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NA TURE 



[April 8, 19 -)9 



heading of meteorological optics — hales, the rainbow, 

 coronas, scintillation — are dealt with, although neces- 

 sarily very incompletely, yet almost too fully in view 

 of the character of the book. Indeed, although for 

 the first principles of the subject one is referred to 

 an elementary treatise (this volume opens with a 

 general exposition of the Gauss theory), its chief fault 

 lies in the extent of the ground covered. On every 

 topic it leaves the reader with a tantalising thirst for 

 more information. It is, perhaps, only on the theory 

 of caustics, here treated with exceptional fulness, 

 that one comes away satisfied. 



This is only to repeat that it is a text-book designed 

 for class use. Such a book, which treats the theory 

 from the point of view of a close interest in the 

 practical questions involved, is undoubtedly stimu- 

 lating and of high value in the hands of a capable 

 teacher. 



CHEMICAL CRYSTALLOGRAPHY. 



Chemische Krystallographie. By Prof. P. Groth. 



Vol. ii., Die anorganischen Oxo- und Sulfosalze. 



Pp. vii + 914; with 522 figures. (Leipzig: Wilhelm 



Engelmann, 1908.) Price 34 marks. 



WITH this, the second, volume. Prof, von Groth 

 completes that half of his great work which 

 deals with inorganic salts. The fact that it has ap- 

 peared within two years of the publication of the first 

 volume is, even when every allowance is made for the 

 assistance which we believe has been placed at his 

 disposal, eloquent testimony to the remarkable in- 

 dustry displayed by the author. To absorb, digest, 

 and arrange in orderly sequence such a mass of data 

 is a gigantic task, and such rapid progress demands 

 unremitting labour and indomitable perseverance. To 

 the great services rendered by Prof. Groth to 

 mineralogy and crystallography, and to those pre- 

 eminent qualifications which mark him out as the 

 obvious man to plan and carry through this important 

 work. Dr. Tutton, in writing of the opening volume 

 (Nature, 1907, vol. Ixxv., p. 529), has referred in 

 graceful and felicitous language. The present writer, 

 who was privileged to serve his novitiate in miner- 

 alogical science in Prof. Groth's laboratory at Munich, 

 feels it would be presumption on his part to add 

 anything to those words beyond his cordial agreement 

 with them. 



To state that Prof. Groth's " Chemical Crystallo- 

 graphy " meets a long-felt want is but a trite and 

 inadequate way of expressing the situation. For years 

 past, students of crystallised substances which are 

 known to occur in nature have, in the well-known 

 and invaluable " System of Mineralogy," which Prof. 

 E. S. Dana prepared as the sixth edition of his 

 father's successive treatises on mineralogy, had before 

 them a coherent arrangement of minerals based upon 

 their chemical and crystallographical properties, and 

 they could readily ascertain what precisely was known 

 with regard to the crystalline characters of any 

 species. Prof. Groth himself has provided an admir- 

 able bird's-eye view of the grouping of minerals in 

 his handy " Tabellarische Ubersicht der Mineralien," 

 NO. 2058, VOL. 80] 



and Prof. C. Hintze is rapidly nearing the final parts 

 of his exhaustive " Handbuch." But the researches 

 of chemists in the laboratory have brought about the 

 formation of a vast number of crystallised substances 

 which have never been found in nature, mostly 

 because of their want of durability for some reason 

 or other, and every year the need for a work that 

 should group together all known crystallised sub- 

 stances, however formed, and give full details of their 

 physical characters, has grown more urgent. 



The general arrangement of the substances is exactly 

 the same as that devised by the author in the 

 " Tabellarische Ubersicht " mentioned above. The 

 nomenclature is chemical, but the mineral names of 

 the natural species are given in brackets. For each 

 species are given as far as possible the physical 

 characters, viz. the specific gravity ; the morphological 

 constants — the axial ratios and the interaxial angles 

 when differing from right angles — the type of crystal- 

 line symmetry, the mode of twinning, the directions 

 of cleavage, and the indices of the forms which have 

 been observed ; the optical characters, including the 

 principal indices of refraction for light of certain 

 standard wave-lengths, the orientation of the optica! 

 indicatrix with regard to the crystal, the angle 

 between the optic axes in the case of biaxial crystals, 

 and occasionally the alteration in these constants 

 caused by a rise of temperature. For artificial salts 

 and mineral species recently discovered the informa- 

 tion is somewhat amplified ; the calculated and 

 observed values of the principal measured angles are 

 quoted, and illustrations of typical crystals are added, 

 the authority for the determinations and the referencf 

 to the original paper being always stated. Dr. 

 Tutton 's classical researches upon certain salts of the 

 alkali metals, potassium, rubidium, caesium, and 

 thallium, and of the ammonium radicle, may be cited 

 as examples of an ideal crystallographical investiga- 

 tion ; for the care in assuring purity of material, the 

 high standard of the apparatus used, the pains taken 

 in the observations, and the completeness of the deter- 

 minations, they stand alone. But although it is rarely 

 possible to obtain crystals large or perfect enough for 

 such accurate work, it is not too much to say that at 

 least the morphological constants of every crystallised 

 substance should be determined, since such a deter- 

 mination, even when the dimensions of the crystals do 

 not exceed half a millimetre, presents no serious 

 difficulty with the instruments now available. 



Prof. Groth prefaces each group with an introduc- 

 tion, in whicli, with his customary clearness of 

 exposition, he discusses the relations subsisting 

 between the component members, and indicates gaps 

 in the data or doubtful points which call for further 

 investigation. These illuminating discussions add 

 greatly to the value and importance of the work, and 

 impart to it an interest and a fascination that would be 

 wanting in a dry compendium of figures and facts. 



It would be impossible within the limits at our dis- 

 posal to attempt any detailed discussion or give more 

 than a broad outline of the contents of the present 

 volume. It is devoted to the o.xo- and sulpho-salts, 

 and includes such important groups as cyanates. 



