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VIII PREFACE TO THE adopted it as being in my own estimation the most advantageous to the student that I could devise, the experience of its utility now induces me to recommend it to him as an instructive method of placing the minerals in his cabinet. In pursuing the at once pleasant and laborious investigations connected with the important characters of cleavage, crystalline form, and measurement, and which were undertaken with the view of rendering the present edition more instructive to the student, it will be imagined that I have myself derived much information ; and, although some new facts relative to these points have resulted, it must be acknowledged that much yet remains for future investigation. If the more accomplished mineralogist should condescend to consult this little work, he will perceive that the measurements of the crystalline forms, and especially of the secondary planes, are not precisely exact, do not on all occasions relatively agree ; for in no instance has it been attempted to correct the geometry of nature by a resort to the more rigid laws of calculation. It has been ascertained, by a comparison of the measurements taken from similar and brilliant planes of diiferent crystals, that, owing to some natural inequality of surface, the same pre cise angle is rarely obtained, and bence those given in the suc ceeding pages cannot be expected to be absolutely exact. Ex perience, however, leads to the conclusion that the limit of error is considerably within one degree,—that it rarely exceeds 40 minutes, and that it is frequently confined to a minute or ttvo. The measurements annexed to the figures will therefore be con sidered only as near approximations to the true value; but where those of the primary form have been obtained from planes produced by cleavage, which is generally noted, when that is the case, in the description of the mineral, they may be con sidered as approximating the truth much more nearly than when taken by means of the natural planes. A considerable propor tion of the whole will perhaps be found sufficiently precise to form a basis for the calculations of the mathematician, and, together with the accompanying figures, to induce the student to examine the forms of crystals, and to delineate and measure the angles formed by the meeting of the planes by which the crystals in his own cabinet are bounded. If errors should be found in the following pages, greater than those above alluded to, they are to be attributed to my own want of exactness in noting the measurements obtained; for, although much care has on all occasions been taken to select the smallest and most brilliant crystals, and to note the results faithfully, it is scarcely to be hoped that errors of this nature have altogether been avoided. It may perhaps be concluded, that by adopting at once the