RECENT ADVANCES IN SCIENCE 41 



cules is comparable with the distance between the atoms in a 

 single molecule, and that the difference is too small for detec- 

 tion by X-ray methods. As Bragg's model of rock-salt is not 

 in accord with the symmetry of the crystal, an alternative 

 structure, containing no atom at the centre of the elementary 

 cube, and taking into account the valency of the atoms, is pro- 

 posed. In this connection Vegard (loc. cit.) states that under 

 certain conditions a lattice may explain the symmetry elements 

 of a crystal where the symmetry of the lattice is not the same 

 as that of the crystal. 



The last-named author has also determined the lattices of 

 the nitrogen and iodine atoms in ammonium iodide and its 

 tetra-methyl derivative and has discussed the probable arrange- 

 ment of the hydrogen atoms. The relations between the topic 

 parameters of these substances cannot be explained by simple 

 substitution as the number of elementary lattices is different 

 in the two cases. 



An investigation of the structure of the rhombic alkali 

 sulphates by Ogg and Hopwood {Phil. Mag. 32, 518, 1916) 

 indicates that the metal and sulphur atoms are probably 

 arranged in a pseudohexagonal lattice of the type suggested 

 by von Federov. The authors hold that the identity of the 

 absolute volume of the unit cells of the lattices of the rubidium 

 and ammonium salts is conclusive evidence against the valency- 

 volume theory of Pope and Barlow. This is further discussed 

 by Tutton (Proc. Roy. Soc. A. 93, 72, 191 7), who reaches the 

 same conclusion. 



BOTANY. By E. J. Salisbury, D.Sc, F.L.S., East London College. 



Anatomy. — Whatever views may be held as to the relative 

 primitiveness of Dicotyledons and Monocotyledons there can 

 be no question but that the two had their origin from a common 

 stock. Research undertaken from different standpoints is 

 constantly bringing to light fresh resemblances or widening 

 our concept of those already known to exist. One of the most 

 obvious of the anatomical differences between the two groups 

 is the presence or absence of intrafascicular cambium. But 

 this distinction is by no means universal, and the collection of 

 records of the occurrence of such a cambium amongst Mono- 

 cotyledons by Mrs. Arber (Annals of Botanty, Jan. 191 7) is a 



