NATURE 



289 



THURSDAY, JANUARY 30, 1896. 



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GROTH'S CRYSTALLOGRAPHY. 

 Physikalische Krystallographie und Einleitung in die 

 krysiallographische Kenntniss der wichtigeren Sub- 

 stanzen. Von P. Groth. Dritte, vollstandtg neu bear- 

 beitete Auflage. Pp. 783. (Leipzig : Wilhelm Engel- 

 mann, 1894-5.) 



EARLY twenty years ago, in 1876, the present 

 writer was acting as Demonstrator in Physics at 

 j the Clarendon Laboratory, Oxford, and chanced to see 



on Prof. Clifton's table a book just published, which 

 differed in subject and treatment from any he had then 

 met with. It contained in small compass (523 pages) 

 that part of Physics which was more especially related 

 to crystallised matter, and without delaying the reader 

 with difficult or abstruse problems, gave in the most 

 fascinating way a general idea of the physical and 

 morphological symmetry of crystals. A few months 

 later, and as a direct consequence of the interest in 

 crystals thus aroused, the writer was advised to submit 

 himself to Mr. Maskelyne as a candidate for the 

 assistantship in the Mineral Department of the British 

 Museum, just vacated owing to health-failure by Mr. 

 Lewis, since appointed Professor of Mineralogy at Cam- 

 bridge. The book was the "Physikalische Krystallo- 

 graphie " of Paul Groth, and the present writer is glad 

 to have this opportunity, when giving a notice of the 

 third edition of this useful book, to put on record his 

 sense of gratitude to the autho*-, a feeling which is 

 doubtless shared by other crystallographic students in 

 this country. 



Prof. Groth was particularly well fitted for the work 

 which he had taken in hand ; he had already initiated 

 and organised the teaching of Crystallography and 

 Mineralogy at Strassburg (1870) ; gifted with great 

 enthusiasm, he had also the rarer power of inspiring his 

 students with the same quality ; in this book he published 

 (1876) the method which he had found to be practically 

 useful in the course of instruction given by him as 

 Professor of Mineralogy in that University. In the next 

 year (1877) the Zeitschrift fiir Krystallographie und 

 Mineralogie was started by Prof. Groth, who took upon 

 himself the arduous duties of editor ; this was the 

 beginning of a new era for the subject. The pages of 

 this periodical have since been at the service of crystallo- 

 graphic students of every country, and the twenty-four 

 volumes already issued are a rich storehouse of the 

 results of recent research. Succeeding von Kobell, Prof. 

 Groth afterwards migrated to Munich, and has since 

 completely reorganised the mineralogical collections and 

 teaching in that University. With twenty-five years' 

 experience as a University Professor, the editor of the 

 many path-breaking memoirs which have appeared in 

 the Zeitschrift could not fail, in the preparation of a new 

 edition of his book, to keep his readers in touch with all 

 that is best in the subject ; and in a living subject like 

 Crystallography, twenty years cannot pass away without 

 the discovery of new facts and new methods, and the 

 invention of new instruments of research. 



In the case of any scientific book which is worthy of 

 NO. 1370, VOL. 53] 



careful study, it must often be the fate of the student to 

 react upon the author ; and in this respect the present 

 treatise is not exceptional. The treatment of the dilatation 

 of crystals on change of temperature, as given in the first 

 edition of the work, appeared to be out of harmony with 

 the ideas there propounded as to crystal structure ; at 

 the suggestion of Mr. Maskelyne, the writer studied this 

 thermal problem from a theoretical standpoint, and re- 

 corded his results in a short paper. Attracted to the 

 subject once more. Prof Groth had the problem practi- 

 cally investigated by one of his students (Dr. Becken- 

 kamp) with the aid of the fine instrument which had then 

 been just made for the University of Strassburg. The 

 conclusions arrived at by Beckenkamp from his experi- 

 ments, whether strictly justifiable or not is a distinct 

 matter, were in complete agreement with those already 

 suggested on theoretical grounds, and the variation of 

 treatment which had been indicated was introduced by 

 Prof Groth in the second edition ; this, indeed, was the 

 only change of any theoretical importance at that time 

 made. The third edition is, however, so different from 

 those which have preceded it, that although the general 

 aim is unaltered, the book is virtually new in its mode of 

 treatment, and demands a more detailed notice than the 

 new edition of a work generally calls for. 



It may be remarked that the title (" Physikalische 

 Krystallographie ") suggests a more restricted survey 

 than is actually undertaken by the author ; for the 

 morphological being regarded as likewise physical 

 characters, geometrical crystallography falls within the 

 scope of the book : the title is intended to be comple- 

 mentary to that of another treatise (" Chemische Krystal- 

 lographie "), a work which Prof Groth has long had in 

 hand, and now looks forward to quickly completing. 



A statement of the numbers of the pages allotted to the 

 various parts of the subject will be useful as suggesting 

 the relative degrees of importance assigned to them. 

 The First Part begins with a discussion of the optical 

 characters of crystals, and extends to no fewer than 163 

 pages ; the thermal characters are allowed 24 pages ; 

 the magnetic and electrical 1 1 pages ; the action of 

 mechanical forces on crystals 43 pages ; and the mole- 

 cular structure of crystals 47 pages. The Second Part 

 is devoted to the geometrical characters of crystals, and 

 occupies 248 pages. The Third and last Part treats of 

 the calculation and graphic representation of crystal 

 forms (65 pages), of goniometers and refractometers (81 

 pages), and of polarising instruments (82 pages). 



There are three great changes introduced in the 

 present edition. In the first place, the optical part has 

 been re-modelled on the basis suggested in the Tract 

 on the " Optical Indicatrix," a work which was some time 

 ago noticed in these columns. As stated by Prof Groth 

 in his preface, " the optical part is no longer based on 

 the theory of an elastic ether, but on the purely geo- 

 metrical treatment proposed by Fletcher ; this method, 

 without involving any mathematical speculations what- 

 soever, suffices to give even for the most complicated 

 optical phenomena, such as conical refraction, that correct 

 insight which is indispensable to the student in his 

 microscopical investigations." The " Optical Indicatrix" 

 itself, it may be here observed, was written not so much 

 for the instruction of the ordinary student as to indicate 



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