M. Plücker, being desirous of finding the extent to which the direction of the fibres in organic bodies might influence their magnetic or diamagnetic properties, was led to inquire whether in crystals the direction of the optic axes, which itself depends upon the arrangement of the particles, might not also exercise some influence. The first submitted to the action of the electro-magnet a thin plate of tourmaline, such as is employed in experiments upon polarization, having its optic axis parallel to its longest length. It was very quickly perceived that the plate was magnetic, by the effect of the iron that it contains; but it was suspended successively in three ways,—first, so that its longest side was vertical, then as that the shortest side was vertical, and finally so that the plate itself was horizontal. In the first case it is directed between the two points of the conical curvatures of the poles like a magnetic body; but, in the other two cases, on the contrary, it took the direction assumed by diamagnetic bodies—that is to say, a direction such that its longest length was perpendicular to the line joining the poles. This direction indicated that the optical axis was repelled by the two poles, and that this repulsion outweighed the magnetic properties of the crystal.[190]
The relation of structure to physical phenomena of essentially different characters is remarkable. Savart, when making crystalline plates of quartz and carbonate of lime vibrate, succeeded in determining a relation between the acoustic figures that are produced in them, and the particular mode of the crystallization of the substance. He found that the direction of the optical axis is constantly connected with that of the principal forms of the acoustic figures.
Mitscherlich has remarked that crystals do not expand uniformly by heat, but that this dilatation is greater in one direction than in another; and that this difference is connected with their crystalline form. M. de Sénarmont has shown that conductibility for heat, which is equal in all directions for the crystals of the regular system, acquires in others a maximum or a minimum value, according to directions parallel to the crystallographic axes; so that the isothermic surfaces, which are spheres in the former case, are, in the other, ellipsoids elongated or flattened in the same direction. The optical axes do not altogether coincide with the principal axes of conductibility for heat; but this appears to be due merely to slight differences in the rate of progression, or the refrangibility of the luminous and calorific rays.
Wiedemann, by employing a fine point through which he made electricity arrive upon a surface that he had powdered with licopodium or red lead, succeeded in determining, by means of the form assumed by this light powder, the conductibility of crystals in different directions.
On a surface of glass, the powder which disperses itself around the points, in consequence of electric repulsion, forms a circular figure traversed by radii. When a plate of gypsum is used instead of glass the figure is found to be elliptical, and the great axis of the ellipse forms a right angle with the principal crystallographic axis, which proves that the electricity distributes itself more easily in a direction perpendicular to the axis than in any other. M. Wiedemann comes to the conclusion that crystals which possess a better conductibility in the direction of the principal axis, all belong to the class of negative crystals: while those which have a better conductibility in the direction perpendicular to the axis are positive, which indicates that the direction of best conductibility for electricity is also that according to which light is propagated relatively with greater velocity.
Tyndale has shown, that if gutta percha which has been rendered fibrous in manufacture is cut so that the fibres are in the direction of this greatest length, or in a direction perpendicular to this greatest length, and placed under the influence of a magnet, they direct themselves equatorially. Ivory cut in the same direction manifests the same conditions, though both these substances are diamagnetic.
The fibrous structure, and the planes of cleavage, thus determine the magnetic condition of a substance. The special properties presented by crystals, in regard to the action exercised upon them by magnets, is due to a particular mode of grouping their particles. This is also the cause of unequal dilatability, and of unequal conductibility for heat and for electricity.
How curiously, therefore, does molecular structure determine the relation of a body to any of the forms of physical force!
We still search in the dark, and see but dimly the evidences; yet it becomes almost a certainty to us, that this stone of granite, with its curious arrangement of felspar, mica, and quartz, presents its peculiar condition in virtue of some law of magnetic force. The crystal, too, of quartz, which we break out of the mass, and which presents to us a beautifully regular figure, is, beyond a doubt, so formed, because the atoms of silica are each one impelled in obedience to one of these two conditions of magnetism to set themselves in a certain order to each other, which cannot be altered by human force without destruction.
All the laws which regulate the forms of crystals and amorphous bodies are, to the greatest degree, simple. In nature the end is ever attained by the easiest means; and the complexity of operation, which appears sometimes to the observer, is only so because he cannot see the spring by which the machine is moved.