of the CrystallograpMc Law of Valency Volumes, 521 



ammonium sulphate*. This is of fundamental importance. 

 It shows us that the replacement in the elementary cell 

 of eight atoms of potassium by forty atoms of the four 

 radicle groups NH 4 , causes no more distension of structure 

 than if eight atoms of rubidium had replaced the eight 

 atoms of potassium. Furthermore, the substitution of eight 

 atoms of caesium for eight atoms of potassium causes double 

 the distension of structure that the substitution of forty 

 atoms of the four radicle groups NH 4 does. 



To the authors this appears to be conclusive evidence 

 against the general truth of the theory of crystal structure 

 based on the closest packing of the constituent atoms or 

 their spheres of influence. 



Further evidence in support of the fact that the elementary 

 cells of each of the above-mentioned crystals contain four 

 molecules, and that the space-lattices of ammonium sulphate 

 and rubidium sulphate are strictly comparable with each 

 other, is exhibited in Table II. 



This table shows the agreement between the values of the 

 glancing angles calculated on the above assumption, and the 

 observed values, for the whole series. The wave-length of 

 the particular line in the X-ray spectrum of palladium used 

 was *584x 10~ 8 cm.t, except in the case of the rubidium 

 and caesium crystals. The reflexions from some of the faces 

 of these two crystals were rather faint, and were taken 

 with slits too wide to entirely cut out the effects due to 

 X=-516xl0- 8 cm. 



In general, there was no difficulty in separating these two 

 lines. 



The complete structure of this series of crystals has not 

 yet been worked out. With the object of doing this, the 

 authors contemplated investigating the related series of 

 crystals — the alkali selenates ; but this work has been 

 interrupted. Some information, however, may be deduced 

 from Table II. 



The absence of the first- and third-order spectra from the 

 (100) planes shows that the spacings are equal to one-half 

 of the sides of the elementary cell. For the (010) planes 

 the fourth order is strongest, the second being found only in 

 the case of ammonium sulphate. The fourth order is also 



* Since the sum of the fundamental valencies of (NH 4 ) 2 S0 4 and Rb 2 S0 4 

 are respectively 2-4 and 12, the volume of the elementary cell of the former 

 should, according- to Pope and Barlow, be double that of the latter 

 crystal. _ 8 _ 8 



*t fteally the two lines X = -5828 X 10 and \ = 5872 X 10 . 



Phil. Mag. S. 6. Vol. 32. No. 191. Nov. 1916. 2 N 



