STRUCTURE OF CRYSTALS WYCKOFF. 209 



different orders, or, what amounts to the same thing, to know how 

 the intensity of reflection depends upon the spacing between like 

 planes of atoms in a crystal. The form of this function is not known 

 now with any degree of certainty. 



Until a knowledge of these two factors governing the amount of 

 reflection of X rays — the effect of atomic number and the effect of 

 spacing — is available it is useless to attack by the means we have 

 been discussing any but the simplest of crj^stals. Even with these 

 it is never certain that the structure selected as agreeing with the 

 experimental data is actually the correct one. It may be urged that 

 if a lack of knowledge of these two " laws " of reflection is the only 

 thing that delays the determination of the structures of many and 

 complicated crystals, then we should immediately proceed to discover 

 the forms of these two expressions. The reason for not carrying out 

 this apparently simple program is not hard to find. The commonly 

 employed spectrometer method of studying the structure of crystals 

 involves the simultaneous use of these " unknowns " ; only in rela- 

 tively few instances has it been possible to get at the arrangement 

 of the atoms in a compound without assuming values for both of 

 these expressions. Since it is only by working with crystals whose 

 structures are known with certainty that progress can be made toward 

 a satisfactory solution of these two problems, and since a use of the 

 answers to these same problems are usually involved in getting the 

 structures themselves, advancement has not been made. 



These two "laws" which we have been discussing are of very 

 considerable importance quite aside from their use in the determina- 

 tion of the structure of crystals. Such meager information as is at 

 present available seems to show conclusively that the forms they will 

 take can only be accounted for by taking into consideration the inti- 

 mate structure of the atoms themselves. 



The diffraction or reflection of X rays by an atom may be roughly 

 pictured as follows : When a beam of X rays of definite wave length 

 strikes an atom the rays may be supposed to accelerate the electrons 

 contained within this atom. X rays which are emitted by such elec- 

 trons are the diffracted (scattered or reflected) radiation. If all of 

 the electrons in an atom were concentrated very close to the center the 

 amount of scattered X rays should be quite closely proportional to 

 the number of electrons in the atom. Such a proportionality is in 

 reality only very roughly fulfilled; in fact, some of the evidence 

 seems to point to quite marked variations from it. This lack of pro- 

 portionality is most simply and easily explained by supposing that 

 the atom, meaning thereby the volume occupied by the electrons sur- 

 rounding the atomic nucleus, is relatively large. The X rays scat- 

 tered by electrons on one side of the atom will interfere, sometimes 

 42803°— 22 14 



