Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/390

Rh 372 dissolved in these waters, or in mineral or sea waters, heated or at the ordinary temperature ; (4) the action of gases ex haling from the earth ; (5) changes referable to volcanic action. Ordinary waters hold in solution, as is well known, more or less of mineral matter. When water containing carbonic acid is passed through a large number of ordinarily occurring minerals, it gives evidence of the presence of ail alkali, or lime, or magnesia; and some of these minerals give the tests even with the first drops. Pure water gives with many of them a similar result, but more slowly. Limestone in forty-eight hours yields soluble ingredients to the extent of 4 to 1 per cent, of the whole mass. The lime, magnesia, and alkalies appear in the condition of carbonates ; and the iron passes from the state of carbonate to that of peroxide during evaporation. The silicates of magnesia, lime, and man ganese are especially ready in yielding to this action. Silica, however, is more soluble in ordinary than in carbonated water. These facts illustrate two important points : (1) that ordinary waters lying upon and filtering through the earth s crust are constantly active in dissolving and decomposing minerals and rocks, and that even species reputed indestructible are thus acted upon ; and (2) that the waters are thus furnishing themselves with agents capable of effecting other chemical changes. These waters penetrate all rocks, as well as percolate through soils. Hence the action is a universal one, everywhere going on ; and the results are universal. Bones, shells, corals, and animal remains generally are also sources of carbonate of lime, phosphates, and fluorides ; and plants may contribute also potash and soda, and sometimes silica. Carbonic acid is a constant ingredient of the atmosphere, and is dissolved by the rains as they descend ; hence this active de composing agent is present in all ordinary waters; but it is also a result of different mineral changes. Sulphate of iron along with vegetable matters gives oxygen to the carbon of the vegetable matter, and thus produces carbonic acid and pyrites or sulphuret of iron ; and the large quantities of pyrites in coal-beds show on how grand a scale this process has taken place. Sulphate of zinc in a similar manner produces carbonic acid and blende or sulphuret of zinc. Bischof observes that the carbonic acid which has thus been eliminated must have been sufficient in quantity to make an atmosphere of carbonic acid equal in height to our present atmo sphere. Again, decomposition of sulphurets produces sulphuretted hydrogen ; this by the oxidating action of the atmosphere forms sulphuric acid, and the sulphuric acid acting on limestone produces gypsum, and liberates carbonic acid. Sulphurous acid is also generated in the neighbourhood of volcanoes, and rapidly becomes sulphuric acid, with the same result. Moreover, silica in waters, if aided by heat, will decompose limestone and liberate carbonic acid. Hence it is that this gas is exceedingly common in exhalations from mineral springs ; indeed it occurs more or less in all waters. The dissolving and decomposing action of carbonated waters is therefore general. The sea also partakes of this character, and, in virtue of the numerous salts which it holds dissolved, is a powerful agent in carrying on the changes to which the process leads. Such changes and the various pseudomorphs to which they give rise have to be regarded as types and evidences of vast metamorphic transfor mations, processes either of decay or of reformation which have modified widespread rock-masses, and which are at the present time altering the structure of the crust of the earth. It is through a study of pseudomorphs, and of the processes which have gone to form them, that mineralogy is to become the germ from which alone the petrological department of geology can have its true development, and become a living instead of a merely speculative PHYSICAL PROPERTIES OF MINERALS. Characters Depending on Light. There are few more interesting departments of science than the relations of mineral bodies to light, and the modi fications which it undergoes either when passing through them or when reflected from their surface. In this place, however, we only notice these phenomena so far as they point out distinctions in the internal constitution of minerals, or furnish characters for distinguishing one species from another. Lustre of Lustre. Though the varieties of lustre admit of no precise minerals. or mathematical determination, they are of considerable value in mineralogy. One highly important distinction founded on them is that between minerals of metallic and non-metallic aspect or character. Transparency and opacity nearly coincide with this division, the metallic minerals being almost constantly opaque, the non-metallic more or less transparent. Minerals which are perfectly opaque, and show the peculiar brilliancy and opacity