[10] p. 167.
[11] Marx. Gesch. p. 97.
[12] Syst. Nat. vi. p. 220.
The circumstance which threw so much difficulty in the way of those who tried to follow out his thought was, that in consequence of the apparent irregularity of crystals, arising from the extension or contraction of particular sides of the figure, each kind of substance may really appear under many different forms, connected with each other by certain geometrical relations. These may be conceived by considering a certain fundamental form to be cut into new forms in particular ways. Thus if we take a cube, and cut off all the eight corners, till the original faces disappear, we make it an octohedron; and if we stop short of this, we have a figure of fourteen faces, which has been called a cubo-octohedron. The first person who appears distinctly to have conceived this truncation of angles and edges, and to have introduced the word, is Démeste;[13] although Wallerius[14] had already said, in speaking of the various crystalline forms of calcspar, “I conceive it would be better not to attend to all differences, lest we be overwhelmed by the number.” And Werner, in his celebrated work On the External Characters of Minerals,[15] had formally spoken of truncation, acuation, and acumination, or replacement by a plane, an edge, a point respectively, (abstumpfung, zuschärfung, zuspitzung,) as ways in which the forms of crystals are modified and often disguised. He applied this process in particular to show the connexion of the various forms which are related to the cube. But still the extension of the process to the whole range of minerals and other crystalline bodies, was due to Romé de Lisle.
[13] Lettres, 1779, i. 48.
[14] Systema Mineralogicum, 1772–5, i. 143.
CHAPTER II.
Epoch of Romé De Lisle and Haüy.—Establishment of the Fixity of Crystalline Angles, and the Simplicity of the Laws of Derivation.
WE have already seen that, before 1780, several mineralogists had recognized the constancy of the angles of crystals, and had seen (as Démeste and Werner,) that the forms were subject to modifications of a definite kind. But neither of these two thoughts was so apprehended and so developed, as to supersede the occasion for a discoverer who should put forward these principles as what they really were, the materials of a new and complete science. The merit of this step belongs jointly to Romé de Lisle and to Haüy. The former of these two men had already, in 1772, published an Essai de Crystallographie, in which he had described a number of crystals. But in this work his views are still rude and vague; he does not establish any connected sequence of transitions in each kind of substance, and lays little or no stress on the angles. But in 1783, his ideas[16] had reached a maturity which, by comparison, excites our admiration. In this he asserts, in the most distinct manner, the invariability of the angles of crystals of each kind, under all the changes of relative dimension which the faces may undergo;[17] and he points out that this invariability applies only to the primitive forms, from each of which many secondary forms are derived by various changes.[18] Thus we cannot deny him the merit of having taken steady hold on both the handles of this discovery, though something still remained for another to do. Romé pursues his general ideas into detail with great labor and skill. He gives drawings of more than five hundred regular forms (in his first work he had inserted only one hundred and ten; Linnæus only knew forty); and assigns them to their proper substances; for instance, thirty to calcspar, and sixteen to felspar. He also invented and used a goniometer. We cannot doubt that he would have been [321] looked upon as a great discoverer, if his fame had not been dimmed by the more brilliant success of his contemporary Haüy.
[16] Cristallographie, ou Description de Formes propres à tous les Corps du Règne Minéral. 3 vols. and 1 vol. of plates.