ANALYSIS OF CRYSTAL-STRUCTURE BY X-RAYS 391 



across the junctions of the yS-hydrogens. This point will be 

 readily visualised when we remember that the molecules lie 

 lengthwise parallel to OC in planes parallel to OCGA. The way 

 in which the crystalline structure reveals the perfect cleavage 

 plane is significant in that it indicates that across the junctions 

 between molecule and molecule, in spite of the fact that it 

 is undoubtedly the forces across these junctions that bind the 

 molecules into the crystal, there are forces far weaker than those 

 valency bonds which unite the atoms of the same molecule. 

 In this statement, we have the essence of the great difference 

 so far observed between the organic and those inorganic 

 structures, such as the alkali halides, with which Bragg's 

 work has made us so familiar. In the latter the identity of 

 each molecule seems to be lost, in fact they may be called 

 " ionic " structures, but in the former we experience little 

 difficulty in picking out complete individual molecules. 



The next step in this work on organic crystals was the 

 investigation of a molecule into which some complexity has 

 been introduced by a substituted group. Bragg considers 

 first the case of acenaphthene, Ci^H^o, 



CH 



c c 

 /\ /\ 



HH HH 



Here the molecules have been made one-sided by the substitu- 

 tion of a group of two carbons and four hydrogens for the two 

 hydrogens on one side. The structure through this change 

 becomes rhombic bipyramidal with four molecules to the unit 

 cell and 



« = 8-32A.U. b = i4'is A.U. €=7-26 A.V. 



The rhombic bipyramidal cell is like those shown in Fig. 2, 

 if ^ is made equal to a right angle ; that is, it is simply a paral- 

 lelepiped all the angles of which are right angles. It possesses 

 the tollowing elements of symmetry : three planes of symmetry 

 parallel to the faces of the simple parallelepiped, and three 

 dyad axes of symmetry which are parallel to the intersections 

 of the three planes of symmetry (and therefore parallel to the 

 three rectangular crystallographic axes a, b, and c). 



The molecules now lie one at each corner of the cell and 

 one at the centre of each of the faces of the cell. But whereas 

 the molecules at the corners of the cell and at the centres of 

 the faces CDEG and OBFA (Fig. 2 [a]) are oriented parallel 



