The solution, on concentration, deposits yellow needle-shaped crystals of TIHO,H,O. Thallous hydroxide is soluble in water, yielding an alkaline solution which gives a brown stain upon turmeric paper. The stain soon disappears, owing to the destruction of the colouring-matter, and is thereby distinguished from the similar stains produced by sodium and potassium hydroxides.

Thallic Oxide, TlO3, is obtained when thallium burns in the air, or when thallium oxyhydroxide, TIO(HO), is heated to 100°. It forms a dark reddish powder, insoluble in water. In warm dilute sulphuric acid it dissolves, forming thallic sulphate

Tl2O3 + 3H2SO4=Tl2(SO4)3+3H2O,

but with hot concentrated acid oxygen is evolved, and thallous sulphate formed—

Tl2O3 + H2SO4=TI2SO1+O2+H2O.

At a red heat thallic oxide is converted into thallous oxide with loss of oxygen.

Thallium Oxyhydroxide, TIO(HO), is formed by the action of potassium hydroxide upon thallium trichloride

TICI,+3KHO=3KCl + H2O+TIO(HO).

Thallous Chloride, TICI, is obtained as a white curdy precipitate when hydrochloric acid is added to a solution of a thallous salt. It is considerably more soluble in hot than in cold water: 100 parts of water at 16° dissolve 0.265 part; and at 100°, 1.427 part of thallous chloride.

Thallic Chloride, TIC, is formed by passing chlorine through water in which thallous chloride is suspended. The solution so obtained, on evaporation in vacuo, deposits colourless transparent crystals of TICI,,2H4O.

When either thallium or thallous chloride is gently heated in a stream of chlorine, a compound is obtained, having the composition TICI,,TICI, or TlCl. If this be further heated, it loses chlorine, and is converted into a yellow crystalline compound of the composition TIC,3TICI, or T1,Cl, thus

2T1C1=Cl2+TI1Cle

Thallous Oxysalts.-The sulphate TI,SO4, and nitrate TINO are best obtained by dissolving the metal in the respective acids. Both salts are soluble in water.

Thallous Carbonate, Tl,CO, is prepared by saturating a solution of thallous hydroxide with carbon dioxide. The salt forms long white prismatic (monosymmetric) crystals, which are moderately soluble in water, giving an alkaline solution.

Thallous Phosphate, TI,PO4, is obtained by precipitation from a thallous solution, by the corresponding potassium phosphate. The monohydrogen phosphate, HTI,PO4, on being heated to 200°, is converted into pyrophosphate

and the dihydrogen salt, on being ignited, yields the metaphosphate

Thallic Oxysalts.-The chief of these are thallic sulphate, Tl(SO); and thallic nitrate, TI(NO3)3. They are obtained by the action of sulphuric acid and nitric acid respectively upon thallic oxide Tl2O3. Thallic sulphate forms colourless crystals of the composition Tl (SO4)3,7H,O. It is decomposed by excess of water, with precipitation of the hydrated oxide; and when heated yields thallous sulphate, sulphur trioxide, and oxygen—

Tl2(SO4)3=TI,SO4+2SO3+02.

Thallic nitrate is deposited in colourless crystals of TI(NO3)3,8H2O, which are decomposed in the presence of much water.

Family A consists of four rare elements.* Titanium, as the oxide TiO2, occurs in the three rare minerals-rutile, brookite, and anastase. The metal is extremely difficult to isolate in a pure state, owing to the fact that it unites directly with nitrogen, forming a nitride.

Zirconium is met with as the silicate ZrSiO4 (or ZrO„SiO) in the mineral zircon. Like silicon, it has been obtained in two forms, crystalline and amorphous. The latter variety, when gently heated, burns in the air, while the crystalline variety requires the high temperature of the oxyhydrogen flame for its ignition.

Cerium occurs associated with lanthanum in the rare minerals cerite and orthite, and with yttrium and ytterbium in gadolinite and wöhlerite.

Thorium is found in the extremely rare minerals, thorite and orangeite, met with in Norway.

Family B. In this family the rare element germanium forms a link between carbon and silicon on the one hand, and tin and lead on the other.

