CRYSTALLOGENIC FORCES.

Crystallisation and Molecular Force distinguished—Experimental Proof—Polarity of Particles forming a Crystal—Difference between Organic and Inorganic Forms—Decomposition of Crystals in Nature—Substitution of Particles in Crystals—Pseudomorphism—Crystalline Form not dependent on Chemical Nature—Isomorphism—Dimorphism—Theories of Crystallogenic Attraction—Influence of Electricity and Magnetism—Phenomena during Crystallisation—Can a change of Form take place in Primitive Atoms?—Illustrative Example of Crystallisation.

“Crystallisation is a peculiar and most admirable work of nature’s geometry, worthy of being studied by all the power of genius, and the whole energy of the mind, not on account of the delight which always attends the knowledge of wonders, but because of its vast importance in revealing to us the secrets of nature; for here she does, as it were, betray herself, and, laying aside all disguise, permits us to behold, not merely the results of her operations, but the very processes themselves.”—Such is the language of an Italian philosopher, Gulielmini; and it is the striking peculiarity of beholding the process of the formation of the regular geometric figures of crystals, the gradual accretion of particle to particle, which induces us to separate crystallization from mere molecular aggregation. Without doubt the formation of a crystal and the production of an amorphous block are due to powers which bear a close resemblance in many points; but they present remarkable differences in others.

Let us take some simple case in illustration. In quiet water we have very finely divided matter suspended, and matter in a state of solution. The first is slowly precipitated, and in process of time consolidates into a hard mass at the bottom, presenting no particular character, unless it has been placed in some peculiar physical conditions; when, as in nature, we have a regular bedding which is intersected by lines of lamination or of cleavage, which we are, from experiment, enabled to refer to the influence of current electricity. The second—the matter in solution—is also slowly deposited; but it is accumulated upon nuclei which possess some peculiar disposing powers, and every particle is united by some particular face, and an angular figure of the most perfect character results. Many pleasing experiments would appear to show that electricity has much to do in the process of crystallization; but it is evident that it must be under some peculiarly modified conditions that this power is exerted, if, indeed, it has any direct action.

The same substances always crystallize in the same forms, unless the conditions of the crystallizing body are altered. It has been supposed that each particle of a crystalline mass has certain points or poles which possess definite properties, and that cohesion takes place only along lines which have some relation to the attracting or repelling powers of these poles. We shall have, eventually, to consider results which appear to prove that magnetism is universal in its influence, and that this polarity of the particles of matter may be referred to it.

Be the cause of crystallisation what it may, it presents to us in appearance a near approach in inorganic nature to some of the peculiar conditions of growth in the organised creation. In one, we have the gradual production of parts and the formation of members due to peculiar powers of assimilation, each individual preserving all its distinguishing features; and in the other, we have a regular order of cohesion occurring under the influence of a power which draws like to like, and arranges the whole into a form of beauty.

This appears to be the proper place for correcting an error too prevalent, relative to the formation of crystals, the development of cells, and the yet more fatal falsehood of referring the great phenomena of Life to any of the physical forces with which we are acquainted.

The Crystal forms, by the accretion of particle to particle, along lines determined by some yet unknown power. There is no change in the character of any particle—like coheres to like; the first atom and the last of the series being identical in character.

The Plant grows, not by the gathering together of similar particles of matter, but by the absorption of a compound particle—by that one which must be regarded as the primary nuclear atom or cell. After this absorption—in virtue of a power which we call life, excited into action by light—the compound particle is decomposed, and one constituent is retained to effect the formation of a new cell, whilst the other is liberated as an invisible air. Here we have a change of chemical constitution effected; and this takes place through the whole period of vegetable growth, from the development of the plumule up to the formation of the latest leaf upon the topmost branch of the most lordly tree.

Life has been referred to electricity and to chemical power—as the effect of a known cause. Without doubt, during the operations of life the whole of the physical powers are necessary to the production of all the phenomena of growth in the vegetable and the animal world. But these powers are ever subsidiary to vital force, and are like attendant spirits chained to do an enchanter’s bidding.

Life is a force beyond the reach of human search, and he who fancies he has a hold upon the principle which produced biological phenomena, has committed himself to as wild a pursuit as he who rashly endeavours to catch a morass-meteor.

Subtile as are the forces of light, heat, and electricity—that of life, vitality, is infinitely more refined, and it must for ever elude the search of the philosopher.

Man is permitted to test and try all things which are created, and to apply to useful ends the discoveries which he may make. But man can never become a creator; and he who would attempt to give sense to an inert mass of matter, by electricity, heat, or light, will prove himself as ignorant of nature’s truth as is the senseless mass upon which he works.

“So far shalt thou go, and no further,” was said equally to the great tide-wave of human intellect, as to the mighty surge of the earth-girdling ocean.

