sulphates, selenates, and tellurates MXO, (M = K,, Ba, &c. ; X = S, Se, Te).

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Manganese, then, shews very feeble analogies with the other elements which are placed in the same group with it.

If the three generalisations stated in par. 446 are applied 452 to Group VII., they would lead us to expect that the unknown members of the even series of this group should resemble manganese, but should on the whole be more distinctly metallic than this element; and that the unknown members of the odd series of the group should resemble the halogen elements, but should be less decisively non-metallic than these elements; the unknown members of series 9 and 11, Group VII., might fairly be expected to form a few salts by the interactions of their oxides with acids.

*There are still some doubts whether these elements are or are not mixtures of two or more distinct kinds of matter.

Boron is said to exist both as an amorphous powder and as metal-like, hard, lustrous, crystals; but recent investigation shews that the so-called crystalline boron is a compound of boron and aluminium,

General formulae and chemical characters of compounds. 454 The compounds BC, BBr, and BF,, have been gasified; these formulae are molecular, No compounds of the other even-series elements have yet been gasified; the formulae given are the simplest that express the compositions of these compounds. Gaseous aluminium and indium chlorides have the molecular composition MCI,, and gallium chloride appears to exist in the gaseous state both as GaCl, and GaCl. There are indications of the existence of a hydride of boron, probably BH; but the compound has not been isolated. The com

pounds of the elements of this family, with the exception of those of boron, have not been very fully investigated: there are many points in the chemical history of boron which require elucidation.

Oxides: M,O,.

Haloid compounds: MX.
Sulphides: M,S,.

Acids: HBO HBO, HBO,; no acids of other elements

are known.

Salts: M,3X; X=SO1, 2NO,, 3PO1, &c.; M = Sc, Y, Yb, not B.

La,

The oxides M,O,, with the exception of BO, are obtained 455 by adding an alkali to the solution of a salt, and heating the hydrated oxide which is pptd. BO, is found in waters in various volcanic districts; it is formed by heating boron in oxygen.

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Boric oxide, B,O,, is acidic; it dissolves in water, and from this solution crystals of ortho-boric acid, H,BO,, separate on evaporation. By heating ortho-boric acid to 100° metaboric acid, H,B,O,, is obtained; and at 160° tetra- or pyro-boric acid, HBO,, is formed. Salts are known derived from each of these acids; borax, one of the most important salts, is sodium tetra-borate Na,B,O,; the ortho-borates are very unstable salts, it is doubtful whether they exist in solution in water. A solution of boric oxide in water reacts as if it contained dibasic metaboric acid, HBO. Boric acid is an extremely weak acid. Dilute aqueous solutions of alkali borates precipitate metallic oxides, not borates, from solutions of many metallic salts; that is to say, these dilute solutions behave as if they contained boric acid and free alkali, pro

456

Borates are

Boric oxide

duced by the action of the water on the borate.
ill-defined salts; many basic salts seem to exist.
combines with a few anhydrides of strong acids to form
compounds in which the boric oxide acts as a base; thus
BO..PO, (BPO) is a stable compound; BO ̧ïSO, also
exists but is decomposed by heat, or by water.

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2 3

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The other oxides M2O (M=Sc, Y, La, Yb) are basic; they dissolve in acids and form salts; they are insoluble in solutions of alkalis. Hydrates of Y,O,(Y,O,.6H,O) and La O. (La 0.3H,O) have been obtained, but not by the direct reaction of water with the oxides. Lanthanic hydrate is said to turn red litmus blue.

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2 3

The sulphides M,S,, where M = B or La, are formed by passing carbon disulphide vapour over heated BO̟ ̧ or LaO̟ ̧; they are easily decomposed by cold water to M.O, and H.S. 457 Haloid compounds of boron, yttrium, and lanthanum, have been prepared. Boron chloride and bromide, BCl, and BBr,, are obtained by heating boron in chlorine and bromine, respectively, or by passing chlorine, or bromine, over a strongly heated mixture of boric oxide and carbon.

458

Boron fluoride, BF,, is formed by heating boric oxide with calcium fluoride to a full white heat.

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Boron chloride and bromide are liquids, boiling without decomposition at 17° and 90° respectively; boron fluoride is a gas. These haloid compounds are all decomposed by water; BCI, and BBr, to solutions of boric oxide and hydrochloric, or hydrobromic, acid; BF, is partly decomposed to boric oxide and hydrofluoric acid, but the latter combines with some unchanged BF, to form fluoboric acid HBF, (= HF.BF). Boron chloride, BCI,, combines with many other compounds to form stable double compounds; e.g. 2BC1,. 3NH; BCI. POCI,; BCI. NOCI; BC11. HCN. Yttrium and lanthanum chlorides are obtained as YC,.6H,O, and 2LaCl,.15H,O, respectively, by dissolving the oxides in HCIAq and evaporating. The anhydrous chlorides have also been obtained. These chlorides form several double compounds chiefly with other chlorides; e.g. YC,.3HgCl,.9H2O; 2LaCl,.3PtCl ̧.24H ̧0; 2 LaCl. 3AuCI. 21HO. The haloid compounds YBr,, YI, 2YF. HO, LaBг.7H ̧O, and 2LaF. H2O, have been isolated. Two oxychlorides of boron, BOCl and BOCI,, are known. The salts of the elements we are considering, so far as they have been examined, belong to the form M..3X where X = SO1, 2NO,, PO,, &c. No definite salts have been obtained by replacing the hydrogen of acids by boron, although

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