284 WHERRY : CRYSTAL FORM AND OPTICAL PROPERTIES 



may be regarded as belonging alternately to carbon atoms shown, 

 and to others lying in adjacent cells; interlocking thus occm-s in 

 these oxygen layers. It also occurs in the nitrogen layers shown, 

 and in both cases is probably associated with residual affinity or 

 secondary valence of these elements. 



In the writer's opinion the valency-volume hypothesis, accord- 

 ing to which the volumes of atoms are proportional to their val- 

 ences, which is much used in the study of molecular structure, is 



Fig. 1. Space-lattice of urea. 



fallacious. Kopp's data on atomic volumes in the liquid state 

 yield the diameters: H, 2.20; 0', 2.45; C, 2.75; 0", 2.85; N, 2.95; 

 or, if the diameter of the H atom is taken as 1.25 X 10"^ cm., 

 those of the others are O', 1.40; C, 1.55; O", 1.60; and N, 1.70, 

 all X 10-^ cm. There is no reason to expect marked changes 

 from these values in solids. 



In the two horizontal directions, front-back and right-left — 

 which are of course equivalent, as required by the symmetry — 



