and allowed to crystallise, the hydrated oxide T10. 2H,O is obtained. When this hydrate is heated to 100o in absence of air TlO is formed. Thallous oxide is very soluble in water and the solution closely resembles KOHAq or NaOHAq in its properties; it has a corrosive action on the skin, a hot and burning taste, turns red litmus blue, absorbs and combines with carbon dioxide, and neutralises acids forming thallous salts, TX. There can be little doubt that the solution contains the hydroxide TIOH, and that the composition of hydrated thallous oxide is better expressed by the formula 2TIOH. HO than by the formula TO. 2H,O.

2 3

The haloid compounds, MC, are obtained; (1) by 463 heating the elements in chlorine, M = Ga and In; (2) by heating an intimate mixture of the oxides M2O, and carbon in chlorine, M = Al and In; or (3) by adding HClAq to the hydrated oxides M,O,, M=Tl. The other haloid compounds are obtained by similar methods.

The vapour density of aluminium chloride agrees with the formula AICI,; that of gallic chloride shews that at moderate temperatures the gaseous compound probably consists of molecules of Ga,Cl, but at higher temperatures it consists of molecules of GaCl,; gaseous indium chloride has the molecular composition InCl,; thallic chloride has not been gasified, at 100° it separates into thallous chloride, TICl, and chlorine. The chlorides, MC,, are all deliquescent solids; they are all, except probably TICI,, partially decomposed by hot water with formation of various oxychlorides of more or less complex compositions. These chlorides all combine with many other chlorides to form double compounds; e.g. AICI,. PCI,; AICI. POCI; AICI. KCl; InCl ̧. 3KCI; TICI. 3KCl.

Thallous chloride, TICI, is formed as a white precipitate when HCIAq is added to a solution of thallous oxide. This chloride is only slightly soluble in water; it has been gasified without decomposition; it combines with various chlorides to form double compounds; the compound 2TICI. PtCl resembles, and is isomorphous with, 2KCl. PtCl ̧.

The sulphides M,S, are obtained by the direct union 464 of their elements at high temperatures. A compound K,S. Tl,S, (possibly a thio-thallate of potassium) is produced by heating thallous sulphide (TIS) with sulphur and potassium carbonate. A similar compound of potassium and aluminium sulphides seems to exist.

Thallous sulphide, TS, is obtained by the direct union

465

466

of its elements, or by adding sulphuretted hydrogen, or ammonium sulphide, to a neutral solution of a thallous salt. A compound TIS. 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, MO,.xH ̧O, 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, TX. The sulphates, with the exception of thallic sulphate, combine with sulphates of the alkali metals to form alums, M. 3SO. X2SO. 24H,O (X = alkali metal except Li.) Thallous sulphate (TI,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 TS,, and the probable existence of an unstable potassium thallatesuggest 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,. xH2O and In,O,.H2O 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 PO; 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

group, and other members most of their other in some

also very similar; boron at one end of the
thallium at the other, are separated from the
of the group; although differing widely in
properties, boron and thallium approach each
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 KCIOs and
HNOgAq 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 MC1, 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.

3

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

Sulphides: CS, CS2, ( CS), Tis,, Ce.S., ThS.

Haloid compounds: MX,, Many element of the family; C,Cl, &c., Ti̟¿Cl ̧, Ce̟Cl; TiCl,.