Atomic Symmetries of Ammonium Chloride. 183 



attacking the assumed mechanism of X-ray diffraction is 

 not likely to yield results of value for the following rea- 

 sons. The datum 10 which has been used here to establish 

 the impossibility of an enantiomorphic hemihedry for 

 ammonium chloride is simply the value of the ratio n 3 /m. 

 The correctness of this determination rests simply upon 

 the idea of atoms as diffracting centers for X-rays, upon 

 the correctness of the determination of the wave-lengths 

 of X-rays, which itself is a consequence of the application 

 of the quantum principle, and upon an approximately cor- 

 rect determination of the number of molecules in a gram- 

 molecular weight of any substance. As a consequence the 

 denial of the determined value of n 3 /m must involve the 

 denial of one of these fundamental factors, the general 

 validity of any one of which is reinforced from many 

 directions in physical science. 



It is of great interest to all students of general crystal- 

 lography to know whether there are any discrepancies 

 between the external symmetry and the structure of crys- 

 tals. To this end a further study of the external sym- 

 metry of ammonium chloride crystals is now needed. 



Summary — -It is shown that not only is the symmetry 

 of the structure that has been assigned to ammonium 

 chloride in conflict with its observed symmetry but that 

 there is no other possible structure which will possess the 

 requisite symmetry. As a result the necessity of further 

 study of the crystallography of this salt is urged. 



Geophysical Laboratory, 



Carnegie Institution of Washington, 

 November, 1921. 



10 The chemically requisite assumption that in a crystal chemically identical 

 atoms must be related in the same way to other atoms is of course likewise 

 involved. 



