368 JOURNAL OF THE WASHINGTON ACADEMY OF SCIENCES VOL. 11, NO. 15 



m is the number of these molecules in the unit, 



p is the density of the crystal, and 



V is the volume of the unit. 

 In this expression the density is known and the value of M is simply 

 the molecular weight of the salt multiplied by the weight of an atom of 

 hydrogen, which has been show^n to be close to 1.64 X 10"-"^ grams. 

 The other term upon the right hand side of this equation, m, can be 

 known, however, only when the crystal structure of the salt is also 

 known. 



Fig. 2. 



In first studying the wave lengths of X-rays the alkali halides were 

 investigated, and of these the one which is from all points of view the 

 most satisfactory to treat is sodium chloride. Since no information 

 immediately useful for the present purpose can be obtained from a 

 consideration of any other available crystals, it will be permissible to 

 limit attention to this one salt. 



The only data for establishing the structure of sodium chloride which 

 are truly independent of assumptions more or less tacitly based upon 

 an imagined structure of some crystal are (1) the observation (from 

 Laue photographs) that only planes all of whose indices are odd are 