of surface of polished metals, are named metallic ; those which possess these properties in an inferior degree are semi-metallic; and those without these properties are non-metallic. Lustre has reference to either the intensity or the quality of the reflected light, considered as distinct from colour. Several degrees in intensity have been named : (1) splendent, when a mineral reflects light so perfectly as to be visible at a great distance, and lively and well-defined images are formed in its faces, as galena, specular iron, or cassiterite ; (2) shining, when the reflected light is weak, and only forms indistinct and cloudy images, as heavy spar or calcite ; (3) glistening, when the reflected light is so feeble as not to be observable at a greater distance than arm s length, and no longer forms an image, as talc ; (4) glimmering, when the mineral held near the eye in full clear daylight presents only a number of small shining points, as red haematite and granular limestone. When, as in chalk or kaolin, the lustre is so feeble as to be indiscernible, the mineral is said to be dull. In regard to the kind or quality of the lustre, the following varieties are distinguished : (1) the metallic, seen in much per fection in native metals and their compounds with sulphur, and imperfectly in glance coal ; (2) adamantine, found in beautiful per fection in the diamond, and in some varieties of blende and cerussite ; a modification is metallic adamantine, as seen in wolfram and black cerussite ; (3) vitreous or glassy, seen in rock crystal, or common glass, or, inclining to adamantine, in flint glass ; sub- vitreous is seen in broken calcite ; (4) resinous, when the body appears as if smeared with oil, as in pitchstone, blende, and garnet ; (5) waxy, like beeswax, as seen in wax-opal and ozocerite ; (6) pearly, like mother-of-pearl, seen in gyrolite, talc, heulandite ; (7) silky, the glimmering lustre seen on fine fibrous aggregates like amianthus, tremolite, chrysotile, krokidolite. These degrees and kinds of lustre are generally exhibited differ ently by unlike faces of the same crystal, but always similarly by like faces. The lateral faces of a right square prism may thus differ in lustre from that of a terminal face. Thus the lustre of the lateral faces of apophyllite is vitreous, while that of the terminal, at right angles thereto, is pearly ; chrysotile is silky when split along the fibres, dull when at right angles to them. The surface of a cleavage plane, in foliated minerals, generally differs in lustre from the sides ; and here again in some cases the latter are vitreous, while the former is pearly, as in heulandite. As shown by Haidinger, only the vitreous, adamantine, and metallic lustres belong to faces perfectly smooth and pure. In the first, the index of refraction of the mineral is 1 - 3 to 1 &quot;8 ; in the second, T9 to 2 - 5 ; in the third, above 2 5. The pearly lustre is a result of reflexion from numberless lamellae, or cleavage planes, within a translucent mineral; and in hydrated minerals, as in the zeolites, it is the result of incipient change, namely, a loss of water v hich ensues upon exposure to the atmosphere. Colour. This is a property which is of very inferior Colour. value. Minerals are so seldom, if ever, absolutely pure that very minute quantities of an intensely coloured impurity may impart colour to a substance inherently colourless, or overpower a feebler colour which may be its own. Some few minerals have colour so strong, or have a constitution so little susceptible of intermixture, that they retain almost unim paired the colour special to them. Such a substance is pyrite ; its brass-yellow colour maybe heightened to gold-yellow by intermixture with copper sulphide, or it may be slightly bleached by arsenic ; but the nature of its composition does not admit of the intrusion of ordi nary colouring ingredients. The yellow of native gold, again, may be paled by impoverishment with the white of silver, down to the dull tint of electrum ; but no foreign colouring matter can intrude itself into a metallic mass. Such substances as these, native metals, sulphides, and oxides, have colours essential to them, dependent on their constitution, and to a great extent characteristic of the species. A second class of minerals are colourless of themselves, and thus very subject to the influence of minute quantities of foreign tinc torial impurity. These are absolutely transparent and devoid of colour -when in crystals, but white and opaque when reduced to powder ; as ice and snow, calcite and chalk, rock-crystal and sand. But such substances are generally coloured ; &quot;muddied&quot; it would be called in the first case, though it is equally so with the others. Such false colour may be imparted in several ways. It may be(l) from their holding dissolved some colouring matter ; (2) from mechanical mixture of colouring substances such as metallic oxides, or minute crystals ( &quot; endomorphs &quot;) of another mineral; or