54 



NATURE 



[July 14, 1923 



By means of the Bragg ionisation spectrometer 

 an examination of this crystal has been carried out. 

 The results ,iic in full agreement with a two-molecule 

 cell (Tabli l.j. 



Table I. 



Of course, we might now argue that this does 

 not prove anything, since the theory of space-groups 

 takes no cognisance of the structure of the ultimate 

 asymmetric units, but confines itself to the number 

 and relative orientations of these necessary to produce 

 one or other of the 230 homogeneous point-systems. 

 Theoretically, it is just as easy to conceive of a crystal 

 of the class C^^ being built from a two-molecule cell 

 as from a one-molecule cell. In the former case it 

 would simply mean that to construct an asymmetric 

 lattice, Nature, at variance with her usual procedure, 

 had used an asymmetric polymer of the chemical 

 molecule instead of the single molecule. 



In spite of this, it is here suggested that CaSgOa . 6H2O 

 is not, after all, the unique example of the triclinic 

 asymmetric class, but only one of the many examples 

 of the pinakoidal class Q\ The following reasons 

 are given for this : (i) in all complex crystals 

 so far examined, the ultimate structural unit has 

 proved to correspond to the chemical molecule ; and 

 (2) there is a mass of evidence to show that the 

 crystal symmetry, as deduced from a study of facial 

 development and etched figures, is often of a lower 

 type than the true structural symmetry as deduced 

 from X-ray data (and generally confirmed by other 

 indications). An interesting paper dealing with this 

 subject has recently been contributed by E. T. 

 Wherry, Am. Journ. Science, September 1922 ; but as 

 a few well-known examples of this pseudo-symmetry 

 we may cite potassium chloride (cubic holohedral, 

 not pentagonal icositetrahedral), diamond (cubic 

 holohedral. not hexakis-tetrahedral), cuprite (cubic 

 holohedral, not pentagonal icositetrahedral), wulfenite 

 (tetragonal holohedral, not pyramidal), and ammonium 

 chloride (not pentagonal icositetrahedral, but either 

 hexakis-tetrahedral or holohedral). The last-named 

 is especially interesting since it has been examined by 

 several workers. It now seems conclusive that its 

 structure is what was originally suggested (Bragg, 

 " X-rays and Crystal-Structure," p. 158), that it 



NO. 2802, VOL. 112] 



belongs not to an enantiomorphous class of the cubi 

 system but to a class showing planes of symmetr\ 

 (probably hexakis-tetrahedral — see R. W. G. Wyckoli, 

 Am. Journ. Science, December 1922). With such 

 crystals as these we must class many of the co-ordi- 

 nation-compounds of the type ^Me"'(X8")i R, . nli^O 

 examined by F. M. Jaeger (" Recherches sur le 

 principe de Pasteur," Rec. d. trav. chim. d. P. B., 

 tome xxxviii.). Though substances of this type 

 are very strongly optically active, many of their 

 crystals appear to lack the characteristics of th' 

 enantiomorphous classes. 



These few examples will suffice to show that it 

 has become unsafe to argue from form development 

 and etched figures, that, for example, the hitherto 

 accepted evidence for placing CaSgOa . 6H2O in the 

 asymmetric class is not trustworthy. It is becoming 

 increasingly clear that the boundary conditions of a 

 crystal are often so different from the conditions 

 obtaining inside the structure that not only th' 

 growth but also the inverse process of solution 

 (etched figures) leads to a definite M«(^gr-estimate (in 

 most cases) of the real internal symmetry. 



On the other hand, all the evidence so far is now 

 in favour of placing CaSjOs . GHjO in the pinakoidal 

 class, since the cell contains two molecules, and, by 

 analogy with other known structures, these may be 

 taken to be centro-symmetrical with respect to each 

 other. It is true that this latter supposition cannot 

 yet be proved, because in the triclinic system the 

 two molecules, provided they are so orientated as 

 to be centro-symmetrical with respect to each other, 

 may occupy any relative positions in the cell. How- 

 ever, X-rays certainly show that the smallest cell 

 which repeats through space contains two chemical 

 molecules, and that there is no evidence that they 

 are alike. The inference then is that they are the 

 inversions of each other and that the complete 

 structure is in reality centro-symmetrical. In this 

 connexion it should be noted that two other triclinic 

 crystals have also been thoroughly examined, namely, 

 anhydrous racemic acid and racemic acid mono- 

 hydrate. If only for chemical reasons, there is no 

 doubt that these two crystals are built of molecules 

 which are inverse to each other, and X-rays again 

 show that the smallest cell which repeats through 

 space contains two chemical molecules only. 



If we knew more about the intensities of X-ray 

 reflections, we should be able to prove that the two 

 molecules in the cell of CaS203 . 6¥LX> are inverse 

 to each other, but for the present this is impossible. 

 For the same reason, no attempt has been made to 

 fix the atomic positions in the cell. An examination 

 of the optical properties would be highly desirable, 

 with the view of detecting rotatory polarisation, 

 should an}' be shown. Such a test would then be 

 practically decisive. 



For the preparation of the crystals which have 

 been examined, I wish to express my indebtedness 

 to Mr. C. P. Proctor, of the Chemical Laboratory, 

 University of Birmingham. W. T. Astbury. 



Physics Department, 



University College, London. 



A Method of Photographing the Disintegration of 

 Atoms and of Testing the Stability of Atoms 

 by the Use of High-speed Alpha Particles, 



W^HiLE the experimental work of Rutherford 

 demonstrates the disintegration of the nuclei of the 

 atoms of six elements of odd atomic number to give 

 long-range hydrogen nuclei, it does not show whether 

 shorter-range products of disintegration are emitted. 

 It occurred to one of the writers several years ago 



