I proceed to review the progress of certain sciences on these principles, and first, though briefly, the science of Mineralogy.
Sect. 2.—Mineralogy as the Analytico-classificatory Science.
Mineralogy, as it has hitherto been cultivated, is, as I have already said, an imperfect representative of the department of human knowledge to which it belongs. The attempts at the science have generally been made by collecting various kinds of information respecting mineral bodies; but the science which we require is a complete and consistent classified system of all inorganic bodies. For chemistry proceeds upon the principle that the constitution of a body invariably determines its properties; and, consequently, its kind: but we cannot apply this principle, except we can speak with precision of the kind of a body, as well as of its composition. We cannot attach any sense to the assertion, that “soda or baryta has a metal for its base,” except we know what a metal is, or at least what properties it implies. It may not be, indeed it is not, possible, to define the kinds of bodies by words only; but the classification must proceed by some constant and generally applicable process; and the knowledge which has reference to the classification will be precise as far as this process is precise, and vague as far as this is vague.
There must be, then, as a necessary supplement to Chemistry, a Science of those properties of bodies by which we divide them into kinds. Mineralogy is the branch of knowledge which has discharged the office of such a science, so far as it has been discharged; and, indeed, Mineralogy has been gradually approaching to a clear consciousness of her real place, and of her whole task; I shall give the history of some of the advances which have thus been made. They are, principally, [315] the establishment and use of External Characters, especially of Crystalline Form, as a fixed character of definite substances; and the attempts to bring into view the connexion of Chemical Constitution and External Properties, made in the shape of mineralogical Systems; both those in which chemical methods of arrangement are adopted, and those which profess to classify by the natural-history method.
CRYSTALLOGRAPHY.
CHAPTER I.
Prelude to the Epoch of De Lisle and Haüy.
OF all the physical properties of bodies, there is none so fixed, and in every way so remarkable, as this;—that the same chemical compound always assumes, with the utmost precision, the same geometrical form. This identity, however, is not immediately obvious; it is often obscured by various mixtures and imperfections in the substance; and even when it is complete, it is not immediately recognized by a common eye, since it consists, not in the equality of the sides or faces of the figures, but in the equality of their angles. Hence it is not surprising that the constancy of form was not detected by the early observers. Pliny says,[1] “Why crystal is generated in a hexagonal form, it is difficult to assign a reason; and the more so, since, while its faces are smoother than any art can make them, the pyramidal points are not all of the same kind.” The quartz crystals of the Alps, to which he refers, are, in some specimens, very regular, while in others, one side of the pyramid becomes much the largest; yet the angles remain constantly the same. But when the whole shape varied so much, the angles also seemed to vary. Thus Conrad Gessner, a very learned naturalist, who, in 1564, published at Zurich his work, De rerum Fossilium, Lapidum et Gemmarum maxime, Figuris, says,[2] “One crystal differs from another in its angles, and consequently in its figure.” And Cæsalpinus, who, as we shall [find], did so much in establishing fixed characters in botany, was led by some of his general views to disbelieve the fixity of the form of crystals. In his work De Metallicis, published at Nuremberg in 1602, he says,[3] “To ascribe to inanimate bodies a definite form, does not appear consentaneous to reason; for it is the office of organization to produce a definite form;” [317] an opinion very natural in one who had been immersed in the study of the general analogies of the forms of plants. But though this is excusable in Cæsalpinus, the rejection of this definiteness of form a hundred years later, when its existence had been proved, and its laws developed by numerous observers, cannot be ascribed to anything but strong prejudice; yet this was the course taken by no less a person than Buffon. “The form of crystallization,” says he,[4] “is not a constant character, but is more equivocal and more variable than any other of the characters by which minerals are to be distinguished.” And accordingly, he makes no use of this most important feature in his history of minerals. This strange perverseness may perhaps be ascribed to the dislike which Buffon is said to have entertained for Linnæus, who had made crystalline form a leading character of minerals.
[1] Nat. Hist. xxvii. 2.