382 SCIENCE PROGRESS 



the function of the water has already been advanced, on the 

 results of the investigation of the absorption of light by the 

 alums (Ann. Phys. (iv.) 50, 339, 1916). 



A somewhat different structure for the alums is put forward 

 by P. N iggl i (Phys. Zeit. 19, 225, 191 8), who has determined 

 the point system from a knowledge of the symmetry and 

 the distance between the successive planes in each series of 

 parallel planes of reflection in the crystal. The oxygen atoms 

 whose position is not determinate from the X-ray results can 

 be located from the data mentioned above. The structure so 

 obtained differs from that of Vegard and Schjelderup so far 

 as the location of the oxygen atoms is concerned. 



The structure of chalcopyrite (CuFeS 2 ) has been determined 

 by C. L. Burdick and J. H. Ellis (Jour. Amer. Chem. Soc. 39, 

 2517, 191 7), who, by regarding the crystal as pseudocubic 

 (a : c = 1 : 0*985), assign a face-centred lattice to the arrange- 

 ment of iron and copper atoms, the planes perpendicular to the 

 unique axis consisting alternately of copper and iron atoms. 

 Taking the two types of atoms separately, each may be regarded 

 as forming two interpenetrating simple tetragonal lattices. 

 From the masses of the atoms and the distances between the 

 successive planes, the density of the crystal is calculated and 

 found to agree with the observed value. 



In a series of papers the work of W. H. and W. L. Bragg 

 is critically reviewed by E. S. Federov (Bull. Acad. Set. Petro- 

 grad, 10, 359, 435, 547, 1675, 1916). According to the available 

 abstracts (Chem. Abs. 12, 8, 191 8, cf. T. V. Barker, Ann. Rep. Prog. 

 Chem. 14, 230, 191 8) the first paper deals with the substances 

 whose structure has already been determined by means of 

 X-rays and gives appropriate structures for several substances, 

 ammonium chloride, sodium chlorate, and cuprite, the investi- 

 gation of which has hitherto been incomplete. It is found that 

 the optical activity of sodium chlorate can only be explained 

 on the assumption that the atom is asymmetric. The second 

 paper enunciates the fundamental law of crystallochemistry 

 that the atoms of a crystal are situated at the points of a single 

 lattice, and hence the position of an atom with reference to any 

 other atom as origin may be expressed in terms of co-ordinates 

 which are either small whole numbers or simple fractions, if 

 the distances from the origin to three atoms not complanar 

 with it are taken as units. The appropriate equations for these 



