1013] on Great Advance in Crystallography 697 



phenomenon, namely, striation of the spots obtained by reflection 

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

 tions from the cubic cleavage planes. The striations are, in fact, 

 true interference bands, due to interference of the reflections from 

 equally spaced parallel planes of the space-lattice. By the kind 

 courtesy of Prof. Barkla, two of these interesting photographs are 

 projected on the screen. On the assumption that the X-rays are 

 waves, and that the reflecting plane is one passing through corre- 

 sponding portions of 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 XaCl 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, 

 0*6 X 10-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 8 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, 

 1-2 X 10"^ mm. ; and if 16 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, 1), the wave-length 

 would be still longer, about the seventy millionth of a millimetre, 

 1*5 X 10"^ 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 2 x 10"^ mm. 



The most trustw^orthy recent estimations of the size of a molecule 

 of rock salt indicate a diameter about 3 x 10"'^ mm. Hence the 

 diameter of a crystallographic molecule 8NaCl would be 6 x 10"'' mm., 

 and of 16NaCl about 7*5 x 10"'' mm. 



It should be emphasized, in concluding the account of this 

 fascinating new field of research, that all these reflections occur in 

 the Ijody of the crystal, and are not surface effects. Cleavage planes 

 usually afPord 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.* 



The experimental proof of the existence of the space-lattice 

 imparts all the more confidence in approaching the other great 

 advance which has lately been achieved. The completion of the 

 catalogue of crystallographically measured substances by Prof, von 

 Groth, provokes tne question : What more is needed in order to 

 enable a crystallized substance described in the book to be recognized 

 by means of a few measurements on the goniometer "t For it is now 



* See Appendix, last page. 



3 A 2 



