150 MOLECULAR GROUPING 



i, 2, 3, and on it, as base, erect a cylinder at right angles to 

 the plane of projection, it is plain that the particles wilt lie 

 on its surface in spiral order, which is right-handed (like 

 a corkscrew) when 3 is twice as far above the plane of i 

 as 2 is, but left-handed when 3 is twice as far below that 

 plane as 2 is. The spirals show these opposite senses 

 whether looked at from above or below ; such a right- and 

 left-handed spiral can in no way be made to coincide, for 

 one is the reflected image of the other, and they are 

 therefore called enantiomorphic. 



Finally, symmetry, if it exists in the molecule, may be 

 raised by the crystal 1 . Two tetragonal pyramids, placed 

 symmetrically on the same base, if they have the form of 

 half octahedra, become an octahedron and so possess the 

 symmetry of the regular system with nine planes of 

 symmetry, while the individual molecules have only four. 



The symmetry present in the molecule may therefore be 

 either raised or lowered by arrangement in the crystal ; for 

 a greater number of compounds in which the opposing 

 influences, tending to raise and lower the symmetry, partly 

 neutralize, the remark of Buys-Ballot holds that the simpler 

 the composition, and consequently greater the symmetry 

 of the molecule, the greater the symmetry of the crystal. 

 The following table 2 allows of forming an opinion on this 

 point : 



Mean Symmetry .... 8-4 7.9 6-2 4.7 



As an expression of the entire condition, we have chosen 

 the mean symmetry, obtained by multiplying each percen- 



1 See, among others, Barlow, Zeilschr.f. Krysl. 1898. 



2 Zeitschr.f. Phys. Chum. 14. i, 548. 



