There is also included an account of the remarkable work of Lehmann and his fellow workers on “Liquid Crystals,” and the bearing of these discoveries on the nature of crystal structure is discussed in so far as the experimental evidence has gone. Similarly, the theory of Pope and Barlow, connecting crystalline structure with the chemical property of valency, is referred to and explained, as this theory has called forth deep and widespread interest. In both cases, however, the author has been careful to avoid any expression of opinion on purely theoretical questions for which there is as yet no definite experimental evidence, and has confined himself strictly to indicating how far such interesting theories are supported by actual experimental facts. No forbidding mathematical formulæ and no unessential technical terms will be found in the book, the aim of the author being to make any ordinarily cultured reader feel at the conclusion that the story has been readily comprehensible, and that crystallography is not the abstruse and excessively difficult subject which it has so generally been supposed to be, but that, on the contrary, it is both simple and straightforward, and full of the most enthralling interest, as well for the exquisite phenomena with which it deals, as for the exceedingly important bearing which it has on the nature, both chemical and physical, of solid matter. If any of its readers should be so impressed with the value of work in this domain of science as to be desirous of joining the very thin ranks of the few who are engaged in it, they will find a guide to practical goniometry and to the experimental investigation of crystals in all its branches and details, as well as the necessary theoretical help, in the author’s book on “Crystallography and Practical Crystal Measurement” (Macmillan & Co., 1911), and also an account of the author’s own contributions to the subject in a monograph entitled “Crystalline Structure and Chemical Constitution” (Macmillan & Co., 1910). A. E. H. TUTTON. _January 1911._ CONTENTS PAGE Preface v CHAPTER I. Introduction 1 II. The Masking of Similarity of Symmetry and Constancy of Angle by Difference of Habit, and its Influence on Early Studies of Crystals 10 III. The prescient Work of the Abbé Haüy 22 IV. The Seven Styles of Crystal Architecture 33 V. How Crystals are Described. The Simple Law limiting the Number of possible Forms 50