520 



NATURE 



[July 17, 19 13 



volume between the atomic spheres. 5 As the spot 

 figure is holohedral it would appear to be due to the 

 space-lattices of similarly placed atoms of either (but 

 in each lattice only one) element, rather than to the 

 spheres of the combined system of atoms. 



This latter conclusion is further borne out by the 

 result of the new work by Laue on quartz. The 

 photograph now shown, so kindly sent by Prof. Laue, 

 exhibits the trigonal nature of the symmetry very 

 clearly, and Prof. Laue informs me that the same 

 figure is afforded by both right and left quartz, so 

 that it does not reveal the hemihedral character of 

 quartz, but possesses the full holohedral symmetry of 

 the trigonal space-lattice, and exhibits the threefold 

 nature of the axis of symmetry which is perpendicular 

 to the plate and along which the X-rays were directed. 



Prof. Laue has also experimented with the crystals 

 of a number of other cubic substances, and, like zinc 

 blende, they all show holohedral symmetry about a 

 tetragonal axis. 



W. L. Bragg has found that stronger photographs 

 of the same nature can be obtained from mica, using 

 nearly grazing incidence, and it is by use of this 

 fact that Mosely and Darwin have been able to study 

 the reflected rays electrically, and found them to 

 resemble ordinary X-rays. By the kindness of Mr. 

 Bragg, a diagram of his apparatus and a positive 

 lantern slide of one of his mica spot photographs are 

 exhibited on the screen. 



Incidentally these experiments appear likely to 

 throw light on the much-debated question of the 

 nature of the X-rays. As all the experiments unite 

 in indicating that a fraction of the X-rays suffers 

 reflection at the planes of atoms parallel to the more 

 important possible crystal faces, all being planes of 

 atomic points of the space-lattice, it would appear 

 that the X-rays are some type of wave-motion, or at 

 any rate some kind of pulse with an extended wave- 

 front. Vet after reflection they retain the same cor- 

 puscular character which Prof. W. H. Bragg has 

 shown they possess. For the liberation of a high- 

 speed electron from an atom traversed bv the X-ray 

 cannot be explained, according to Rutherford, unless 

 it be supposed that the energy of the X-ray is con- 

 centrated over a minute volume, and can be given 

 up in an encounter with a sintrle atom. Hence these 

 experiments show that the X-rays possess at the 

 same time the apparently opposite properties of ex- 

 tension over a wave-front and concentration in a 

 corpuscular point. 



