216 ANNUM, REPORT SMITHSONIAN INSTITUTION, 1920. 



A DISCUSSION OF SOME OF THE STRUCTURES THUS FAR STUDIED. 23 



By making the seemingly plausible assumption that chemically 

 simple compounds have their atoms in some sort of a simple ar- 

 rangement in space, it is possible, as we have seen, to determine the 

 crystal structures of quite a large number of compounds. It is per- 

 haps worth while to consider to what results the structures obtained 

 with the aid of this assumption will lead. A knowledge of the 

 arrangement of the atoms in a crystal is essential to the adequate 

 treatment of a large field of problems. The crystallographer who is 

 concerned with finding some explanation for cleavage, or for a kind 

 of twinning, will find an acquaintance with the crystal structure in- 

 dispensable ; the manner of the arrangement of the atoms in a com- 

 pound forms a rational basis upon which to build a discussion of the 

 relative occurrences of crystal faces. 24 Furthermore, the physicist 

 who is interested in the structure of the atom must produce a model 

 which will account for the structures of crystals. Yet again the 

 chemist may look to these same structures for information as to 

 the nature of the forces which are binding atoms together, for evi- 

 dence of the existence or nonexistence of the chemical molecule, and 

 for some light upon the nature of what we have been accustomed to 

 call the valencies of the atom. 25 Quite recently attempts have been 

 made to build upon these determinations of the structure of crystals 

 a quantitative measure of such physical properties of the solid as its 

 compressibility, as well as to try to explain, upon the basis of a 

 definite model of the atom, some of the simpler structures. 26 The 

 problems of the chemist, while by no means necessarily the ones 

 whose ultimate solution will be most easily obtained, are, neverthe- 

 less, the ones to which the sort of information furnished by a knowl- 

 edge of the structure of crystals is most readily applicable. The data 

 as yet at hand are so meager — and the determinations themselves are, 

 as already emphasized, of sufficient uncertainty — that the conclusions 

 at which we arrive must be only provisional. 



Metals. — The structures of a relatively large number of metals 

 have been studied. Most of them appear to have the arrangements 

 that would result from the close packing of bodies spherical or 

 nearly spherical in shape. There are two forms of this close pack- 

 ing of spheres: (1) The closest packing, which has hexagonal sym- 



2 » Not all of the crystals that have been studied will be discussed. Some, like fluorspar, 

 CaF 2 , can not as yet be satisfactorily considered ; many other determinations seem to be of 

 more or less doubtful value. 



34 Ralph W. G. Wyckoff, Am. J. Sci., 50, 317, 1920. P. Niggli discusses from this point 

 of view the development of faces by crystals. Zeit. anorg. Chem., 110, 55, 1920. 



26 For instance, see I. Langmuir, J. Am. Chem. Soc, 38, 2221, 1916; Ralph W. G. Wyck- 

 off, J. Wash. Acad. Sci., 9, 565, 1919. 



38 M. Born and A. Lande\ Verh. deut. Phys. Ges., 20, 210, 1918 ; and several other papers 

 by the same authors. 



