vi PREFACE. the Editors trust that by ordinary attention on the part of the reader, they will not occasion any embarrassment when he compares them with the figures given by that author. In preparing this Work, the Editors have availed themselves of the opportunities afforded by specimens from different col lections, of remeasuring a great number of the crystals de scribed ; they have thus been enabled to correct several errors of form, and, having deduced the angles by calculation from the best observations, they believe that the forms and angles inserted in this work are as nearly accurate as they can at present be rendered. In the compilation of this volume, the Editors have relied chiefly on Zippe s edition of Mobs, and on Hausmann; but they have also availed themselves of such other sources of information as appeared best calculated to increase the accuracy of the descriptions. The analyses are almost entirely selected from Kammelsberg’s “ Handworterbuch des chemischen Theils der Mineralogie,” and the four Supplements to that work. In making the selection, the Editors have been guided by his criticisms, and have endeavoured to avoid encumbering the book, and misleading the reader by the insertion of analyses palpably inaccurate. They are also indebted to Professor Rammelsberg for an early communication of the results of his elaborate chemical examination of tourmaline subsequent to the appearance of the fourth supplement. x It has been the practice in other mineralogical treatises to omit all the optical characters of minerals except those of colour and lustre; but it has appeared to the Editors that some account of the characters and phenomena which depend upon refraction and polarization would confer a higher interest on the study of minerals, and they have accordingly inserted many notices of these curious properties. The geometrical expressions for computing the symbols of crystals, and the angles the faces make with each other, are more numerous than is usual in elementary treatises, and they will probably meet