192 



KNOWLEDGE. 



June, 1915, 



become established on a finn, experimental basis. Thf book 

 under review brings forward more evidence to confirm the 

 entity of individual atoms. The two component branches 

 of this book arc complicated, but i'rofessor Bragg's well- 

 known ability of opening up abstruse avenues of knowledge 

 to those who arc not specialists loads us to expect a volume 

 containing much information brought together in an 

 agreeable manner. This expectation has been fully realised. 

 \Vhile the niathcmatical side of the work is not unduly 

 prominent, enough has been introduced to render the facts 

 easily comprehensible. 



The work owes its initial stages to the researches of Lauc, 

 Friedrich, and Knipping, who not only demonstrated that 

 X rays were propagated by means of waves similar to light- 

 waves, but also were able to show that the space-lattice 

 arrangement of particles in crj-stals could act as a grating 

 for tlie diffraction of x rays. Although mathematical 

 difficulties have hindered the practical development of 

 the original procedure, a simplified two-dimensional method 

 devised by the authors has proved very fruitful. The 

 essentially new idea of the Bragg method is due, according 

 to the preface, solely to W. L. Bragg, and is the conception 

 of the " reflection " of x rays. This reflection differs from 

 ordinary reflection of light from a polished surface in that 

 it arises from the diffraction of the rays by the successive 

 planes of particles in a crj'stal. The results are interpreted 

 by means of the very simple expression n\=2dsm 6, where 

 X is the wave-length of the " monochromatic " x rays, 

 n a whole number giving the " order " of the spectrum 

 (analogous to the spectra given by a line grating), d the 

 distance between two successive parallel planes of atoms in 

 the crj'stal, and e the angle between these planes and the 

 incident stream of x rays. Where a number of spectra can 

 be obtained, it is obvious that the measurement of 6 leads 

 to the determination of d and X, the latter being of the 

 order of 10-* centimetres. 



Perhaps the most generally interesting feature is the 

 elucidation of the internal structure of a number of crystals. 

 A comparison of the spectra obtained from sylvine and rock- 

 salt indicates the degree of intimacy with w'hich the crystal 

 structure may be explored. Sylvine gives spectra which 

 show that the structure may be referred to the simple cube- 

 lattice, while, on the other hand, rock-salt must be referred 

 to the face-centred cube-lattice. This observed dissimilarity 

 in the case of two isomorphous substances can only be 

 explained by assuming that it is the atoms, and not the 

 molecules, which act as diffracting centres, and that the 

 scattering power varies as the atomic weight. Further, 

 since a structure formed by the interpenetration of two 

 face-centred cube-lattices would reduce to a simple cube- 

 lattice when the two atoms are of similar mass, the appar- 

 ently simple structure of sylvine is due to the approximate 

 equality of the atomic weights of K and CI. The second 

 assumption finds verification in the case of the rhombohedral 

 carbonates, where the first spectrum for the octahedral 

 faces diminishes as the atomic weight of the metal increases, 

 until in the manganese and iron salts, where the mass of the 

 metal is approximately equal to that of the CO, group, the 

 first spectrum vanishes. 



It is interesting to note, in connection with the Pope- 

 Barlow theory, that, while the structure of copper conforms 



to the closest packed system of polyhedra, that of the diamond 

 is represented by a very loosely packed system. In the latter 

 case the structure assigned by the authors reconciles the 

 conflicting evidence regarding the existence of polarity in 

 the crystal. 



The utility of the Laue method in rendering the quanti- 

 tative nature of the Bragg method more accurate is exem- 

 plified in the case of iron-pyrites, where the iron atoms are 

 arranged on a lace-ccntrcd cube-lattice, the sulphur atoms 

 lying unsymmctrically on a system f)f non-intersecting 

 trigonal a.xes. The sulphur atoms divide the latter in the 

 ratio 4 : 1 according to the spectrometer results, and in the 

 ratio 776 : 224 according to the Laue method. 



Although this work has proved exceedingly fruitful, it is 

 evident that many experimental difficulties remain to be 

 overcome, particularly where the number of different atoms 

 in llic molecule exceeds three. So far as it has gone, however, 

 the results afford striking confirmation of the structural 

 theories of Federov and Schoenflies. 



A mild criticism might be offered regarding a nomen- 

 clature which applies optical terms, such as " monochro- 

 matic " and " white radiation " to denote radiations of 

 homogeneous and heterogeneous wave-lengths respectively. 



The letterpress, though generally excellent, is marred by 

 a lew arithmetical misprints, and wc also note a mistake 

 on page 85, where more care might have been taken in 

 deahng with isotopes. A. S. 



A. F. 

 NATURAL HISTORY. 



Wonders of Wild Nature. — By Richard Kearton. 174 



pages. 72 plates, of which four are in colour. 8-in. xSJ-in. 



(Cassell & Co. Price 6/-.) 



jNIuch of Mr. Richard Kearton's work has been upon the 

 rarer of our British birds, which are generally to be looked 

 for in the least accessible and distant parts of these islands. 

 His present book deals, to a considerable extent, with 

 natural history near London ; while chapters are devoted to 

 the birds of Holland and Norway. In the first-mentioned 

 country some interesting British birds and visitors, hke 

 the spoonbill and purple heron, were met with and photo- 

 graphed ; while in Norway, while the same held good, 

 several common British migrants, which do not breed with 

 us, were photographed on their nests. There is plenty of 

 interesting incident in the letterpress. 



Mr. Kearton's readers will remember his taking photo- 

 graphs from inside an artificial cow ; but in Norway, when 

 getting photographic records of redwings' nests, he was 

 attacked by a very real bull. Those who are engaged in the 

 workof bird-protection will beglad to hear that the Sanctuary, 

 established for the great skua in the Shetlands some fifteen 

 years ago, has been most successful. The way in which Mr. 

 Kearton persuaded a hen Dartford warbler to alight on his 

 hand to feed one of her fledglings may also be mentioned. 

 We think that Mr. Kearton's book shows that there is still 

 a \ast amount of natural-liistory material to draw upon 

 for the benefit of nature-lovers and the public. We are 

 kmdly permitted to reproduce two of the illustrations, of 

 which there are seventy-two from photographs by the 

 author and his daughter, Miss Grace Kearton (see Figures 

 168 and 169). W. M. W. 



NOTICES. 



CUMBERLAND NATURE RESERVE ASSOCIATION. 

 — The Secretary of the Cumberland Nature Reserve 

 Association, Mr. Linnaeus E. Hope, F.L.S., of the Municipal 

 Museum, Carlisle, appeals for subscriptions to the watchers' 

 fund. In 1914, the first year of its existence, the Association 

 superNdsed several eyries of the peregrine falcon, the buzzard, 

 and the raven, from which all the broods were successfully 

 reared. It also provided funds for the Kingsmoor Nature 

 Reserve, CarUsle, where the local Committee has secured 

 this piece of common land as a nature reserve. 



PHOTO-MICROGRAPHY.— We have received from 



Messrs. Kodak, Limited, the third edition of an introduction 

 to photography with the microscope, which was originally 

 brought out in 1907, when Messrs. Wratten & Wainwright 

 prepared a set of colour filters for use in photo-micrography. 

 As is well known, ^Messrs. Kodak have taken over the work 

 of the last-mentioned firm, which now forms their Wratten 

 division. The advantages which arc gained by use of the 

 filters, especially in conjunction with the special M plate, 

 are very great, as we shall show in uur next number 



