It is thus obvious that we have now arrived at a stage in the history of crystallography when more experimental data, and many more measurements of the most carefully conducted character, on pure materials and excellently developed crystals, are most urgently needed, in order to decide these important, indeed fundamental, questions, the present state of which the author has endeavoured to present with judicial impartiality. When one looks around, and sees the almost complete lack of opportunities for the training of investigators in this rapidly growing branch of science, the importance of which to chemistry and physics is increasing every day, while the field is ripe for the harvesters, one is inclined to feel depressed with the thought of the opportunities which are being lost. Our country has, in this science at any rate, a fine record, having with few breaks led the van of progress from the time of Wollaston, the inventor in the year 1809 of the reflecting goniometer, and of Miller, the originator of our method of describing crystals and the pioneer of accurate experimental work, down to the present day. It may be, also, that our country’s reputation is safe at this moment. But it is in the hands of a band of investigators so small, and often of the private and not professional nature, carrying on the work for sheer love of it and deep interest in it, that the wonder is that so much has been done, and it is the provision for carrying on our national tradition of leadership in crystallography in the future that is a matter for the deepest concern. If the perusal of this book should have awakened sufficient interest in the minds of some of its readers to prompt them to offer themselves as recruits to this small band of investigators, and especially if it should have inspired the zeal and enthusiasm of a few young students looking around for a promising and fascinating field of work, and, finally, if it should prove to be of assistance in obtaining the means of training such recruits with the help of the best and most accurate experimental apparatus which can be obtained, the author’s main objects in writing it will have been attained. INDEX Airy’s spirals, 197 Alpine snow-field, 49 Alum, ammonium iron, 43; cæsium, 42; potash, 5, 16, 17, 77; vicinal faces of potash, 248, 251 Amethyst quartz, 222–229; as example of pseudo-racemism, 231, 232 Ammonium chloride labile crystallisation, 248; iron alum, 43; NH_{4} group, and its isomorphism with alkalies, 82, 83, 131; oleate, liquid crystals of, 259–263; oleate, twins of, 262 Ampère’s researches on ammonia, 83 Anatase, crystal of, 37 Antimony oxide, dimorphism of, 87, 88 Apatite, crystal of, 37 Aragonite, biaxial interference figure of, 189, 190 Armstrong and Pope on sobrerol, 153 Arsenic oxide, crystals of, 5, 88 Astatic systems of molecular magnets of Lehmann, 271, 272 Asymmetric carbon atom, 144, 145