Thorina differs from the other earths in the following properties: From alumina and glucina, by its insolubility in liquid potash; from yltria, by its solutions being purely astringent to the taste, without any sweetness, and by the property of being precipitated at a boiling heat, except when prevented by too great an excess of acid. It differs from zirconia in the following respects: 1st, Because, after being ignited, it is still soluble in acids. 2d, It is not precipitated by sulphate of potassa, which throws down zirconia, even from solutions containing a considerable excess of acid. 3d, Thorina is precipitated by oxalate of ammonia, which is not the case with zirconia. 4th, Its combination with sulphuric acid crystallizes readily, while sulphate of zirconia forms, when pure and dried, a gelatinous transparent mass, without any trace of crystallization.

SECTION XII.

Zirconium.

THE base of zirconia, or zirconium, is still unknown, though investigated by Sir H. Davy in the same manner as the bases of other earths. When potassium was brought into contact with ignited zirconia, potassa was formed, and dark metallic particles were diffused through the alkali.

Zirconia.

1. This earth was discovered by Klaproth in the year 1789, in a precious stone from the island of Ceylon, called Jargon or Zircon, and has since been detected in the hyacinth.

MM. Dubois and Silveira have described the method of obtaining zirconia from zircons as follows: powder the zircons very fine, mix them with two parts of pure potassa; and heat them red hot in a silver crucible for an hour. Treat the substance obtained with distilled water, pour it on a filter, and wash the insoluble part well; it will be a compound of zirconia, silica, potassa, and oxide of iron. Dissolve it in muriatic acid, and evaporate to dryness, to separate the silica. Redissolve the muriates of zirconia and iron in water; and te

separate the zirconia which adheres to the silex, wash it with weak muriatic acid, and add it to the solution. Filter the fluid, and precipitate the zirconia and iron by pure ammonia; wash the precipitates well, and then treat the hydrates with oxalic acid, boiling them well together, that the acid may act on the iron, retaining it in solution while an insoluble oxalate of zirconia is formed. It is then to be filtered, and the oxalate washed, until no iron can be detected in the water which passes through. The earthy oxalate is, when dry, of an opaline colour; after being well washed, it is to be decomposed by heat in a platinum crucible.

When thus obtained, zirconia is perfectly pure, but is not soluble by acids. To render it so, it must be re-acted on by potassa as before, and then washed until the alkali is removed. Afterwards dissolve it in muriatic acid, and precipitate by ammonia. The hydrate thrown down, when well washed, is perfectly pure and easily soluble in acids. (Ann. of Phil. New Series, i. 74.)

II. Zirconia has the following properties:

1. It has the form of a fine white powder, which, when rubbed between the fingers, has somewhat of the harsh feel of silica. It is entirely destitute of taste and smell. Its specific gravity exceeds 4.

2. It is insoluble in water; yet it appears to have some affinity for that fluid, for it retains, when slowly dried after precipitation, one third its weight, and assumes a yellow colour and slight transparency, like that of gum arabic.

3. It is insoluble in pure liquid alkalis; nor does it even combine with them by fusion; but it is soluble in alkaline carbonates.

4. Exposed to a strong heat, zirconia fuses, assumes a light grey colour; and such hardness, on cooling, as to strike fire with steel, and to scratch glass, or even rock crystal.

5. Its action on other earths has not been fully investigated. 6. It dissolves readily in acids. Its solution in muriatic acid, when sufficiently heated, becomes milk white, and runs in some measure into a jelly, especially if concentrated to a certain point by evaporation.

7. From the muriatic solution of zirconia, oxalic acid throws down a white precipitate, which is re-dissolved by an excess of the acid.

8. It is precipitated from its acid solutions, by the neutral succinates and benzoates, in copious white bulky flocks, which are again readily dissolved by an excess of succinic acid. It is also thrown down from its solutions by tartaric acid, malic acid, and tartrate of potassa.

9. From a sufficiently neutral solution of zirconia, ferrocyanate of potassa throws down a greenish blue precipitate, which, on adding muriatic acid, becomes more blue, but after some time changes into celadon green.

10. Hydrosulphuret of ammonia produces, in the muriatic solution of zirconia, a dark olive or blackish green precipitate in very loose flocks. This precipitate may be washed with water without changing colour; but, when exposed to sunshine, it becomes white.

