GEOPHYSICAL LABORATORY. 133 



explosion must have resulted through which the entire accumulation of energy- 

 would have found release in a single catastrophe. This obviously was not the 

 case. If, on the other hand, the resistance of the mountain to the accumulated 

 pressure within had somehow been weakened slightly by action from without, 

 such as an earthquake crack, and this development of structural weakness 

 had been followed by a succession of explosions extending it gradually, then 

 we would seem to reach the counterpart of the present phenomenon. 



ATOMIC STRUCTURE OF CRYSTALS. 



During the past year there has been prepared for publication a complete 

 elaboration of the 230 space-groups which form the basis of all studies of 

 atomic structure in crystalline matter. 



With the gradual introduction of orderly methods of determining crystal 

 structures by the use of X-rays, a thorough analysis of the systems resulting 

 from an application of the theory of space-groups has become increasingly 

 necessary. Publication No. 318 of the Carnegie Institution of Washington, 

 entitled "The Analytical Expression of the Theory of Space-groups," is intended 

 to supply such an analysis for general use in crystal-structure study. In a 

 complete and unique determination of the structure of a crystal, as opposed 

 to the mere suggestion of a possible arrangement of atoms which will conform 

 to certain limited data, it is imperative that the investigator have available 

 a knowledge of all of the atomic arrangements that are possible from the 

 standpoint of the observed symmetry of the crystal. This information is 

 furnished by a study of all of the special cases of the space-groups. Because 

 in the study of a single crystal it is usually necessary to consider several 

 space-groups, and also because the available methods of study are gradually 

 gaining the power to attack successfully other systems than cubic crystals, 

 it has seemed not only expedient but necessary to make a complete treatment 

 of the space-groups of all crystallographic symmetries. 



Particular care has been taken to express the results of this study in appro- 

 priate detail, not only for facility of reference but also because it is realized 

 that the majority of future students of crystal structure will hardly wish to 

 take the time to become conversant with the details of the theory of space- 

 groups and so will be in a position to use results of the theory only so far as 

 they are completely tabulated. Although the primary purpose of the work 

 has been the presentation of a complete analytical expression of the space- 

 groups, the almost total absence from English literature of any discussion of 

 this phase of geometric theory has suggested the desirability of incorporating 

 a brief discussion of the characteristics of the space-groups themselves for 

 the benefit of those who may desire to gain some idea of their nature without 

 wishing to study the theory itself in detail. A discussion of so much of the 

 material as appeared necessary to accomplish this purpose has consequently 

 been included in short chapters prefatory to the principal tabulations. 



This careful elaboration of the possible group patterns should be of the 

 greatest service to students of atomic structure. Indeed, since the prepara- 

 tion of such a volume became known a number of requests have been received 

 for advance copies of portions of it, and photostat copies of many of the 

 manuscript tables have been supplied to workers in this field. 



