The animal and vegetable tribes cease to continue the functions of life: death ensues, and a complete disorganisation takes place; but this is not the case in the mineral world: the crystal being the result of a constantly acting force is not necessarily liable to decomposition.
Nevertheless, we sometimes find in nature that crystals, after arriving at what may be regarded as, in some sort, their maturity, are, owing to a change of the conditions under which they were formed, gradually decomposed. In our mines we discover skeletons of crystals, and within the hollow shell thus formed, other crystals of a different constitution and figure find nuclei, and the conditions required for their development. Again, to give a striking instance, the felspar crystals of the granitic formations are liable to decomposition in a somewhat peculiar manner. In decomposing, these crystals leave moulds of their own peculiar forms, and it not unfrequently happens, in the stanniferous districts of Cornwall, that oxide of tin gradually fills these moulds, and we procure this metallic mineral in the form of the earthy one. Then we have the curious instances of bodies crystallising in a false form under change of circumstances. We find, for example, Pseudomorphism, (or false-form), as this class of phenomena is named, occurring by the removal of the constituent atoms of one crystal, while another set—which naturally assumes a different form—takes their place, yet still preserving the original shape. It often happens that copper pyrites will, in this manner, exhibit the angles of an ordinary variety of crystallised carbonate of iron. These curious changes may be familiarised by supposing a beautiful statue of gold, from which some skilful mechanic removes particle by particle, and so skilfully substitutes a grain of brass for every one of gold removed, that the loss of the precious metal cannot be detected by any mere examination of its form.
Crystalline form is not strictly dependent upon the chemical nature of the parts forming the crystal. The same number of atoms, arranged in the same way, produce the same form. Substances much unlike each other will assume the same crystalline arrangement. Magnesia, lime, oxide of cadmium, the protoxides of iron, nickel, and cobalt, combined with the same acid, present similarly formed bodies. These Isomorphic (like-form)[37] peculiarities are exceedingly common, and the discoverer of the phenomena, Mitscherlich, announced the above law. It cannot, however, be regarded as a philosophical expression of the fact, and requires reconsideration—chemical elements of a dissimilar character may have the same law of aggregation, and thus produce the same form, without having any relation to the number of atoms.
We also find compounds which have two distinct systems of crystallisation. This property, Dimorphism, is very strikingly shown in carbonate of lime, which occurs in rhombohedrons, in calc spar, and in rhombic prisms in arragonite. The molecular arrangements here are not, however, of equal stability, and one form is evidently forced upon the other, and is abandoned by it on the slightest disturbance. When a prism of arragonite is heated it breaks up into the rhombs of common calc spar, at a temperature far below that at which the carbonate of lime is decomposed; but no alteration of temperature can convert calc spar into arragonite.
Crystals are found in the most microscopic character, and of an exceedingly large size. A crystal of quartz at Milan is three feet and a quarter long, and five feet and a half in circumference, and its weight is 870 pounds. Beryls have been found in New Hampshire measuring four feet in length.[38]
In the dark recesses of the earth, where the influences which produce organisation and life cease to act, a creative spirit still pursues its never-ending task of giving form to matter.
The science of crystallogeny,[39] embracing the theoretical and practical question of the causes producing these geometric forms, has in various ways attempted to explain the laws according to which molecules arrange themselves on molecules in perfect order, giving rise to a rigidly correct system of architecture. But it cannot be said that any theory yet propounded is sufficiently exact to embrace the whole of the known phenomena, and the questions,—What is crystallogenic attraction, and what is the physical nature of the ultimate particles of matter,—are still open for the inquiries of that genius which delights in wrestling with the secrets of nature.
The great Epicurus speculated on the “plastic nature” of atoms, and attributed to this nature the power they possess of arranging themselves into symmetric forms. Modern philosophers satisfy themselves with attraction, and, reasoning from analogy, imagine that each atom has a polar system.
Electricity, and light, and heat, exert remarkable powers, and accelerate or retard crystallisation according to the conditions under which these forces are brought to bear on the crystallising mass. We have recently obtained evidence which appears to prove that some form of magnetism has an active influence in determining the natural forms of crystals, and we discover that magnetism exerts a peculiar influence in relation to the optic axes of crystals, which is not exerted in lines at right angles to these. Electricity appears to quicken the process of crystalline aggregation—to collect more readily together those atoms which seek to combine—to bring them all within the limits of that influence by which their symmetrical forms are determined; and strong evidence is now afforded, in support of the theory of magnetic polarity, by the refined investigations of Faraday and Plücker, which prove that magnetism has a directing influence upon crystalline bodies.[40]