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OF THE ARRANGEMENT IN WHICH MINERALS ARE DESCRIBED IN THE FOLLOWING PAGES. In the absence of that organization which so admirably serves as a guide to the generic differences in animals and plants, we must seek for some other basis on which to found an arrange ment of minerals. No one has yet been, nor does it seem pos sible that one should be, constructed, that is altogether satisfac tory ;—one in which there is not much that is arbitrary. The characters of minerals are of two kinds, Physical and Chemical: every system must be founded on one or other of these, or upon their combination. Of the Physical characters, the most valuable, because the most certain, when it exists, is Structure; and since by it alone we may often recognise a mineral, it is highly deserving of the earliest attention of the student. There are, however, many mi nerals in which no regular structure is visible. If, therefore, we would depend on the physical characters, we must look for some other amongst them ; but there is none so invariable as structure. Therefore any arrangement that is made to depend on the phy sical characters can only be founded on a comparison of a num ber of them ; but many, if not most, of these characters, are sub ject to some, and often to a considerable degree of variation, even in the same substance. The physical characters, therefore, are not of that precise, in variable, and universal application which alone would justify their adoption as the basis of an arrangement. One of the chief difficulties attendant on the plan of arranging minerals according to their composition is, the uncertainty which exists in particular substances, as to what their absolutely essen tial constituents are. It may, however, be understood, that what soever enters into the composition of a mineral, that does not al ter the external form and internal structure of that substance in its purer state, is not an essential element, but an accidental in gredient. Thus, among earthy minerals, the various colouring