It must not be forgotten that a striking difference exists between the productions of the mineral and the other kingdoms of nature. Animals and vegetables arrive at maturity by successive developments, and increase by the assimilation of substances, having the power of producing the most important chemical changes upon such matter as comes within the range of their influence; but minerals are equally perfect in the earliest stages of their formation, and increase only, as previously said, by the accretion of particles without their undergoing any change.

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]

It has been found that crystals of sulphate of iron, slowly forming from a solution which has been placed within the range of sufficiently powerful magnetic force, dispose themselves along certain magnetic curves, such as are formed around a magnet by steel filings; whereas the crystals of the Arbor DianÆ, or silver tree, forming under the same circumstances, take a position nearly at right angles to these curves. Certain groups of crystals have been found in nature, which appear to show, by their positions, that terrestrial magnetism has been active in producing the phenomena they exhibit; indeed, nearly all our mineral formations indicate the influences of this, or some similarly acting power.[41]

During rapid crystallisation, some salts—as the sulphate of soda, boracic acid, and arsenious acid crystallising in muriatic acid—exhibit decided indications of electrical excitement; light is given out in flashes. We have evidence that crystals exhibit a tendency to move towards the light, and that crystallisation takes place more readily, and progresses with greater activity in the sunshine than in the shade. Professor PlÜcker has recently ascertained that certain crystals—in particular the cyanite—“point very well to the north, by the magnetic power of the earth only. It is a true compass needle; and, more than that, you may obtain its declination.” We must remember that this crystal, the cyanite, is a compound of silica and alumina only. This is the amount of experimental evidence which science has afforded in explanation of the conditions under which nature pursues her wondrous work of crystal formation. We see just sufficient of the operation to be convinced that the luminous star which shines in the brightness of Heaven, and the cavern-secreted gem, are equally the result of forces which are known to us in only a few of their modifications.

Every substance, when placed under circumstances which allow of the free movement of its molecules, has a tendency to crystallise. All the metals may, by slowly cooling from the melting state, be exhibited with a crystalline structure. Of the metallic and earthy minerals, nature furnishes us with an almost infinite variety of crystals, and, by a reduction of temperature, yet more simple bodies assume the most symmetric forms. Water, in the conditions of ice and snow, is a familiar and beautiful example; and, by such extreme degrees of cold as are artificially produced, many of the gases exhibit a tendency to a crystalline condition.

May not the solid elementary atoms be susceptible of change of form under different influences? May not the different states under which the same bodies are found—as, for example, silica, carbon, and iron—be due entirely to a change in the form of the primitive atom?

Admitting the probability of this, we then easily see that the central molecule, formed of an aggregation of such atoms, uniting by particular faces, would present a determinate form; and that the resulting crystal, a mass of such molecules, cohering according to a given law, at certain angles, would present such geometric figures as we find in nature, or produce in our laboratories, when we avail ourselves of processes which nature has taught us.

If we take a particle of marble, and place it in a large quantity of water acidulated with sulphuric acid, it dissolves, and a new compound results. The marble disappears—the eye cannot detect it by form or colour: the acid also has been disguised—the taste discovers nothing sour in the fluid. We have, in combination with the water, the lime and sulphuric acid; but that combination appears to the eye in no respect different from the water itself. It is colourless and perfectly transparent, although it holds a mass of solid matter which previously would not allow of the permeation of a ray of light. Let us expose this fluid to such circumstances that the water will slowly evaporate, and we shall find forming in it, after a time, microscopic particles of solid, light-refracting matter. These particles gradually increase in size, and we may watch their growth until eventually we have a symmetric figure, beautifully shaped, the primary form of which is a right rhomboidal prism. Thus in nature, by the action, in all probability, of vegetable matter on the sulphates held in solution by the water of the great rivers and the ocean—aided by our oxidizing atmosphere—sulphuric acid is produced to do its work upon the limestone formations, and from this combination would result the well-known gypsum, or plaster of Paris, which ordinarily exists as an amorphous mass, but is often found in a crystalline form.[42]

This is a very perfect illustration of the wonderful process we have been considering, and in which, simple though it appears to be, we have set to work a large proportion of the known physical elements of the universe. By studying aright the result which we have it in our power to obtain in a watch-glass, we may advance our knowledge of gigantic phenomena, which are now progressing at the bottom of the ocean, or of the wondrous agencies which are in operation, producing light-refracting gems within the secret recesses of the rocky crust of our globe.

The force of crystallisation is a subject worthy of much consideration. If we examine our slate rocks, through which little veins filled with quartz crystals are spread, we shall see that the mechanical force exerted during the production of these crystals has been capable of rending those rocks in every direction. Those fissures formed by the first system of crystalline veins, in order of time, are filled in by another set of crystalline bodies, which equally exert their mechanical power, and thus produce those curious intersections and dislocations which were long a puzzle to the geologist. The simplest power, slowly and constantly acting through a long period of time, may become sufficient, eventually, to rend the Andes from base to summit, or to lift a new continent above the waters of the ocean.