of its elements, or by adding sulphuretted hydrogen, or ammonium sulphide, to a neutral solution of a thallous salt. A compound TI,S. As,S,, analogous to K,S. As,S,, is known.

Salts of the form M,3X (X = SO,, 2NO,, CO,, PO,, &c.) are obtained by dissolving the different hydrated oxides, MO̟....îHO, in acids and evaporating. The salts are generally soluble in water; the thallic salts are easily decomposed, sometimes even by solution in water, and are readily reduced to thallous salts, TI,X. The sulphates, with the exception of thallic sulphate, combine with sulphates of the alkali metals to form alums, M.. 3SO,. X,SO,.24H,O (X = alkali metal except Li.) Thallous sulphate (Tl,SO,) forms an alum with aluminium sulphate, Al,. 3SO. TI,SO. 24H,O.

Several basic salts of aluminium, and a few of the other elements, are known.

The thallous salts, Tl,X, are generally stable bodies; they are similar to, and usually isomorphous with, salts of potassium.

Of the four odd-series elements of Group III., three, viz. aluminium, gallium, and indium, are evidently very closely related; the fourth, thallium, is to a great extent separated from the others.

Thallium shews marked similarities with the alkali metals which belong to Group I.; at the same time the properties of some of its compounds-e.g. the acidic character of Tl,S,, and the probable existence of an unstable potassium thallate— suggest relations with the most negative element of Group III., viz. boron.

Boron, which is the first member of the group, to some extent summarises the properties of the other members. Oxide of boron is acidic; aluminium oxide is acidic towards strong alkalis; the solubility of Ga,O. H ̧O and In,O,.xH2O in concentrated potash solution shews that these oxides have feebly marked acidic functions; an acidic oxide of thallium probably exists. Oxide of boron also shews basic functions towards some acidic oxides, e.g. SO, and PО; all the other oxides of the group are basic towards most acids. The positive character of boron is shewn in its interaction with steam at high temperatures.

Neither the group-character nor the family-character distinctly preponderates in Group III. The even-series elements from scandium to ytterbium form a closely related class; the odd-series elements from aluminium to indium are

also very similar; boron at one end of the group, and thallium at the other, are separated from the other members of the group; although differing widely in most of their properties, boron and thallium approach each other in some respects.

The melting points of these elements have not been determined; cerium is said to fuse considerably above 500° but under 900°, and zirconium at a higher temperature than cerium. Carbon, titanium, and thorium, have not been melted.

Even-series elements

General chemical properties.

CARBON.

Heated in air or
oxygen, burns
to CO and CO2;
diamond burns
only at very
high tempera-
tures in oxygen.
Combines di-
rectly with H
when electric
sparks are
passed from
carbon poles in
atmosphere
of H.
Combines di-
rectly with S at
high tempera-

tures.
Combines with
many metals to
form bodies re-
sembling alloys.
Graphite is oxi-
dised by heating
with KCIO3 and
HNO3Aq to
graphitic acid
CH4O5; no

other form of

carbon gives
this acid.

Exhibits allo

tropy very
markedly.

Atom is tetra-
valent in gas-

eous molecules.

General formulae and characters of compounds. Carbon is 468 characterised by the enormous number of compounds which it forms by combining with some or all of the elements hydrogen, oxygen, and nitrogen.

The chlorides MCI,, except CeCl, and the fluorides MF, have been gasified and their vapour-densities determined; the oxides CO and CO2, and the sulphide CS,, have also been gasified; the formulae of these compounds are therefore molecular; the formulae of the other compounds are the simplest that express their compositions.

None of the elements of the family except carbon forms any compound with hydrogen: the hydrides of carbon are exceedingly numerous.

Oxides: CO, (? TiO); (? C,O,), Ti,O,, Ce,O,; MO,, M = any element of the family.

Sulphides: CS, CS,, ( CS), TiS,, Ce,S,, ThS..

Haloid compounds: MX,, M = any element of the family; C,Cl, &c., Ti¿Cl¿, Ce,Cl; TiCl.

469

Acids: H,CO,Aq; salts of the form M,TiO, and M.ZrO, are known (M = an alkali metal); the hydrates TiO,.xH2O and ZrO2. xH2O are acidic.

Salts carbon does not form salts by replacing the hydrogen of acids; M.2X (X=SO1, 2NO,, &c.) M = Ti, Zr, Ce, Th; Ce,3X, and Ti̟,3X.

Oxides. The dioxides MO, are produced by heating the elements in oxygen; the dioxides of titanium, zirconium, cerium, and thorium, are also obtained by precipitating solutions of salts of these elements by ammonia, and drying and heating the hydrated oxides so obtained. Carbon dioxide is most easily obtained by decomposing a metallic carbonate by an acid, e.g. CaCO,+ 2HClÃq = CO2+ H2O+ CaCl,Aq. The dioxides, with the exception of CO,, are solids, insoluble in water, some of them insoluble also in most acids; carbon dioxide is a colourless, odourless, gas which can be condensed to a liquid, and, at a very low temperature, to a snow-like solid.

Carbon dioxide dissolves freely in water; the solution reddens blue litmus and interacts with alkalis to form salts M,CO, from these alkali carbonates, carbonates of most metals can be obtained. An aqueous solution of carbon dioxide probably contains carbonic acid, H,CO,; but this compound has not been isolated; the sulphur compound thiocarbonic acid, H,CS,, is known as a solid.

2°

By precipitating solutions of salts (M2X) of titanium, zirconium, cerium, and thorium, hydrated dioxides MО ̧.жHO are obtained. Many of these hydrated oxides seem to exist; the following, obtained by drying under different conditions, are among the more important, TiO,. H,O, TiO,.2H2O; ZrO,. H2O, ZrO,. 2H,O; ThO,.2H,O; 2CeO,.3H,O. These hydrated dioxides are soluble in acids, and from these solutions salts of the form M. 2X (X=SO,, 2NO,, &c.) are obtained. The solution of CeO,.xH2O in acids seems to contain cerous salts Ce,. 3X, as well as ceric salts Ce2X.

Most of the dioxides MO,.xH2O exhibit acidic properties. None of them dissolves in solutions of alkalis to form salts. When however titanium dioxide, TiO,, is fused with sodium carbonate, a quantity of carbon dioxide is evolved corresponding with that calculated on the assumption that sodium titanate Na,TiO, is produced; the fused mass is separated by water into NaOHAq and NaHTIO,. Zirconium dioxide, ZrO2, fused with Na,CO, behaves similarly to TiO,; the zirconates Na,ZrO, and Na,ZrO are said to have been