is done to protect the plates from corrosion by the acid when the battery is not in use. Looking-glasses are "silvered" with an amalgam of mercury and tin.

Mercury gives off vapour very slowly at ordinary temperatures, but on heating in a test-tube it is readily volatilized. Some of the vapour will condense on the cool part of the tube as a bright metallic ring.

When mercury is exposed to a high temperature in contact with air for several days, a red oxide is formed on the surface. This red oxide, if strongly heated, is split up again into oxygen and metallic mercury.

The experiment may be performed, and the oxygen gas collected, by using the apparatus shown in Fig. 104.

112. Silica or Silicon Dioxide (SiO2).—Silica is the only known oxide of silicon, and is the most abundant binary compound found in the earth's crust. It is found in nature in a variety of forms, of which quartz, sand, flint, and agate may be taken as examples.

m

Quartz is a common crystalline form of silica, the crystals being in the form of hexagonal prisms, bounded at the ends by hexagonal pyramids. Complete crystals, with both ends perfect and all the faces equally developed, are rare. A cluster of them may often be found springing from a common base and having the free ends of the FIG. 109.-Form prisms terminated by pyramids (Fig. 109). The angle between adjacent faces of a crystal always forms 120°. Quartz is so hard that it will scratch glass, and a knife will not scratch it. A pure transparent kind of quartz is known as rock-crystal, and is used under the name of Brazilian pebble to form lenses for spectacles.

taken by quartz crystal: p, pyra. mid planes; m, prism planes.

Sand is largely composed of grains of quartz that have been worn more or less round by rubbing. Sandstone has been formed by the grains becoming united under great pressure by some cementing substance.

Flint and Agate are non-crystalline or amorphous varieties of silica. Flint is a dark-coloured impure variety of silica that is found occurring in nodules and sheets. The nodules can be broken into fragments with sharp cutting edges, and such fragments were formerly used with a piece of steel for obtaining fire. Agate is an extremely hard variety of silica, formed of layers having different colours. It is used for making ornaments, and also for making mortars in which substances are pounded.

Opal is an amorphous variety of silica containing water. It often shows a beautiful play of colours, and is much used for ornaments.

All the forms of silica are quite insoluble in water, and also in all acids except hydrofluoric acid. The amorphous variety, however, is somewhat soluble in alkaline solutions. This may be shown by trying the action of water and the common acids on the varieties of silica mentioned above.

Silica united with the oxides of other metals forms compounds called silicates. Many rocks, as will be shortly learnt, are largely composed of silicates. Most silicates are very insoluble substances, but the silicates of sodium and potassium are soluble, and are often spoken of as soluble glass.

Experiment 95.-Obtain some soluble glass composed of sodium silicate. Note its glassy appearance when broken. Powder a portion and boil it with a little water. A viscous alkaline solution will thus be obtained. To this add strong hydrochloric acid, when a gelatinous mass of silica separates

out.

CHAPTER XII.

THE MINERALS OF THE EARTH'S CRUST.

113. The Crust of the Earth. The term "crust of the earth" is used to denote the exterior portion of the earth that can be observed and examined, that is, the " upper or outer layers of the earth's mass." We have no means of knowing what is now the real structure and condition of the earth's interior; but by inspecting quarries, railway cuttings, sea cliffs, ravines, wells, mines, and the material sent out by volcanoes, we can learn something about the outer parts of the solid earth, and it is to this outer portion that we refer when we speak of the "crust." The total thickness of the earth's crust penetrated by man is only a few thousand feet, and the highest mountains are not more than 30,000 feet high. Although our reasoning may lead us to infer something about the rocks to a depth of nearly fifty miles, yet, considering that the centre of the earth is nearly 4000 miles from the surface, we have only knowledge of but a small fraction of the earth's radius.

114. Rocks and Minerals.-Every one has noticed that the solid parts of the earth consist of distinct substances, such as clay, limestone, chalk, sand, coal, peat, granite, etc.; and to these several substances which form the materials of the earth's crust we give the name rock. Some are hard and firm, others are soft and loose; but all alike are called rocks. Hence we see that while in ordinary language the word "rock" denotes a great mass of hard stone, in geology a rock is any mass of natural substance forming part of the earth's crust. In this sense, loose sand, gravel, and soft clay are as much rocks as hard limestone and granite.

