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OF THE PHYSICAL CHARACTERS. XVII duced by cleavage to the primary form. All may be cleaved into obtuse rhomboids, differing from each other in measure ment. If the planes of one of them meet at the angles of 105° 5' and 74° 55', it is carbonate of lime ; if the second measures 106° 15' and 73° 45', it is bitterspar ; the third, measuring 107° and 73°, is carbonate of iron. Hut comparatively few substances can he known by so simple a process; some cannot be cleaved with regularity ; we must then resort to other characters ; and it is frequently only by a comparison of several of these that the desired object is attained. It is therefore essential that the characters belonging to each should be faithfully detailed in describing it, since there is no book to which the beginner can resort, that will enable him to distinguish the generality of mi nerals with facility. § 3. The characters belonging to most simple minerals may be said to be numerous. If its parts cohere, it possesses some degree of hardness; by trying its hardness, we may discover the ease with which it breaks, or its frangibility ; and we may or may not perceive that it possesses a regular structure; if the structure be regular, we discover the forms into which it. may be divided, and amongst them, that from which all the rest are derived, or, its primary crystal. These regular forms may be termed the geometrical characters of the substance ; although, along with numerous others, they are commonly included under the term of physical or external characters. § 4. These characters are extremely important; but with whatever precision they are given, we should still be far from a competent knowledge of the real nature of the substance, without the aid of the chemist. Hence the characters of mi nerals may be said to be of two kinds ; Physical or external, and Chemical. 1. OF TIIK PHYSICAL CHARACTERS. § 5. These characters are numerous, and require to be well defined, in order that the same language may always convey the same definite idea : there exist, however, and often in the same substance, such very nice shades of difference in certain of them, that much at last is necessarily left to experience. The learner will find that, after a laborious endeavour to discover by written description what a mineral is, it will be much more easy to dis cover what it is not; and at all times he will reap an infinitely greater and more speedy advantage from personal instruction than from books. Such, however, as .can resort only to the latter, will find that an attentive observation of the physical characters, and a comparison of them in different minerals, will forward the acquisition of knowledge.