[36] Young’s Lectures on Natural Philosophy and the Mechanical Arts. Lecture 49, On the Essential Properties of Matter.
CHAPTER V.
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.