Form, the most important, perhaps, of the external characters, includes the figures of their crystals, and the various particular shapes which many of them, even in their uncrystallized state, often assume. The texture of fossils, as dia. covered by their fracture, affords another and very important discriminating cha racter. The fracture may either present a surface continuous or uninterrupted ; or it may present a surface composed of an aggregation of distinct parts, by which th e continuity is more or less broken. T he former is denominated the compact, the latter has been termed the jointed fracture; and each is subdivided into a number of varieties. Minerals are like. wise discriminated by their hardness. The degree of it in a fossil is judged of with most certainty by the comparative facility or difficulty of impressing it. Four de grees of it are marked by Werner; the " hard," in which the substance is not capable of being scratched with the knife, but gives sparks when struck by the steel; " semi-hard," when it does not strike fire with steel, and may be scraped by the knife ; "soft," when it may be easily scraped with the knife, but receives no impression from the nail ; and " very soft," when it is scratched by the nail.
Hwy determines the degrees of hard ness according as one fossil impresses another. In one division those are placed which scratch quartz ; the individuals be longing to this, are arranged, as much as possible, in the order of their relative hardness, so that, when placed in a co lumn, each will impress those beneath it. The second class are those which will scratch glass; these are arranged in a similar manner. In a third, those which scratch calcareous spar : and in a fourth, those which make no impression upon it.
Tenacity is that property which relates to the cohesion of the integrant particles of solid minerals, which, existing in differ. ent degrees, give rise to the distinctions of brittle, malleable, and the intermedi ate degree of sectile.
The frangibility denotes the facility with which a mineral may be broken. It exists in different degrees, which are marked by the common terms of difficult. ly frangible, easily frangible-, Ac cording to the Wernenan system, the specific gravity is thus described : a mi neral is said to be " supernatent," which is lighter than water, and will swim upon its surface ; it is called " light," when the specific gravity is between 1.0 and 2.0: " rather heavy," when the specific gra vity is between 2.0 and 4.0 : " heavy, where it varies from 4.0 to 6.0 : and
"very' heavy," when the specific gravity is shove 6.0. The better way is that which we have adopted under the several genera, of stating the numbers denoting the real specific gravity as found by the hydrostatic balance. To these external characters are added others of less im portance, which are derived from proper ties peculiar to a few minerals, such as that of adhering to the tongue, soiling the finger, feeling hard or unctuous, giv ing a particular streak on paper, giving, when struck, a peculiar sound, feeling told when applied to the tongue, having taste, or emitting some perceptible odour.
With respect to the chemical charac ters : " fusibility" is generally deter mined by the action of the blow-pipe, as we can thus operate on a small fragment, and perceive easily the appearances pre dented on fusion. Some minerals are perfectly fusible by it ; others melt with facility ; some fuse with intumes cenco.; others decrepitate or exfoliate when urged by the flame, or lose their colour ; in some the fusion is partial ; sometimes the result is a kind of !scoria: in many cases it is a complete vitreous globule, transparent or opaque, and of various colours. These appearances are diversified, by adding to the substances various fluxes, as borax, and the phos phates of soila and ammonia. The action of acids affords another chemical charac ter of fossils, by observing whether they effervesce when touched by the acid, or whether, when a small fragment is im mersed in it, it is partially or entirely dissolved; if the solution is fluid or gela tinous ; and what appearances it pre sents from the action of re-agents. Di luted nitric acid is generally used in these trials. To the characters taken from cer tain physical properties are referred the phosphorescence,eIecteicity,and maret ism of minerals. Phosphorescence is pe culiar to some minerals, and is therefore a property well adapted to assist in their discrimination. In some it is excited by attrition, more or less strong ; in others, by exposing them to beat. The electri cal state, either positive or negative, ix excited in some minerals by friction, in others by heat ; and iron, in many states of combination, is discovered by its mag netic power. An advantage is sometimes taken of what are denominated empirical characters; of these the most important is that derived from the natural associa tion of minerals; some being found in the same situation, and even blended with each other : while there are others which have never been observed to oc cur together.