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 KCIO, and
HNOgAq to
graphitic acid
C1H4O5; 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, CS2, (? C2S ̧), TiS,, Ce,S,, ThS.

Haloid compounds: MX,, Many element of the family; C,Cl, &c., Ti,Cl, CeCl; 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. H2O are acidic.

Salts carbon does not form salts by replacing the hydrogen of acids; M.2X (X=SO1, 2NO2, &c.) M = Ti, Zr, Ce, Th; Ce,3X, and Ti,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 CO2, 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.

2

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.

By precipitating solutions of salts (M2X) of titanium, zirconium, cerium, and thorium, hydrated dioxides MO,.xH2O are obtained. Many of these hydrated oxides seem to exist; the following, obtained by drying under different conditions, are among the more important, TiO. H2O, TiO,.2H,O; ZrO2. H2O, ZrO2. 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 CeO2.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

337 obtained. Thorium dioxide, ThO2, does not decompose Na,CO2 when heated with this salt, nor does it form salts by fusion with solid potash or soda. The fact that when hydrated cerium dioxide, CeO2.xH2O, is precipitated by adding potash to a solution of ceric sulphate, Ce(SO4)2, the whole of the potash cannot be removed from the precipitate by washing with water indicates the possible formation of an unstable potassium cerate.

Carbon monoxide, CO, is obtained by heating the dioxide. with carbon (CO2 + C = 2CO), or by the interaction of (1) formic acid, or (2) oxalic acid, with concentrated hot sulphuric acid; (1) H,CO,+H,SO, = CO + H,O.HSO4,

(2) H,C,O,+ H,SO, = CO + CO,+H,O. H,SO,.

This oxide is a poisonous gas; salts of formic acid H2CO2 can be obtained from it, and as the oxide is produced from the acid, the oxide is sometimes called formic anhydride.

The sesquioxides Ti0, and CeO are obtained by reducing the higher oxides MO2; in the case of titanium this is done by heating TiO, to redness in dry hydrogen, in the case of cerium it is better to heat cerous oxalate, Ce(C2O4)3, in hydrogen. Ti,O, is oxidised to TiO, by continued heating in air; Ce,O, is very easily oxidised by mere exposure to air. Both oxides dissolve in acids forming salts; a large series of cerous salts Ce2. 3X is known; only titanous sulphate, Ti2(SO) ̧.8H2O, seems to have been obtained as a definite

solid.

The sulphides, with the exception of CS2, have not 470 been much examined. Titanium disulphide, TiS, is a dark coloured stable solid, obtained by the interaction of titanium tetrachloride and sulphuretted hydrogen at high temperatures. Thorium disulphide, ThS,, resembles TiS,; it is obtained by passing hydrogen mixed with hydrogen sulphide over heated ThO2. Cerium sesquisulphide, Ce,S,, is a golden coloured solid formed by passing sulphuretted hydrogen over hot CeO,. None of these sulphides exhibits any tendency to form thiosalts by interacting with alkali sulphides, or by fusion with alkalis.

Carbon disulphide is a mobile, very refractive, liquid, boiling at 47°; its vapour is very inflammable. It is obtained by the direct union of its elements at high temperatures. This sulphide is the anhydride of thio-carbonic acid HCS,; when CS, interacts with concentrated Na,SAq sodium thio-carbonate Na,CS, is formed; the acid H,CS, is obtained as a dark

M. E. C.

22