THE FORMATION, GROWTH, AND HABIT OF CRYSTALS."

By PAUL GAUBERT, D. Sc.,

Assistant in Mineralogy at the Natural History Museum, Paris.

A crystal arouses the interest of the observer not only by the regularity of its forms, the perfection of its surfaces and angles, its transparency, and its brilliancy, but also by the manner in which it grows, heals its wounds, is dissolved, and modified under the influence of the inclosing medium. To some authors the crystal, from certain points of view, appears analogous to living forms, and seems to undergo a sort of evolution.

Its formation, its growth, the variations of the faces under the influence of the inclosing medium, have been the object of numerous researches which have greatly modified our conceptions regarding them. The purpose of this article is to show the present state of our knowledge concerning these diverse and interesting questions of crystallogeny.

I.

As early as the seventeenth century Leeuwenhoek, who examined under the microscope everything that in his time lent itself to this line of observation, followed the formation and growth of the crystals of various substances (as sugar, tartar, sea salt, etc.). He was led to conclude that the cubic crystals of sea salt are formed of other minute cubes, themselves made up from cubes, the existence of which one has to accept through analogy with what is seen, since they are invisible under any magnifying power. Later, Baker, Ledermüller, and others also examined under the microscope the branched and varied forms that appear when a substance crystallizes on a sheet of glass; but it is to Nicholas Le Blanc that we owe the first systematic and effective researches in crystal genesis, and particularly in the variation of the form. In his very interesting work "De la Cristallotechnie" he gives methods for the preparation of crystals, and in particular does he set forth the process of renewing the solution,

a Translated by permission from Revue Scientifique, Paris, 48th year, No. 3, January 15, 1910.

of "feeding" the growing solids that they may attain a relatively considerable size.

In what form do we see the crystal with the aid of the highest magnifying power? Does it present from the first the form that it will have later? The biologists were the first to take up the matter of the formation of the crystalline "germ;" that is, of the form which it presents at the instant when it first becomes visible; and most of them have admitted the existence of an embryonic state, or a state in which the constitution and form are different from that of the crystal properly called; although this idea has been contradicted by Frankenheim, to whom we owe numerous crystallogenic observations. Vogelsang, in 1867, took it up again and made numerous ingenious and varied experiments to show its correctness. His observations are generally exact, but he has unfortunately misinterpreted them. To show the embryonic state of the crystal, Vogelsang tried to make the bodies crystallize under special conditions with the purpose of retarding their formation so as to enable him to observe all the steps of development. With this purpose in view he added to a sulphur solution a viscous body, Canada balsam. There were produced little spheres, to which Vogelsang gave the name of globulites, and which were thought to represent an embryonic stage. These globulites unite to form particular groups, each of which has received a special name, and at the expense of which the crystal would be produced only at a later stage.

Moreover, Vogelsang rests his experimental researches on observations made with crystallites of varied forms existing in a few rocks rich in silica and more or less vitreous, and in the slags of blast furnaces; but as was later shown by M. O. Lehmann, who made numerous researches on the formation of crystals, these globulites are but drops supersaturated with sulphur, and consequently have nothing in common with the crystalline state.

Brame, as well as Vogelsang, studied sulphur, but in a molten condition. He observed little supermelted drops (utricules) to which he attributes a considerable rôle in crystallization. His ideas differ from those of Vogelsang, but nothing in his experiments substantiates the existence of an embryonic state.

The observations of M. O. Lehmann have shown that the crystal possesses from the beginning a form identical with that which it has when it has attained larger dimensions. T. V. Richard and E. H. Archibald have employed the cinematograph to follow out the formation of the crystal, and obtained only figures of completely formed individuals.

I myself have made a great many experiments, and have always found that the first visible particle had all the properties of the crystal. It is, nevertheless, not to be disputed that in some cases

there takes place what Vogelsang and his predecessors have observed with sulphur or other bodies, but who worked with supersaturated drops or amorphous particles, or little spherulites of unstable form, which later underwent modifications into more stable forms, and the normal development of which can then be followed.

Nevertheless, in spite of the observations of Frankenheim, O. Lehmann, and others, the idea of the embryonic state of the crystal has not disappeared from science, and the hypothesis of Vogelsang, supported by De Schoen, Cartaud, and others, resting on misinterpreted observations, still finds some credit.

II.

When the crystal is once formed-that is, becomes visible under the microscope-how does it grow? Several cases may be presented: First, the mother liquid is in a state of rest, the cooling or evaporation is extremely slow, and the crystalline particles are built up by diffusion alone. In this case the growth is too slow to be constantly followed under the microscope. In the second case the liquid is cooled or evaporated with such rapidity that the quantity of matter deposited on the crystal produces an enlargement microscopically visible. Movements in the liquid are thereby set up. It is an established fact that currents, called "currents of concentration" passing over a crystal, deposit a thin coating of substance, followed by a second, and so on, until, for example, one can see on a crystal of lead nitrate, having a diameter of half a millimeter, as many as twelve of these successive layers deposited. If the process were suddenly interrupted and the crystal examined any observed face would not be a plane, but would show a sort of step arrangement, of which the highest step would indicate the point of contact of the current of deposition.

