Immagini della pagina
PDF
ePub

throughout its length; while different varieties of cast iron will present congeries of plates, and a structure more nearly approaching the crystalline. The peculiar toughness of the one, and the fragility of the other, derive their explanation from this difference of texture.

§ 117. The regular direction and antagonism of such forces is again displayed in a very interesting manner, by the action of quicksilver upon small bars of tin. The force of adhesion between these metals is very strong, and is capable of entirely overcoming the force of cohesion. If a small square bar of tin be laid horizontally, be just covered with mercury, and, to preserve uniformity of action, be frequently turned upon its different faces, at the expiration of twenty-four hours minute fissures will begin to appear along its lateral and terminal edges. As the process continues the cracks will widen, and about the third day they will open to such a degree as to show that the bar is resolved into four equal, trihedral, rectangular prisms, with two equal angles. These may readily be separated from each other by the point of a knife, as well as two similar pyramids from the two extremities, with angles at their basis of 45°. It is indifferent to this result whether the square form be given to the bar, by casting, hammering, or the filing down of any other shape.

Here it would appear, that as the mercury penetrates the four faces of the bar, the original force of cohesion is weakened, a new force of cohesion between the particles of the amalgam draws them towards the centre of the four rectangular prisms into which we may conceive the bar to be divided, and causes the mass to separate along the lines of least resistance, which coincide with the edges (30).

(30) Dissected bars.

This figure shows the four trihedral bars which result from the disintegration of a square bar of tin by the action of mercury.

In this figure is represented the regular crack which occurs in the edge of the bar previously to its final separation, together with the small pyramid detached from the summit.

§ 118. A similar effect may be produced by careful hammering upon bars of malleable metals. If square bars be hammered upon the edges, and the blows repeated round them, so as to give them a cylindrical shape, they soon become what is technically called rotten, and break into fibres; but when the blows are directed parallel to their faces, they are capable of great extension; but many of them, by alternate hammering upon each face, ultimately split along the edges in a manner very similar to that which is produced by the heterogeneous adhesion of the mercury to the bar of tin. When it is desired to give a round form to any part of a square bar, it is effected by confining it in a kind of form or mould, and the mechanical force thus applied produces an equality of pressure. It is by a similar precaution to equalize the pressure, that metals are capable of being drawn into fine wires, through circular holes in steel plates.

A curious and interesting discussion has lately arisen with regard to the alteration of structure which the iron axles of carriages are liable to from the perpetual tremulous vibration to which they are exposed upon the railways. The toughest and most fibrous wrought iron is always selected for the purpose, but it is believed that under this vibration the particles slowly assume a crystalline arrangement like cast iron, and ultimately become brittle. This important point is now undergoing the careful examination which it so well deserves, both in a scientific and practical point of view.

§ 119. The phenomena of crystalline structure which have now been described, have given rise to two rival hypotheses with regard to their cause, and to account for the forms of the molecules which are thus capable of being built up into the beautiful geometrical solids, which have in all times excited the admiration of the curious.

a

Guided by the mechanical dissection of different crystals, of which we have already given an account, M. Haüy imagined that the form which was ultimately obtained by following up the cleavage till the new formed planes met together in a symmetrical manner, and which in the case of calcareous spar is rhombohedron of definite angles (27), constituted the primitive form of the substance, and was the form, in fact, of its ultimate molecules. From aggregations of these, externally modified according to geometrical laws, he conceived all secondary forms

of the same substance might be produced. There is no difficulty, in fact, in admitting that inconceivably small parallelopipedons of all varieties of angles, may be built up into masses which would have the same relations of sides and angles. He further supposed that the external modifications of this structure, which produce the numberless varieties of secondary forms, but which are still limited by certain definite geometrical relations to the primitive form, originate from decrements, or deficiencies of particles, taking place in the act of crystallization, on different edges and angles of the primitive form. Thus, if upon a compound cube made up of a large number of small cubes, we place layers of cubic particles, decreasing each by a row of particles parallel to the edges, till a pyramid is constructed upon each, terminating in a single particle, the figure will become converted into a dodecahedron, with twelve equal rhombic faces (31). If the decrement were to take place upon the angles, instead of the edges of the original cube, the figure would be converted into an octohedron. By decrements of more than one row of particles, and by intermediate and mixed decrements taking place, according to the laws of symmetry, it may be shown that an almost infinite variety of secondary forms may be constructed, all of which would be related by geometrical laws to the primitive form, and all of which might be assumed by the substance to which it belongs. No objection arises to this hypothesis, from the non-appearance of the ori

(31) The structure of the rhombic dodecahedron from cubic particles is shown in this figure:

[graphic]

ginal form in these secondary structures, for by multiplying the number of particles, and diminishing their size continually, we soon arrive at a limit where the primitive solid becomes insignificant, and, in fact, is actually lost in the comparison.

§ 120. But we have already seen that the mechanical dissection of some substances affords more than one ultimate form, as in the case of fluor-spar. Both the octohedron and the tetrahedron result from the cleavage of this remarkable substance; which, then, is to be taken as its primitive form,-the form of its integrant particles? Neither of them, by their aggregation, can fill space like parallelopipedons. They may be both symmetrically arranged, so as to afford to the eye the external forms of the secondary crystals, which may be geometrically calculated from their various decrements; but the tetrahedral arrangements would be regularly interspersed with octohedral vacuities; and the octohedral with tetrahedral cavities. Such structures, moreover, would be unstable, and contrary to all the established laws of attraction in matter (32).

(32) The following figures present the construction of the tetrahedron and octohedron, each with tetrahedral and octohedral particles.

[graphic][graphic][merged small][merged small][graphic][graphic][merged small][merged small]

The phenomena of dissection would be quite opposed to the hypothesis that solids thus attract one another by their edges, and not by their sides: for a cube so put together of octohedral particles would necessarily split in directions parallel to the faces of the cube, and not to those of the octohedron. The fact is, that M. Haüy himself was not unaware of the difficulties of his system, regarded as a physical explanation of the phenomena of crystallization; but, without insisting upon this view of the subject, as some of his disciples have injudiciously done, he made use of it in a legitimate way, to present together in a general point of view a great number of particular facts; and it enabled him to reason from the known to the unknown in a truly philosophic manner. Like "a geometrical diagram, it had its important uses in the advancement of science, but it had no physical meaning*.”

Dr. Wollaston proposed by another ingenious hypothesis, which had been already developed to a considerable extent by Dr. Hook, to obviate the necessity of admitting the interstitial vacuities in those arrangements which the tetrahedral and octohedral particles afford. He suggested that the elementary particles of this class of substances might be considered to be perfect spheres, and to assume that arrangement which would bring as near to each other as possible.

The relative position of any number of equal balls in the same plane, when gently pressed together, every three forming an equilateral triangle, is familiar to every one; and it will be evident, from inspection of a number of small shot placed together on a plate, that if they were cemented together, and the stratum thus formed were afterwards broken, the lines in which they would be disposed to break would form angles of 60° with each other (33).

It will be seen that the tetrahedral structure is interspersed with octohedral vacuities, and the octohedral with tetrahedral spaces; so that one might be conceived to be taken out of the other.

All bodies may be considered as attracting each other, as if the whole of their matter were condensed into their centres of gravity, which therefore would in all cases be drawn as closely as possible together: under this law solid tetrahedra and octohedra would apply themselves to each other by their sides, and not by their edges.

(33) The arrangement of balls pressed together in the same plane.

* WHEWELL's History.

« IndietroContinua »