Structure of Sodium Hydrogen Acetate. 197 



the first order from the large unit containing 24 chemical 

 molecules. 



The Laue photographs show clearly the absence of 

 planes of symmetry. The crystals of this compound 

 must then have either tetartohedral or paramorphic 

 hemihedral (pyritohedral) symmetry. It was further 

 observed that all of the planes giving reflections in the 

 first order region on the basis of the large unit have 

 two odd and one even indices. This points to an underly- 

 ing body-centered lattice. 5 The four space groups 

 T 3 , T 5 , T h 5 and T h 7 have the appropriate symmetry and 

 are built upon r c ". Distinction between the first three 

 of these is impossible upon the basis of the diffraction 

 effects to which they give rise. Crystals corresponding 

 T h 7 would give no reflections 5 in odd orders from planes 

 of the forms {0M} y where both h and I are odd. Neither 

 upon the Laue photographs nor upon the transmission 

 spectrum photograph were any such planes found in odd 

 orders, even though some were in suitable positions for 

 reflection. This would make it necessary to assign to 

 crystals of sodium acid acetate the symmetry of T h 7 . The 

 unit is so large, however, that with moderate degrees of 

 tilt from symmetrical Laue photographs the few planes 

 which reflect in the first order region have complicated 

 indices and are of weak intensity. Consequently in order 

 to place this assignment of symmetry beyond any legiti- 

 mate questioning, it would be desirable to study Laue 

 photographs from crystals inclined farther from the 

 symmetrical position than those here investigated. It 

 did not, however, seem worth while to make these addi- 

 tional experiments at the present time. 



Accepting this assignment to the space group T h 7 as 

 correct, the general coordinate positions of the atoms in 

 sodium acid acetate are defined. Depending upon what 

 equivalence is assumed for the two acetate groups and 

 for the atoms within these groups, all of the atoms of the 

 crystal will be arranged according to either the 48 

 generally equivalent positions 6 of T h 7 or the 24 equivalent 



5 Ralph W. G. Wyckoff , see the first article by the writer in this number 

 of this Journal. 



These general positions are stated in abbreviated form by A. Schoen- 

 flies, Krystallsysteme und Krystallstruktur, p. 551, Leipzig, 1891. Also in 

 detail by P. Niggli, Geometrisehe Krystallographie des Discontinuums, p. 

 368, Leipzig, 1919. 