These successive deposits have no interspaces and the crystal may be perfectly transparent. If the crystal of lead nitrate is, however, subjected to the influence of two or of several currents of concentration, the corresponding coatings laid upon it start from different points in the periphery and may not be of the same thickness. Ordinarily they do not join exactly at their point of meeting. In this way are then produced inclusions and the crystal is no longer transparent, but becomes milky. On a glass plate it is easy to produce at will a transparent or milky crystal of lead nitrate. In the experiment it is necessary to agitate the crystal very slightly with a needle in order to subject it to the influence of one or several currents. These concentration currents produce other peculiarities (vicinal faces, etc.), which it would take too long to describe in detail. I shall confine myself to calling attention to the influence they may

have upon the faces of the crystal. The introduction of matter to one face only of a crystal causes it to develop unequally, and since all crystals of the same bath or magma are not influenced in the same way, they may present a number of different forms. The crystals formed at the bottom may be different from those which are deposited on the side walls or at the surface.

III.

If these concentration currents can completely change the habit of a crystal by producing elongation in one direction, the nature of the faces is not modified; an octahedral crystal always appears in octahedrons. But there are two other influences which modify the faces. One of them is the rapidity of crystallization, the other the constant absorption of foreign matter by the crystal in process of growth. Still other factors may intervene, but they are only indirectly concerned.

It has long been known that crystals formed rapidly possess simple faces, while those which have grown slowly are more complicated. Thus in nature the crystals rich in inclusions, sometimes of large size, are poor in faces, while the small crystals of the same substance, but transparent, are generally limited by a large number of faces. These differences are due to the rate of crystallization, the influence of which has been made known by the experiments of Frankenheim, Lecoq de Bosbaudran, O. Lehmann, and myself. In rapid crystallization the crystals have faces with simple symbols; in slow crystallization these same simple faces persist, but the angles and edges have been truncated and beveled, giving rise to new facets, and I have shown that in certain cases these facets make their appearance always in the same order. With varying rates of crystallization the dominant forms obtained in the case where the crystallization was rapid persist with more or less extensive development, but it may be otherwise in the case where the faces are modified by the regular absorption by the crystal during growth of foreign matter added to the mother liquor. This fact is easily made evident, as I have demonstrated, by adding a little coloring matter.

Rome de l'Isle and Berniard have observed that the crystals of sodium chloride formed in urine are regular octahedrons instead of cubes, such as crystallize from a pure mother liquor. Vanquelin and Fourcroy showed later that this curious modification is due to the urea present. Boydant also established a few phenomena of the same class, and tried without success to ascertain why the mere presence of a foreign substance can be thus effective.

P. Curie developed a remarkable and attractive theory, which apparently furnished the key to this curious modification. He

claims that the capillary action existing between the liquid and the crystal intervenes, an effect varying with the nature of the faces belonging to the diverse forms and with the nature of the liquid. Basing his belief on Gauss's theory of capillarity, he concludes that such faces develop or require the minimum expenditure of capillary energy. The dominant forms must consequently be conditioned by those faces the constant capillarity of which is the least. The addition of a foreign substance altering the different capillary constants may consequently induce modifications of form.

It appears, indeed, that the capillary forces must act, but up to this time there is no fact known which proves that they intervene sufficiently to modify the forms, in spite of the experiments of M. Berent carried out in the laboratory of Sohncke; moreover, I shall describe later an observation showing they are without influence.

IV.

The crystals of one substance rarely form synchronously with those of another dissolved in the same mother liquid, and it is on this property that chemists base their action when they attempt to purify bodies by repeated crystallizations; but there are exceptions, as in the well-known coloration of hydrated nitrate of strontium by extract of logwood, which was accomplished by Senarmont. Since then M. Lehmann and I have proved a few other cases of coloration of crystals by artificial organic dyes.

By making use of the artificial coloration of crystals so as to indicate the presence of foreign matter which has crystallized with the colorless substance I have been enabled to show that the absorption caused modification in form.

The absorption of foreign matter by crystals in process of formation is accomplished in two different ways: First, the coloring matter enters into the composition of the crystal, whatever may be its degree of dilution, and is shared between the crystal and the liquid; second, the coloring matter is taken up by the crystal only when the liquid becomes saturated.

The two processes may go on simultaneously. The study of certain properties of colored crystals, particularly polychroism, and the law of division, shows that the coloring substance in the first case is found in the crystal in the same state as in the liquid; in the second, on the contrary, the coloring matter is in the crystalline state, and we have to do then with a regular grouping of the crystalline particles of the colorless substance with those of the coloring material added to the mother liquor.

Lead nitrate is colored by methylene blue in the second manner; it appears in cubic crystals with the triglyphic striæ of pyrite in