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Rh together. But in the meantime we are still ignorant in what measure the optical properties of minerals depend either on their physical or their chemical nature: so that we may have substances, not externally distinguishable from Herschel's Leucocyclite or Brewster's Tesselite, and yet, we shall not be able to tell whether we are to call them by such names, till we have subjected them to the very optical experiments by which the phenomena are elicited. And if we find that they are thus, by definition, Leucocyclite and Tesselite, we shall still be ignorant whether our specimens agree in chemical composition with those which suggested the names to Herschel and to Brewster. Such are the inevitable embarrassments which arise from defining without possessing a system; from naming objects without knowing their relation to other objects.

If we want decisive evidence of the way in which the possibility of good names necessarily implies much previous knowledge, we may find such evidence in the progress of Geology. The terms now used in that science, to designate the various strata, albeit harsh and rugged in many instances, are of signal use and value, because they express the result of a laborious examination and classification of the real materials of the earth. Gneiss and killas, coral rag and cornbrash, are of service in enunciating intelligible general propositions with regard to the structure of this and other countries; and therefore sound harmonious to a philosophical ear. And their music is but little impaired by the consideration that they are not susceptible of exact definition; or that the literal meaning of the terms used does not suggest the most characteristic attributes of the thing signified. At one of the meetings of the Geological Society of London, a memoir was read on "The Green Sand" by an eminent member of the Society. At these meetings, the readings are followed by oral discussions, usually conducted with a rare mixture of acuteness and good breeding. On the occasion just mentioned, a distinguished geologist, well known both for the extent of his knowledge and the fastidiousness of his taste, stated that he had three objections to the Title of the paper:—First, to the article The, since there are several green sands: second to the adjective Green, since the stratum spoken of is more commonly red: third to the substantive