Talc, we see, is the softest, and diamond the hardest. Thus “diamond cut diamond” has passed into a proverb expressive of the difficulties one “sharp” person has to circumvent or “cut out” another. Diamond is used by glass-cutters. When geologists wish to express the degree of hardness of any substance, they mention it with reference to the foregoing list; and if the substance be harder than fluor-spar, but not so hard as felspar, they determine its hardness five, or perhaps between five and six, or between four and five, according as it is harder or less hard than apatite. Thus hardness, or power of cohesion, resistance to exterior force and pressure, is a prime characteristic of the mineral kingdom. The file is the best test.
We now come to another phase of the physical character of our minerals—cleavage. This is the term employed to express the facility of cutting in a certain direction which in the mineral is its direction of cleavage. Take mica, for instance. There is no difficulty in separating mica into thin layers; we can do so with our fingers. The layers, or flakes, or laminæ are so arranged that they exhibit less cohesion in one direction than when tried in other ways. We cut with the grain, as it were in the direction of the fibre when wood is concerned. Here we have another popular saying expressive of this,—“against the grain,”—which signifies an act performed unwillingly and unpleasantly. Cleavability, therefore, means cutting with the grain, as it were, and various minerals are possessed of different degrees of cleavage. It sometimes happens that electric excitement is observed when cleavage takes place. One place will become positive, and the other negative. Mica, arragonite, and calcareous spar will exhibit this action after cleavage or pressure. When a crystal of tourmaline is heated, it will develop positive electricity at one end of its principal axis, and negative at the other. Even if it be broken, the extremities of the fragments will exhibit similar phenomena, and so far like a magnet, which, as we have seen, possesses this attribute of “polarity.” But a curious fact in connection with this is that, if the heating cease the polarity ceases for a second or two, and yet as cooling goes on the polarity is restored, with the difference that the positive end has become negative, and the end previously negative has come over to the opposite pole. Electricity, therefore, must be hidden away in every portion of our globe, and will some day be proved to be the mainspring of all life.
Fracture in minerals is also to be noticed. Those substances which we cannot laminate we are obliged to break, and we may require to break a mineral in a direction different from or opposed to its direction of cleavage. Under such circumstances we must break it, disintegrate it, and observe the fracture. Sometimes we shall find the surfaces very even, or uneven, or what is termed conchoidal. This is observable in the breaking of flint. There are various ways in which minerals display fracture, and the particular manner and appearance denotes the class to which the mineral belongs.
We may pass over the question of the specific gravity of minerals, as we have in a former part explained this. It is important, however, to ascertain the specific gravity. As a general rule, minerals containing heavy metals are of high specific gravity.
But the relation of minerals (crystals) with regard to light is of great interest and importance. When we were writing of polarization, we mentioned the faculty a crystal has for double refraction, by which it divides a ray of light into two prolonged rays taking different directions, the plane of vibration of one being at right angles to that of the other. This property is not possessed by all crystals. Some act as ordinary transparent media. Some crystals transmit only one polarized ray, and tourmaline is called a polarizer; and if light be passed through it to another polarizer, it will be transmitted if the latter be similarly held; but if the second be held at right angles to it the ray will be stopped. We can easily understand this if we suppose a grating through which a strip of tin is passed; but the strip will be stopped by bars at right angles to it. The coloured rings in crystals can be observed when a slice of a double refracting crystal is examined. The rings are seen surrounding a black cross in some instances, and a white cross in another. The effect when examined in the polariscope is very beautiful. Selenite is probably the best crystal for exhibiting colours.
Minerals sometimes reflect, sometimes refract light; they are said to possess lustre and phosphorescence. All these properties may be considered as belonging to the crystals which are transparent, semi-transparent, translucent, or opaque, according to the degrees in which they permit light to pass through them. All minerals are electric or non-electric, and the variety can be ascertained by rubbing and placing the mineral near the electrometer. But all do not exhibit magnetic properties. Taste and smell are strongly marked in some minerals—salts, for instance, and sulphur; some are soapy to the touch, some appear cold to the fingers. Chemistry is very useful to us in determining the nature of the mineral, and the amount of it enclosed in the substance under examination. These delicate operations are termed qualitative and quantitative analysis. The application of heat is increased by means of the blowpipe, which is in effect a small bellows. We can thus, and particularly by means of the oxy-hydrogen blowpipe, obtain a very intense heat with little trouble. When the fragments of a mineral are held in the flame by platinum “tweezers,” or tongs, then the fusibility of the substance, and the colour of the blow-pipe flame will be of great assistance in determining the nature of the mineral. It is also curious to observe the different forms into which the various substances expand or contract under the influence of the blowpipe. We may have a rugged slag, an enamel, or a glass, or a bead, or “drop” of metal. The varied substances produce various colours—yellow, green, orange, or red, according to circumstances. Strontia is a vivid red, copper is green, lime orange, and so on.
Fig. 456.—The blowpipe.
It is very little use to attempt a study of mineralogy without some acquaintance with chemistry. In dealing with minerals, and in studying geology, we must try to keep our knowledge of chemical science in our minds, and thus fortified we can more easily understand the steps leading to the classification of minerals. It is impossible to teach mineralogy or geology from books. Nature must be studied, the specimens must be seen, the earth must be examined. The advance in mineralogy may be—probably will be—slow, but crystals will teach something; and when we can pass a viva voce examination in chemistry and crystallography, expressing, by the symbols, the various substances under discussion, we shall have made a considerable advance in the science. We shall have an idea of the component parts of various substances, and be able to class the various minerals according to their chemical constitution. Beginning with the metalloids, we shall pass to the metals and various compounds, salts, resinous substances, etc., such as amber.
It is impossible in the space at our command to describe all the minerals, and yet it is necessary to enumerate the most important. We may, therefore, take them in the following order. It should be added that most of the simple minerals occur in comparatively small quantities, but sometimes we find them in aggregate masses (rocks). We append a table.