May 15. 1913] 



NA'l URE 



267 



the present stale of speculation and hypothesis, 

 and hopes for the coming of a great mind which 

 will unify the whole. 



Luftelektrizitat. By Dr. Karl Kahler. Pp. 151. 

 (Berlin and Leipzig : G. J. Goschen'sche Ver- 

 lagshandlung G.m.b.H., 1913.) Price 90 

 pfennigs. 

 This is one of a long series of cheap, instruc- 

 tive books published by the firm of G. J. Goschen. 

 The principal contents are the earth's potential 

 gradient, forty-four pages ; the electric con- 

 ductivity of the atmosphere, thirty-five pages ; 

 electric currents in the air (including the ordinary 

 lair-weather vertical current, electricity brought 

 down by rain and snow, and lightning), twenty- 

 nine pages ; and the radio-active phenomena of 

 the atmosphere, twenty-eight pages. Two other 

 shorter sections deal respectively with the electric 

 effects of sunshine and theories as to the source 

 <>l atmospheric electricity. There are eighteen 

 figures in the text, including some interesting 

 Potsdam records of potential gradient during calm 

 and disturbed weather. The author is a member 

 of the staff of the Royal Meteorological-Magnetic 

 Observatory at Potsdam, and is a recognised 

 expert on the subject of which he treats. Con- 

 sidering its size, the book gives an excellent 

 account, clear as well as concise, of the whole 

 subject. German results loom somewhat more 

 largely than they probably would in a text-book 

 writ ten in France or England, but there are a 

 good many references to non-German writers, 

 including Chauveau, Simpson, and C. T. R. 

 ■\Yilson. 



Lecons sur les Hypotheses Cosmogoniques pru- 

 lessdes 4 la Sorbonnc. By H. Poincare. 

 Edited by H. Vergne. Pp. lxx + 294. Second 

 edition, with a Portrait and a Memoir on H. 

 Poincare by E. Lebon. (Paris : A. Hermann 

 et Fils, 191 3.) Price 12 francs. 

 The first edition of this work was given an ex- 

 tended notice in the issue of Nature for May 2, 

 1912 (vol. lxxxix., p. vi). The present issue has 

 been enriched by a portrait of Prof. Poincare, and 

 by the inclusion of a biographical notice and critical 

 estimate of the eminent savant's work by M. E. 

 Lebon, who has in addition made a few necessary 

 corrections in the text. 



A Manual of Agricultural Chemistry. By H. 



Ingle. Third edition. Pp. vii + 397. (London: 



Scott, Greenwood and Son, 1913.) Price 75. 6d. 



net. 



The first edition of Mr. Ingle's book — reviewed in 



the issue of Nature for July 10, 1902 (vol. lxvi., 



p. 245) — dealt with the chemistry and physics of 



subjects relating exclusively to English agricul-' 



ture. In the present edition, however, reference 



has been made to the chemistry of crops of tropical 



and sub-tropical countries, as well as to questions 



of stock-feeding in other lands. In addition, the 



book has been revised, and to bring it up to date 



some portions have been re-written. 



NO. 22;2. VOL. qi] 



LETTERS TO THE EDITOR. 

 [The Editor does not hold himself responsible for 

 opinions expressed by his correspondents. Neil her 

 can he undertake to return, or to correspond with 

 the writers of, rejected manuscripts intended for 

 this or any other part of Nature. No notice' is 

 taken of anonymous communications.] 



Some Phenomena Connected with Reflected X-Rays. 



The diagrams shown in Figs. 1 and 2 represent 

 photographs obtained when X-rays were reflected on 

 rock-salt and quartz. The spots lying on the left 

 of the vertical line are due to the rays which have 

 passed directly through the crystal, while those on the 

 right are produced by reflected rays. It is easy to 

 see from Fig. 2 that there are five spots due to' the 

 impact of the reflected rays. The middle spot and 

 the two on the outside are more intense than the 

 two others. The optical axis of the quartz specimen 

 lies in this case in the plane of incidence forming an 

 angle of 1' with the surface of the crystal. 



An explanation of the different spots of reflection 

 shown in Fig. 2 can be given with the help of Fig. 3, 



which is meant to show a cross-section of a hexagonal 

 crystal cut at right angle to the optical axis. The 

 points of intersection of the lines drawn in full may 

 represent the positions of the molecules in the crystal. 

 Considering this figure we can assume that the 

 more intense rays are reflected by the layers which 

 are parallel to the lines drawn in full, and the weaker 

 ones by layers which are parallel to the dotted lines. 

 It is obvious that in the first case the molecules lie 

 closer together than in the second. 



In addition, in Figs. 1 and 2 there are seen a series 

 of lines which seem to converge towards the points of 

 impact of the reflected rays, and are distributed 

 in a way which is very similar to a spectrum obtained 

 with visible light by means of two crossed gratings. 

 Provided that in our case the phenomenon is due 

 to an effect similar to that of crossed gratings, the 

 directly reflected rays must be regarded as spectra of 

 the order zero. On account of the diffusion of the 

 lines, however, it is not possible at present to deduce 

 from this the wave-length of the X-rays. 



E. Hupka. 



Phvsikalisch-teehnische Reichsanstalt, 

 Charlottenburg, April T2. 



