These are only a portion of the minerals, but it would be scarcely interesting to give the list at greater length. In the foregoing we recognize the metals and various combustible and non-combustible substances familiar to us, existing, as people say sometimes, in “lumps.” But if any one will take the trouble to examine a “lump,” he will find the shape is definite and even. These regular forms of the minerals are called CRYSTALS, from the Greek word krustallos, ice. The term was originally applied to quartz, for in olden times it was thought that quartz was really congealed water. We can define a crystal as “an inorganic solid bounded by plane surfaces arranged round imaginary lines known as axes.” It must not be imagined that crystals are small bodies; they may be of any size. There are crystals of many hundredweight; and although the usual crystal is comparatively small, it may be any size.
Crystallization has occurred by cooling, or by other natural means; and we can form crystals by evaporation from certain salts deposited in water. So we may conclude also that the evaporation of water in the early periods deposited many forms of crystals. We have crystals in the air, such as snowflakes, which are vapours crystallized. Carbon, when crystallized, is the diamond. Boron is very like it. Oxygen cannot be crystallized. Alumina makes sapphires and ruby with silica. Alumina and earth give us spars, tourmaline, and garnets. Limestone also has beautiful forms, as in Iceland spar. Crystals, therefore, are certain forms of nature, corresponding in the inorganic kingdom to the animals and plants of the organic.
Let us look a little more at these. Here we have a group of crystals of different forms. Earths are metals combined with oxygen, and the principal earths are alumina, lime, and silica. To these three we are chiefly indebted for the ground we live on, and from which we dig so many useful metals and other minerals. Earths are coloured by the substances mixed with them. We can thus find copper, silver, gold, lead, etc., by noting the appearance of the soil. True earths are white. Strontia and baryta are also earths, and the latter is used in firework manufactories. Our chief assistants are Alumina, which furnishes us with bricks and slate; Lime, which gives us marble or stones for building in a carbonate form. Quicklime, by which is meant lime freed from the carbonic acid, is well known; and plaster of Paris is only lime and sulphuric acid in combination. The Silicates, such as sand and flint, are in daily demand. Agate, cornelian, Scotch pebbles, rock-crystal, etc., belong to the same family. Even our gems are crystallized earths, and, as already stated, diamonds are merely carbon.
Stone, as we know, is quarried; that is, it is dug out of the earth. But perhaps many readers do not know why a stone-mine is called a “quarry.” Most kinds of stone (granite and marble are the exceptions) are found in layers, or strata, rendering them easy of removal. The blocks of stone are cut with reference to these layers in a more or less square manner, and “squared up” before they are carried away. Thus the term “quarry,” from an old French word, quarré, or carré, as now written, signifying a square. In granite quarries the stone being very hard is bored, and loosened by means of gunpowder or dynamite blasting. Slate, on the contrary, is easily divided into slabs. We will now resume the subject of Crystals.
Fig. 434.
1.—Emerald. 3.—Garnet. 5.—Diamond.
2.—Agate. 4.—Ruby. 6.—Rock crystal.
We have said that crystals vary in size, and this variety may be traced, in the cases of crystallization from fluids, to the slowness or the rapidity of the cooling process. If the work be done slowly, then the crystals obtain a size commensurate with the time of cooling, as they are deposited one upon the other. The form of minerals is the first important point, and to ascertain their forms and structure we must study Crystallography. We shall find faces, or planes,—the lines of contact of any two planes,—called edges, and the angles formed where these planes meet. We may add that crystals have, at least, four planes, making six edges and four angles. Nearly all crystals have more than this, for the forms are, if not infinite, very numerous, and are divided into six (by some writers into seven) different systems or fundamental forms from which the varieties are derived. The axis of a crystal is an imaginary line drawn from an angle to the opposite one.
The first form, the monometric, or cubic system, with three equal axes at right angles, is represented by fig. 436. This crystal is limited by eight equilateral triangles. It has twelve edges and six angles. If we describe a line from any one angle to an opposite one, that line is called an axis, and in the case before us there are three such axes, which intersect each other at right angles.[23] Such crystals are regular octohedra. There are irregular forms also, whose axes do not come at right angles, or they may be of unequal length. The substances which we find crystallized in this form or system are the diamond, nearly all metals, chloride of sodium (salt), fluor-spar, alum, etc.