Professor Pfaff has remarked that in most of their properties there is a striking resemblance between zirconia and oxide of titanium. The only re-agent, which acts in a strikingly different manner upon solutions of oxide of titanium and zirconia, is tincture of galls, which, from the common solution of oxide of titanium, throws down a reddish brown precipitate, whereas from solution of zirconia it occasions a deposition of yellow flocks. (Ann. of Phil. xiii. &3.)

SECTION XIII.

Silicium.

In his attempts to obtain the base of silica, or silicium, in a state of perfect separation, Sir H. Davy was unsuccessful; but the results of his experiments leave little room to doubt that this earth is, like the rest, compounded of oxygen with a pe culiar base.

Berzelius has since decomposed silica, by fusing it with charcoal and iron in a blast furnace. He obtained an alloy

of iron and silicium, which, by the action of a diluted acid, gave more hydrogen than the same weight of iron.* This process was successfully repeated by Stromeyer, and the properties of the different alloys investigated. He recommends the fusion of 7 parts of iron, 5 of silica, and from+to+ths of a part of soot. From the results of acting on the alloy by dilute acids, Berzelius infers silica to consist of

And Sir H. Davy deduces the proportions to be 31 of metal to 30 oxygen. These numbers, however, can be considered in no other light than as approximations. The base of silica Sir H. Davy believes not to be a metal, but a substance most resembling boron; and, like it, bearing an analogy to charcoal, sulphur, and phosphorus. In the present imperfect state of our knowledge of silicium, it would be premature to assign the weight of its atom. If silica be a protoxide, its equivalent will be 16, and that of silicium, or silicon, as it is called by Dr. Thomson, will be represented by 8; but these numbers must be considered as open to correction.

Silica.

I. Siliceous earth, or silica, may be obtained tolerably pure from flints by the following process:- Procure some common gun-flints, and calcine them in a crucible in a low red heat. By this treatment they will become brittle, and easily reducible to powder. Mix them, when pulverized, with three or four times their weight of carbonate of potassa, and let the mixture be fused in a strong red heat, in a crucible. The materials must bear only a small proportion to the capacity of the crucible; and the heat must at first be very moderate, and slowly increased. Even with this precaution, the mass, on entering into fusion, will be apt to overflow; and must be

81 Ann. Ch. 179. See also his account of an attempt to analyze si lica, in 40 Phil. Mag. 201.

pressed down as it rises by an iron rod. When this effervescence has ceased, let the heat be considerably raised, so that the materials may be in perfect fusion during half an hour, and pour the melted mass on a copper or iron dish. We shall thus obtain a compound of alkali and siliceous earth. Dissolve this in water, filter the solution, and pour it into diluted sulphuric or muriatic acid. An immediate precipita tion will ensue, and, as long as this continues, add fresh portions of the solution. In precipitating the alkaline solution of silica, more acid must be used than is sufficient to engage the alkali; and the alkaline liquor must be added to the acid, and not the reverse; for, in the latter case, the precipitate will be glass, and not silica. Let the precipitate subside, pour off the liquor that floats above it, and wash the sediment with hot water, till it comes off tasteless. Then dry it.

Silica, obtained by this process, though pure enough for the following experiments, may still contain a portion of alumina. To separate the latter earth, boil the precipitate with diluted sulphuric acid, to which a little sulphate of potassa may be added. The alumina will thus be dissolved; and, the silica may be freed from the solution of alum by repeated washings with water. Even silica, however, that has been most carefully washed, still gives traces of potassa on the application of electro-chemical powers. (Davy.) According to Mr. Brande, silica may be obtained of sufficient purity for most purposes by heating rock-crystal to redness, quenching it in water, and then reducing it to fine powder.

II. Siliceous earth, as thus obtained, has the following qualities:

(a) It is perfectly white and tasteless. It is infusible by the intense heat of Voltaic electricity; but was melted by Dr. Clarke with the oxygen and hydrogen blow-pipe. To a certain degree it appears to be volatile, for a filamentous substance, collected from iron furnaces, and resembling amianthus, was found by Vauquelin to be pure silica.

(b) When mixed with water, it does not form a cohesive mass like alumina, but has a dry and harsh feel to the fingers.

* Phil. Trans. 1815, p. 370.