Rocks are formed of various materials called minerals. If we take a piece of sandstone rock, or a piece of granite, we shall probably be able to notice that the rock is made up of different substances.

Obtain and examine a specimen of coarse-grained sandstone. On looking at it carefully, especially if we use a magnifying glass, we see that it is composed of little rounded grains of a glassy-looking substance cemented together. In some specimens these grains are larger than in others. This cementing material is not the same in all sandstones, but in

our specimen it is formed of calcium carbonate, for when we drop a little dilute hydrochloric acid on the rock there is an effervescence of a gas (carbon dioxide) and the cementing material is dissolved. But the little rounded grains, which consist of quartz, are not affected by the acid. Sandstone, then, consists of quartz grains cemented together by calcium carbonate or some other cementing substance. In some sandstones the cementing material and the grains themselves are coloured yellow or brown by rust (oxide of iron).

Now take a piece of granite, and break it with a hammer to get a cleancut face. On looking at this face we see that the rock is made up of three different substances.

One of these has a glassy appearance like the grains in the sandstone, and is so hard that we cannot scratch it with a knife. This is quartz. Another of the substances is of a dull white or pinkish colour. It lies in long smooth-faced crystalline patches, which easily break along a number

of smooth parallel surfaces having a pearly lustre. It can be scratched with difficulty by the point of a knife. This substance is called felspar. The third substance consists of bright glistening plates, sometimes of a dark colour, which can be easily scratched, and which readily split into transparent leaves. This is mica (Lat. mico, to glisten). Notice that these substances do not occur in any definite order, but are scattered about through

the stone irregularly, the felspar occurring in some specimens in larger crystals than in others.

Hence we see that granite consists of a mixture of three substances, called quartz, felspar, and mica, the felspar being in greatest quantity. Each of these substances possesses properties more or less peculiar to itself, such as hardness, solubility in acids, specific gravity, crystalline form, way of splitting, etc. Moreover, each of these substances has a constant chemical composition.

Thus quartz is a binary compound known in chemistry as silica, and a molecule always consists of one atom of silicon united to two atoms of oxygen (SiO2). The calcium carbonate is represented by the formula CaCO3. It is a ternary compound, formed of the three elements calcium, carbon, and oxygen, chemically united. It may also be regarded as formed of the two oxides, CaO, calcium oxide or quick-lime; and CO2, carbon dioxide (CaO,CO2).

The felspar of the granite contains four elements. Its formula may be thus represented: Al,O,,K,O,6SiO2. This shows it to be made up of three oxides, Al,O,, alumina; K,O, potash; SiO2, silica, that is, it is a silicate of alumina and potash.

115. Minerals. Inorganic bodies which have a definite

Rock-forming Minerals.

145

chemical composition and constant physical properties, are called minerals; and rocks are formed either of one mineral, or more generally of a mechanical mixture of minerals. Hence we give the following definition of a mineral:-A mineral is a naturally formed inorganic substance which has a constant chemical composition and constant physical properties. This definition may be understood to include such substances as coal and chalk, which are the mineralized remains of plants and animals respectively. Even water and the gases of the atmosphere may be said to belong to the mineral kingdom of nature, as plants and their parts are said to belong to the vegetable kingdom, and animals and their parts to the animal kingdom.

116. Chief Rock-forming Minerals.-The total number of rock-forming minerals is large, but many of them are rare and form but a small part of the earth's crust. The following table gives the percentage by weight of the most important minerals:

The Percentage of Minerals in the Earth's Crust.

Though felspar is found to be the most abundant mineral, it is worthy of note that silica (SiO) is the most abundant compound, for it not only occurs separately as quartz to form 35 per cent. of the earth's crust, but it occurs in combination with other oxides to form felspar, mica, and talc.

Analysis of the minerals shows the chemical elements entering into their composition, and from this a calculation has been made of the proportion by weight of the elements present in the outer layers of the earth.

Percentage of Elements in the Earth's Crust.