sodium hydroxide does. Boric acid is in some respects like aluminum hydroxide, which is an acid or a base, according to circumstances.

Boric acid is volatile with steam, as was indicated in § 395. Its solution in ethyl alcohol burns with a bright green flame. When boric acid is heated it loses water, giving finally the anhydride, B2O3. This redissolves readily in water, giving the acid.

399. Borax. The ordinary borates are derived not from the ortho acid, HBO3, but from tetraboric acid, H2B4O7. This is related to the ortho acid just as the polysilicic acids are to orthosilicic acid (cf. $392).

4 H3BO3-5 H2O H2B407.

Borax is sodium tetraborate, Na2B4O7. It comes into the market as crystals which are either Na2B4O7. 10 H2O or Na2B4O7.5 H2O.

When heated, borax loses its crystal-water and swells to a porous mass; on stronger heating, this melts to a clear glass. Molten borax dissolves the oxides of metals, and with some of them gives characteristic colors; hence, its use in the laboratory to form the "borax bead," and in the arts to clean the surfaces of metals before soldering and welding, and before coating metals with enamel (cf. § 394).

We may look upon the borax molecule as 2 NaBO2+B2O3, i. e., as sodium metaborate plus boron trioxide. The oxides of metals react with the boron trioxide to give other metaborates (cf. §§ 162, 357, and 391).

Borax is used, also, to prevent the decomposition of organic substances, in medicine, in the manufacture of hard-water soaps, and to increase the gloss of starch.

400. Exercises.

1. Name some of the scouring mixtures which consist essentially of silica.

2. Suggest a possible reason for the fact that silicon dioxide is a hard, infusible solid, while the corresponding oxide of carbon is gaseous.

3. Name some polishing, or abrasive, agents besides silica and carborundum. How are diamonds polished?

4. From some of the facts named in this chapter suggest a way in which carbon dioxide might be liberated from calcium carbonate in the earth's crust.

5. What class of substances must be present in underground waters so that silica may be held in solution? What substance is needed to hold calcium carbonate in solution?

6. From what polysilicic acid would a silicate of the formula CaSiO be derived? Show the relation of this acid to normal silicic acid.

7. What are the proportions, by volume, of factors and products in the equation representing the combustion of hydrogen silicide (cf. § 389)?

CHAPTER XXIX.

METALS.

401. Metals and Non-Metals. In our previous work we have studied chiefly non-metallic elements. There is, however, no sharp distinction between metals and non-metals, but rather a gradual change from one class to the other (cf. § 378). But just as oxygen, chlorine, and sulphur have a distinct character, which no one would mistake for that of a metal, so there are certain elements having a typical metallic nature.

Metals are usually opaque, and their polished surfaces are good reflectors of light; hence the metallic luster. They are good conductors of heat and electricity. With oxygen and hydrogen the metals form bases, and by replacing the hydrogen of acids, i. e., ionic hydrogen, they form salts.

Some elements, e. g., antimony, are both acid-forming and base-forming; they are often called metalloids.

Besides the general properties already given, metals possess other properties in varying degrees. Thus, some metals, e. g., sodium and lead, are soft; while others, e. g., chromium and manganese, are hard. Sodium and lithium are light enough to float on water; while gold is 19.3, and platinum 21.5 times as heavy as water. Metals having a specific gravity less than 5 are called "light" metals; those above 5 are called "heavy" metals.

OCCURRENCE OF METALS.

367

402. Occurrence and Extraction of Metals. The solid elements and compounds found in nature are called minerals. The minerals and mixtures of minerals from which metals are obtained are called ores. Some metals, e. g., gold and copper, occur free, that is, uncombined with other elements; but most metals are found as oxides or sulphides. Some occur as carbonates, hydroxides, chlorides, sulphates, phosphates, etc.

If metals occur free, they may be separated by mechanical means from minerals mixed with them. An illustration is the crushing of an ore in a stampmill, and the washing away of the lighter materials. Copper and gold are extracted in this way, although chemical methods are employed with inferior ores of these metals.

The most common method of extracting metals is to reduce the oxide with carbon (charcoal or coke). This is the case with iron. If the ores used are not oxides, they are usually converted into oxides by "roasting" (cf. § 252). Sulphides, hydroxides, and carbonates may thus be changed into oxides.

Another method of reducing an oxide is to heat it in a stream of hydrogen. The oxygen and hydrogen unite and escape as steam, while the metal is left. This is a good laboratory method; but it is too expensive for commercial use.

Chlorides are sometimes reduced by heating them with sodium. Aluminum was formerly obtained in this way from its chloride.

Several of the metals, e. g., aluminum, sodium, and calcium, are obtained by the action of the electric current upon some of their compounds.

403. Alloys. When two or more melted metals form a uniform mixture (cf. § 78), the mixture is called an alloy. If mercury is one of the metals, the alloy is called an amalgam. Alloys resemble solutions in many ways. Thus, melted tin and lead, like alcohol and water, mix in any proportion, but zinc and lead, like ether and water, only to a limited extent (cf. § 82). If a metal is mixed with even a small per cent of another, it undergoes a marked change in properties; the melting temperature, the malleability, and the conductivity for heat and electricity are lowered (cf. § 374); the hardness is increased (cf. §§ 367, 458, 464, and 470). Copper containing 0.8% of arsenic has only 30% of the electric conductivity of pure copper.

404. Classification of the Metals. The metals. may be classified and studied according to several systems. One of these systems is that of "Qualitative Analysis," a branch of Chemistry in which we identify the common metals and acid radicals present in a mixture.

The Qualitative scheme is based upon the properties of the sulphides (cf. § 255). These may be classified as,

(1) soluble, including sulphides of Groups I and II (see below);