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THIRD EDITION. IX figures and measurements given by Ilaiiy, and other mineralo gists, much chance of inaccuracy might have been prevented. But it must be observed, that where the primary form is not a regular geometrical solid, such as are the cube, regular octa hedron, and six-sided prism, the means resorted to for deter mining the true measurements—namely, that of subjecting the planes obtained by cleavage to the reflective goniometer—is a more certain method than that adopted by Haiiy; and it will be perceived that a very large proportion of all the primary forms are not regular geometrical solids, such, for instance, as * the oblique and doubly oblique prisms, and the very numerous class of rhombic prisms. Where the cube, regular octahedron, six-sided prism, and other regular solids, are the primary forms, I have adopted the measurements given by Haiiy, acknowledg ing them in all cases by annexing the letter H, or by some other mode; first, however, verifying them in most cases by the re flective goniometer. In a very few instances, the authority of the Comte de Bournon has been resorted to, but not without ac knowledgment. It will of course be understood that where no authority is mentioned the measurements have been obtained by the reflective goniometer, and, from what is said above, that they must be considered only as approximations.* In regard to the figures to which the measurements are an nexed, it may be observed, that these are not in all cases the representatives of single crystals, for in some of them are asso ciated the planes observed on two or three; thus occasionally rendering the form more complicated than any single crystal I have seen, but not more so than may probably be found here after. This mode has been adopted, as offering to the student the greatest assistance that I could devise, since it combines at one view all the observed planes, without increasing enormously the bulk and consequent expense of the work, as must have been the case if all the varieties of form had been given sepa rately. As to the drawing of the figures, it remains to be add ed, that they are not given as the result of a laborious exe cution by the assistance of the rules of geometrical projection ; but, in the general, only as diagrams, wanting the precision which in that case might have been claimed for them, and drawn without any other rule than such as the hand and eye could furnish. The letters on each plane of the larger figures have been so * In some, though comparatively few instances, the crystals of a sub stance have not been found sufficiently bright for the use of the reflec tive goniometer; the common goniometer has then been resorted to, and the measurements taken by it are always distinguished by having the letters c. g. annexed to them.