Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/364

Rh 346 MINERALOGY &quot;VTATURAL objects which are homogeneous in their _LN mass, and in which no parts formed for special pur poses can be distinguished, are termed &quot; minerals &quot; ; and the branch of natural science which treats of these is termed mineralogy. Minerals differ from the structures treated of in botany and zoology in the three following par ticulars. (1) They differ in the mode of their formation; this has been accomplished, not by assimilation of matter, producing growth from within, but by augmentation of bulk through accretion of particles from without. (2) Minerals are not heterogeneous. While the objects treated of in the other departments of natural history consist of beings possessed of life, and having parts which, being mutually dependent, cannot be separated from one another without a more or less complete destruction of the individual, the objects treated of under the department of mineralogy have so uniformly consistent an individuality that they are not destroyed by any separation of parts, each portion or fragment possessing the same properties and the same composition as the whole. And (3), while those beings which are possessed of life have their component elements grouped into complexes, for the most part capable of more or less freedom of motion and susceptible of change, minerals have a constitution resulting from chemical attrac tions alone and an arrangement of their parts, under physical influences, which has resulted in rigidity and an absence of all tendency to change. FORM OF MINERALS CRYSTALLOGRAPHY. Definiti- The most precise definition of a mineral would be an tion of a inorganic body possessed of a definite chemical composition, mineral. an( j usua |iy O f a regular geometric form. Of these, the second is in one respect the direct outcome of the first ; while many of the most important physical properties pos sessed by minerals are outcomes of the second. Both the geometric form and the composition of minerals are produced and modified under the influence of general laws. Mineral bodies occur in the three physical conditions of solid, liquid, and gas. Those now found in the last two states are few in number, and are of altogether inferior interest to those which occur as solids ; but there is reason to believe that the minerals we know as solids once existed in the liquid or gaseous state, and that their pre sent structure was determined in the process of solidifica tion. All bodies thus formed may be divided into two great classes: Amor- 1. Amorphous bodies, or such as do not possess a de- phous finite and characteristic geometrical form. These (when Ddies. transparent) refract light singly in every direction (except when under stress) ; they are equally easy or equally diffi cult to break in all directions ; when broken they exhibit a conchoidal or an earthy fracture ; they are equally hard throughout all their parts ; they are equally elastic in all directions ; they conduct heat with equal rapidity and in equal amount in all directions. Crystals. 2. Crystalline bodies, or such as occur in definite geo metrical forms bounded by flat surfaces. These present greater facilities of separation of their particles, or &quot;cleav age,&quot; in certain directions lying in determinate planes than they do in others ; most of them are neither equally hard nor equally elastic in all directions, conduct heat more rapidly in certain directions than they do in others, and, when transparent, refract light doubly except in certain directions. Fig. 1. Mineral bodies are found in both of the above classes ; and the same mineral body may occur in both the amorphous and the crystalline condition. This is seen in the piece of gold shown in fig. 1, where the upper portion has a sharply angular and a well-defined shape, while the lower presents curvilinear and rugged outlines, similar to one another in no part. Under favouring circumstances, it i.s possible that every substance whose composition is capable of being represented by a definite chemical formula i.e., which has an unvarying composition- may be capable of assuming a definite crys talline form. Size and Form of Crystals. They are of every size from Size of over a yard in diameter to mere specks requiring a high crystal: power of the microscope to reveal their existence. Beryls have been obtained in America more than 4 feet in length by 2 1 in thickness, weighing 2| tons. Equally large crystals of apatite have been found in Canada. There is a rock crystal at Milan 3^ feet long by 5J in circumference, weighing 870 ft). The highest perfection of form, and hence of other properties, is only found, however, in crystals of moderate or of small size. Variety of Form, and Constancy of Form. The same Variety mineral may be found in different localities, or sometimes in the same locality, exhibiting an almost endless variety of forms. Calc-spar occurs at a Scottish locality in acicu- lar pyramidal crystals of which the length may be ten or more times as great as the width (fig. 2) ; in flat plates as thin as paper, in which the length is not the hundredth part of the width ; also in prisms, pyramids, and rhombohedra, which at first sight (as in figs. 3, 4) seem Fig. 4. destitute of any relationship to each other. This substance has elsewhere been noted in several hundred forms. The minerals fluorite, pyrite, and baryte have each been observed in over a hundred diverse forms. Nevertheless, however ^ great the number, all the forms, in the case of each mineral, may be reduced or referred to a single type, Reiatio by the simple process of examining its internal structure to parer or the mode of arrangement of its molecules. This is form&amp;lt;