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waters on cooling deposit their silica on surrounding objects as petrifactions. It is generally believed that chalcedony and the various agates and onyxes were formed in this way.

372.* What are the properties of silica ?

Silica is a snow white, gritty powder, which, after being strongly heated, is quite insoluble in water and acids, except hydric fluoride. It dissolves easily in a boiling solution of potassic hydrate. In nature it is found crystallised as quartz. It is very hard, for it scratches glass. It melts when exposed to the oxyhydrogen flame.

373. Describe how you would render silica soluble in aqueous hydrochloric acid, and how the soluble modification of this substance can be converted into the insoluble form. Jan. 1869.

I should first heat the silica with sodic carbonate, thereby forming a sodic silicate, and afterwards add to it hydric chloride, which partly decomposes the silicate and partly dissolves the separated silicic hydrate. Now, if I evaporate this liquid to perfect dryness by a temperature of about 100° C., it loses its solubility, becoming silicic dioxide.

The equations of this twofold operation are:2NaCl + H2SiO3

1. Na2SiO3 + 2HC1

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374. A crystal is given to you, and you are requested to ascertain by a chemical experiment whether it is a

diamond or a common rock crystal. Describe an experiment which will furnish the required information. June 1874.

I should heat the crystal to redness and then plunge it into a jar containing oxygen. If the crystal be a diamond it would burn with flame, with the formation of carbonic dioxide; whereas if it be the rock crystal it would not burn.

375. A splinter of a colourless, transparent solid body is given to you. How could you ascertain, (a) by physical tests, (b) by chemical tests, whether this is a glass or diamond?

By Physical Tests.-1. Glass, when dropped in glycerine, is scarcely seen, because both have nearly the same index of refraction; whilst diamond, having a much greater index of refraction, is clearly seen in the glycerine.

2. Diamond has a great dispersive power; glass has but little.

3. Diamond scars glass, but glass does not scratch the diamond.

4. Glass breaks with great facility, but diamond does not.

By Chemical Tests.-1. Glass becomes tarnished by vapours of hydric fluoride, and diamond does not.

2. Diamond burns in oxygen, and glass does not do so.

376. How could you prove that the constituent of flint has properties similar to other acids, and how can this substance be obtained dissolved in water? June 1876.

I could prove that the constituent of flint has

properties similar to those of other acids by heating in a platinum crucible some sodic carbonate. The acid of the salt (carbonic dioxide) is expelled, and the silica of the flint takes its place, forming sodic silicate. This shows that silica must be an acid, for otherwise it could not expel an acid.

To obtain silica dissolved in water, add to sodic silicate hydric chloride. This decomposes the silicate, forming a gelatinous precipitate of silicic hydrate, which may be dissolved in water.

377. Flint is said to be composed of silica, which, although it has no sour or acid taste, is also called silicic acid. Why is it so called? Jan. 1866-Jan. 1874.

Why is silica called silicic acid? Jan. 1882.

It is so called from the similarity of its action to that of other known acids. Thus it displaces anhydrous acids from their compounds when heated with them, and likewise stronger acids displace it from its compounds in solution. E.g. if sodic carbonate and silica be heated together, they form sodic silicate with evolution of carbonic dioxide. But the expelled gas-carbonic dioxide—is an acid; therefore the body that expelled that gas from the salt it was combined with, must also be an acid.

CHAPTER XIX.

PHOSPHORUS (P).

(Usually a Triad, as in PH,; frequently a Pentad, as in PCl.)

[blocks in formation]

Or 31 grammes of P and 71 of P2O5 measure 11.2 litres. of P and 71 of P2O5 measure 444 c. in.

31 grains

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