JULY 27, I916] 



NATURE 



451 



7, pp. 1-53J, under the title " Les cristaux de 

 ^ ^e," an interesting resumd of what is as yet known 

 of this the commonest of substances, in which he 

 points out the lacunae that remain and the questions 

 that are still unsettled. He has himself studied no 

 fewer than 3000 photomicrographs of actual ice 

 crystals under the microscope, the photographs being 

 taken from the fine collections formed by A. W. 

 Bentley, G. Nordenskiold, and F. Hallberg. There 

 appear to be three different types of habit of natural 

 crystals of ice, viz. lamellar, rod-like, and acicular, 

 of which the first Is by far the commonest. The 

 author hazards the suggestion that the other two 

 result from the transitory presence in the air of 

 certain unstable gases, but hesitates to say which. 

 From a study of the tapering, rod-like crj'stals he 

 concludes that ice belongs to the tourmaline class 

 of the hexagonal system, which is characterised by a 

 trigonal polar axis of symmetry. No measureniients 

 which will permit of the determination of a satisfac- 

 tory value for the ratio of the crystallographical axes 

 have yet been published; that quoted in the text- 

 books is based upon some extremely rough observa- 

 tions made by Nordenskiold, and is quite untrust- 

 worthy. In the rod-like and acicular types twinning 

 about 0001 is common, as is shown by the existence 

 of groups in which two tapering ends are aligned 

 in contact. It is well known that laboratory experi- 

 ments have produced different kinds of crj'stals of 

 ice. For instance, water when containing more than 

 50 per cent, of alcohol forms cubic crystals on freezing. 

 For a smaller percentage such crystals, if formed, 

 are unstable, and their occurrence in Nature would 

 therefore appear doubtful. Certain photomicrographs 

 taken by Bentley and by Hallberg appear to suggest 

 cubic svmmetr>-, but. failing optical tests, it remains 

 uncertain whether they may not be merely distorted 

 forms of the ordinarj' type of crystals. 



The following volumes are announced for early 

 publication in the " University- of Chicago Science 

 Series " by the University of Chicago Press (London : 

 Cambridge Universitv Press) :—•• The Origin of the 

 Earth," T. C. Cha'mberlin ; '"The Isolation and 

 Measurement of the Electron," Prof. R. A. Millikan ; 

 and "Finite CoUineation Groups," Prof. H. F. Blich- 

 feldt. Other volumes in preparation for the same 

 series are: — "The Evolution of Reptiles," S. W. 

 Williston; "Food Poisoning," E. O. Jordan; "The 

 Problem of Individuality in Organisms," C. M. Child; 

 "The Development of a New System of Organic 

 Chemistrv, based on Dissociation Concepts," J. U. 

 Nef and J. VV. E. Glattfeld; "The Living Cycads," 

 C. J. Chamberlain ; '• Mechanics of Delaved Germina- 

 tion in Seeds." W. Crocker; "The Rigidity of the 

 Earth and of Materials," A. A. Michelson ; "The 

 Problem of Fertilization," F. R. Lillie; and "Linear 

 Integral Equations in General Analysis," E. H. 

 Moore. 



Messrs. Const.able and Co.. Ltd., announce the 

 following books of science: — "The Flying Machine 

 from an Engineering Standpoint," F. W. Lanchester 

 (the James Forrest Lecture, 19 14. including a discus- 

 sion concerning the Theory of Sustentation and the 

 Expenditure of Power in Flight) ; " Some Modem 

 Methods of Ventilation, with Special Reference _ to 

 Public Buildings," R. Grierson, illustrated; "Mining 

 andMineVentilation," J. J. Walsh, illustrated; "Prac- 

 tical Surveying," E. McCullough, illustrated; "Colour 

 and its Applications," M. Luckiesh, illustrated; 

 "Atoms," J. Perrin, translated by D. L. Hammick. 

 illustrated; and a new and enlarged edition of 

 "Manual of Reinforced Concrete," C. F. Marsh and 

 W. Dunn, 



NO. 2439, VOL. 9>] 



OUR ASTRONOMICAL COLUMN. 



Po.vs-Winnecke's Comet and the Meteoric Shower 

 OF June 28. — Mr. Denning writes : — " That the remark- 

 able display of June 28 was due to the earth passing 

 through or very near a cometar)' orbit appears highly 

 probable. The elements of the meteoric shower present 

 some resemblances to those of Pons-Winnecke's comet 

 of 1819, which has a period of about 58 years, and 

 last reached perihelion on September i, 1915. Any 

 meteoric shower connected with Pons-Winnecke's 

 comet, though not visible in past years, may well be 

 perceptible in future times. In 1869 the perihelion 

 distance was 0-7815, or about 2 15 millions of miles inside 

 the earth's orbit, but in 1915 the P.D. had increased 

 to 09725, or only 4 millions of miles inside our orbit. 

 Planetary- perturbations have effected changes in the 

 orbital elements of the comet, and- brought it so near 

 to us at one section that a meteoric rencontre seems 

 very likely." 



Differential Measurement. — Mr. H. H. Plaskett 

 has made an interesting study of some questions in- 

 volved in measures of the distance between a pair of 

 lines with the object of tracing the origin of differ- 

 ences found when different observers measure the 

 same spectrograms, with special reference to spectro- 

 graphic determinations of the solar rotation (Journal 

 Roy. Ast. Soc. of Can., vol. x.. No. 5). He finds the 

 chief source of these differences is the " attitude " of 

 the observer, and defines two modes of measurement, 

 the "attentive" and the "automatic," according as 

 the measures are made under the influence of pre- 

 possessions or otherwise. It must suffice to add that the 

 automatic mode is found to possess the desirable ad- 

 vantages of speed, accuracy, and economy of effort, but 

 is marred by high fortuitous error (apparently, greater 

 p.e.). Mr. Plaskett is of the opinion that the highest 

 accuracy can only be attained in replacing differential 

 measures by determinations of changes of wave- 

 length with the help of standard absorption lines. A 

 very successful application of these results has already 

 been made at Ottawa (Nature, May 18). 



Wave-lengths in the Iron Spectrum. — Interfer- 

 ometer measures of the wave-lengths of some 400 lines 

 in the spectrum of the iron arc in the region covered 

 by the international secondary' standards have been 

 made by Messrs. Burns, Meggers, and Merrill in 

 continuation of the revision of wave-lengths under- 

 taken at the L'nited States Bureau of Standards 

 (Scientific Paper No. 274). The poles used were either 

 of electrolytic or Norwegian iron, and only iron lines 

 were measured. The reductions were based on the 

 international secondary standards, and the final wave- 

 lengths were corrected by means of a smoothed 

 curve obtained by plotting the differences between 

 observed and normal wave-lengths of the standard 

 lines. The mean difference is stated to be about one 

 part in four millions. Three interferometers have 

 been used in measuring each line, and, in addition to 

 the work on wave-lengths, considerable attention has 

 been devoted to observations of the physical character- 

 istics of the lines. Thus, more than five hundred 

 lines have been divided into four groups according 

 to the limiting path difference at which interference is 

 shown, and the data have been examined in connec- 

 tion with pole-effect, intensity, and pressure shifts. 

 It appears that lines showing negative pole shift are 

 never sharp; lines of faint or moderate intensit\* are 

 sharper than strong lines, whilst the lines shifted 

 by pressure are more likely to be broad than un- 

 affected lines. 