It appears to the lecturer that the simpler explana- 

 tion is that we are truly dealing with waves, but 

 that the wave-lengths of the X-rays are excessively 

 short, approaching atomic dimensions, and that 

 the amplitude of the effective waves is actually 

 smaller than the reflecting atom. This view 

 that the X-rays are waves is further supported by 

 the results of some experiments just completed by 

 Barkla, in which a diverging pencil of X-rays was 

 directed on a crystal of rock-salt, and the issuing rays 

 received on a photographic plate in the same manner 

 as in the experiments already described. The de- 

 veloped plate shows a new phenomenon, namelv, 

 striation of the spots obtained bv reflection from the 

 planes of atoms of the space-lattice, especially in the 

 reflections from the cubic cleavaee planes. The 

 striations are, in fact, true interference bands, due to 

 interference of the reflections from eaually spaced 

 parallel planes of the space-lattice. By the kind 

 courtesy of Prof. Barkla, two of these interesting 



« Prof. T. W. Richard? shows (lite, clt.) how four molecules of ZnS, each 

 composed of an atom of zinc and an atom of sulphur of very d : fferent 

 volumes, can form the cuhic crystal unit of an edifice possessing cubic 

 systematic symmetry, the different volumes of thettwo kinds of atoms causing 

 it, however, to exhihit hemihedral class-symmetry, 



NO. 2 28l, VOL. 91I 



photographs are projected on the screen. On the 

 assumption that the X-rays are waves, and that the 

 reflecting plane is one passing through corresponding 

 portions ot single NaCl molecules — which agrees with 

 the choice of a representative point from each simple 

 molecular grosser unit, or of a similarly situated atom 

 of one of the two chemical elements present in each 

 molecule NaCl to act as such representative point 

 of the space-lattice — Barkla has calculated that the 

 wave-length is the one hundred and sixty millionth 

 of a millimetre, o-6xio~ s mm. If the grosser unit 

 be polymolecular, the wave-length works out larger, 

 being proportional to the cube root of the number of 

 atoms in the molecule. If eight molecules form the 

 grosser unit of sodium chloride crystals, as suggested 

 by some chemists, the wave-length is found by Barkla 

 to be twice this value, namely 12 x io -8 mm. ; and 

 if sixteen molecules of NaCl are comprised in the 

 grosser unit, as would be the case if Barlow and Pope's 

 structure for the cubic binary compounds be correct 

 (the space-lattice in the case of rock-salt being that 

 of the simple cube, No. ij, the wave-length would 

 be still longer, about the seventy millionth of a milli- 

 metre, 1-5 x io _s mm. Now it is very interesting that 

 these values are of the same order as those derived 

 from determinations of the velocity of electron 

 ejection, which varied from 1 to 2xio -8 mm. 



The most trustworthy recent estimations of the 

 size of a molecule of rock-salt indicate a diameter 

 about 3X io -7 mm. Hence the diameter of a crystal- 

 lographic molecule SNaCl would be 6xio -7 mm., 

 and of i6NaCl about 7-5 x io~ 7 mm. 



It should be emphasised, in concluding the account 

 of this fascinating new field of research, that all these 

 reflections occur in the body of the crystal, and are 

 not surface effects. Cleavage planes usually afford 

 stronger results merely because they are generally 

 primary planes of high reticular density. The effect 

 is sometimes heightened by conducting the X-rays at 

 nearly grazing incidence ; but this is by no means 

 necessary, and in Laue's experiments several of the 

 planes were inclined as much as 30 to the incident rays. 6 



The experimental proof of the existence of the 

 space-lattice imparts all the more confidence in ap- 

 proachine the other great advance which has lately 

 been achieved. The completion of the four-volume 

 catalogue of crvstallographicallv measured substances 

 by Prof, von Groth provokes the question : What 

 more is needed in order to enable a crystallised sub- 

 stance described in this book to be recognised by 

 means of a few measurements on the goniometer? 

 For it is now proved up to the hilt that, except in 

 the cases of cubic crystals identical in angles in 

 accordance with their perfect symmetry, everv solid 

 crystallisable substance is characterised bv its own 

 peculiar crystalline form and interfacial angles. This 

 is quite true, even to the last minute of angular 

 measurement, when the conditions of crystallisation 

 are ideal. When thus perfect, even isomorphous sub- 



with X-rays and 

 xci., pp. in. its. 



n'd 161). 



delivered, the following furth 

 ive been described in Natu 



H. B. Keene has obtained « 

 alogous results t 



corresponding 



ixpenments 

 1 Nature (igi3, vol. 

 ained with crystals of 

 of Laue. Friedrich. and 

 holohedral systematic 



symmetry in each case. T\ Terada has found that the transmitted rays may 

 be rendered optically visible by means of an ordinary fluorescent screen, 

 provided the pencil of ray* he from 5 to 10 mm. in diameter and the crystal 

 adequately transparent to the rays ; this latter he found to be the case with 

 crystals of alum, borax, cane-sugar, fluorspar, mica, rock crystal, aod rock 

 salt, in thicknesses of j to 10 mm. M. de Eroglie has obtained spot diagrams 

 similar to those of Laue. Friedrich, and Knipping with fluorspar, magnetite 

 (using an octahedron face), and rock salt ; but all the spots were striated 

 wiih parallel fringes Finally, Owen and Blake have obtained what appears 

 to be a line spectrum of X-rays by using the surface of a crystal of gypsum 

 as a diffraction grating. The lines were always the same with different 

 crystals, using the same X-ray bulb, but the different lines varied in intensity 

 with the hardness (degree of vacuum) of the hulb. The evidence from the 

 action of crystals on X-rays is thus accumulating that the X-rays are waves 

 of exceedingly short wave-length. 



