Volume Xcviii.

DIRECT REPRODUCTIONS OF AUTOCHROME PHOTOGRAPHS OF SCREEN PICTURES IN POLARISED LIGHT.

FIG. 90.—Screen Picture in Polarised Light, with Nicols crossed, of a thick Plate perpendicular to the Axis of a naturally twinned Crystal of Quartz, the left half being of right-handed Quartz and the right half of alternately left and right-handed Quartz, the Planes of Demarcation being oblique to the Plate. ]

FIG. 97.—Crystals of Benzoic Acid in the Act of Growth, as seen on the Screen in Polarised Light with crossed Nicols. ]

THE INTERNATIONAL SCIENTIFIC SERIES

CRYSTALS

BY A. E. H. TUTTON

D.Sc., M.A. (NEW COLLEGE, OXON.), F.R.S.

VICE-PRESIDENT OF THE MINERALOGICAL SOCIETY MEMBER OF THE COUNCILS OF THE CHEMICAL SOCIETY AND THE BRITISH ASSOCIATION FOR THE ADVANCEMENT OF SCIENCE

WITH 120 ILLUSTRATIONS

LONDON KEGAN PAUL, TRENCH, TRÜBNER & CO. L^{TD} DRYDEN HOUSE, GERRARD STREET, W. 1911

PREFACE

The idea underlying this book has been to present the phenomena of crystallography to the general reading public in a manner which can be comprehended by all. In the main the sequence is that of the author’s evening discourse to the British Association at their meeting at Winnipeg in the summer of 1909. It is hoped, however, that the book combines the advantages of sufficient amplification of the story there told to make it an adequately detailed account of the development of the subject, and of the immense progress which has been made in it during recent years, with a full description of the numerous experimental illustrations given in the lecture, involving some of the most beautiful phenomena displayed by crystals in polarised light. Such an account has not been otherwise published, the brief abstract appearing in the Report of the British Association for 1909 giving no account of the experiments, which were a feature of the lecture, owing to the employment of a fine projection polariscope of more or less novel construction, and including two magnificent large Nicol prisms, a pair of the original ones made by Ahrens. The author has been frequently requested to publish a fuller account of this discourse, and as the general plan of it so fully embodies the present aspect of this fascinating science, it was determined, when invited by the publishers to write a generally readable book on “Crystals,” to comply with these requests.

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

VI. The Distribution of Crystal Faces in Zones, and the Mode of Constructing a Plan of the Faces 60

VII. The Work of Eilhardt Mitscherlich and his Discovery of Isomorphism 70

VIII. Morphotropy as distinct from Isomorphism 98

IX. The Crystal Space-Lattice and its Molecular Unit Cell. The 230 Point-Systems of Homogeneous Crystal Structure 111

X. Law of Variation of Angles in Isomorphous Series. Relative Dimensions of Unit Cells. Fixity of Atoms in Crystal 121

XI. The Explanation of Polymorphism and the Relation between Enantiomorphism and Optical Activity 133

XII. Effect of the Symmetry of Crystals on the Passage of Light through them. Quartz, Calcite, and Gypsum as Examples 162

XIII. Experiments in Convergent Polarised Light with Quartz, as an Example of Mirror-Image Symmetry and its accompanying Optical Activity 183

XIV. Experiments with Quartz and Gypsum in Parallel Polarised Light. General Conclusions from the Experiments with Quartz 201

XV. How a Crystal Grows from a Solution 236

XVI. Liquid Crystals 255

XVII. The Chemical Significance of Crystallography. The Theory of Pope and Barlow—Conclusion 283

Index 295

CRYSTALS

(INCLUDING LIQUID CRYSTALS)