Carbon (the typical element) is essentially non-metallic, and forms an acidic oxide. Silicon approaches more nearly to the metals in its physical properties, but its oxide is still acidic, and no compounds are known in which silicon functions as a basic element. Germanium is both metallic and non-metallic; its oxide

For descriptions of these rare elements the student is referred to larger treatises.

unites with acids; and it also combines with alkaline hydroxides, forming germanates corresponding to silicates. Tin is a still more basic element, forming well-marked salts with acids; but it is also acidic, and with alkalies forms stannates.

Carbon and silicon exhibit a close relationship. They both form allotropes, which correspond in many respects. They both unite with hydrogen, forming the analogous compounds CH, and SiH4; and with hydrogen and chlorine they form the similarly constituted compounds, chloroform, CHCl3; and silicon chloroform, SiHCl3.

Tin and lead approach more nearly to each other, especially in their physical properties, than to the other members of the family. They both form compounds, in which the metals function both as divalent and tetravalent elements; although in the case of lead (as often happens with the heaviest metals of a family), the element exhibits much greater readiness to act in the lower state of atomicity. Until quite recently (1893) no compound was known in which an atom of lead is united with four monovalent atoms, although lead ethide, Pb(CHÁ), had been obtained. Now, however, the compound PbCl has been produced, corresponding to SnCl4, which it resembles in many respects; and still more recently (1894) the tetrafluoride has been obtained.

Carbon, as usual with the typical elements, stands apart from the other members of the family in many of its attributes. Thus, its oxides are both gaseous; it also forms a vast number of compounds with hydrogen, oxygen, and nitrogen, the study of which constitutes the science of organic chemistry. This element has already been treated in Part II. (page 285).

SILICON.

Symbol, Si. Atomic weight=28.4.

Occurrence.-Silicon is not known to occur in the uncombined state, although in combination it is the most abundant and widely distributed of all the elements, with the exception of oxygen. In combination with oxygen, as silicon dioxide or silica, SiO, it occurs as flint, sand, quartz, rock crystal, and chalcedony; while in combination with oxygen and such metals as calcium, magnesium, and aluminium, it occurs in clay and soil, and constitutes a large number of the rocks which make up the earth's crust. Silicon, in

combination with oxygen, is also met with in the vegetable kingdom, being absorbed by plants from the soil.

Modes of Formation. (1.) Silicon may be obtained by strongly heating a mixture of potassium silico-fluoride and potassium

The mass, after cooling, is treated with water, which dissolves the potassium fluoride, leaving the liberated silicon.

(2.) This element may also be prepared by heating sodium in a stream of the vapour of silicon tetrachloride

(3.) In an impure state, mixed with magnesium silicide, it may also be obtained by heating a mixture of dry white sand with about four times its weight of dry magnesium powder in a hard glass tube.

As obtained by either of these methods the silicon is in the form of an amorphous, dark-brown powder.

(4.) Silicon is obtained in a crystalline condition by passing a slow stream of the vapour of silicon tetrachloride over aluminium, previously melted in a current of hydrogen; the volatile aluminium chloride passes on in the stream of gas, and the liberated silicon dissolves in the excess of aluminium—

3SiC1, +4A1=3Si+2Al Cl

As the mass cools, silicon is deposited in the form of long, lustrous, needle-shaped crystals.

(5.) The most convenient method for the preparation of crystallised silicon consists in heating in a crucible a mixture of 3 parts of potassium silico-fluoride, 1 part of sodium, and 4 parts of granulated zinc. The regulus so obtained contains crystallised silicon. It is gently heated, and the excess of zinc drained away, the remainder being removed by treatment with acids.

I

Properties.-Amorphous Silicon, as obtained by the reactions Nos. 1 and 2, is a dark-brown amorphous powder, having a specific gravity of 2.15. When heated in the air it burns with the formation of silicon dioxide, which, being non-volatile, coats the particles of the element and protects it from complete oxidation. It burns when heated in a stream of chlorine, with formation of silicon tetrachloride. It is insoluble in water, and in all acids except