392 SCIENCE PROGRESS 



to the plane BDGA, those at the centres of the faces OCDB, 

 AGEF, OCGA and BDEF all lie parallel to the plane OCEF, 

 an arrangement which was necessitated by the observation 

 that the spacing between the planes parallel to BDGA is 

 actually twice the value that would be calculated on the 

 assumption that all the molecules are parallel. A model 

 shows at once that again the cleavage plane passes through 

 the junctions of the /S-hydrogens, while the substitution 

 group on the side of the molecule seems to engage with a 

 neighbouring molecule on its unsubstituted side. Two facts 

 with respect to the structure are left doubtful. They are 

 (i) that the molecule of acenaphthene seems to be rather 

 shorter than the naphthalene molecule, and (2) that there is 

 an indication that alternate planes parallel to ACDF differ 

 somewhat. 



Bragg next considers the two naphthols : 



CH 



CH HCl I IC-O-H 



H 



a-naphthol. /3-naphthol. 



Of these a-naphthol gives 



fl:=i3-iA.U. ^ = 4-9 A.U. c=i3-4A.U. 



with four molecules to the unit cell (again monoclinic pris- 

 matic, Fig. 2). These four molecules are distributed thus : 

 one at each corner of the cell, one at the centre of each of the 

 faces OCGA and BDEF, and one at the middle of each of the 

 edges of the cell. However, alternate planes parallel to 

 OBFA are quite different ; in fact, the combined data from 

 crystal and powder reflections are sufficient to show that there 

 are really four different orientations of the same molecule. 

 When a model is made, it appears that once more these lop- 

 sided molecules lie criss-cross with the cleavage plane passmg 

 markedly through the yS-j unctions. The hydroxyl groups fit 

 into their places very naturally, rather close together, in pairs. 

 There seems to be a special attraction between two oxygens 

 exerted across every alternate plane parallel to OBFA. The 

 other planes parallel to OBFA are cleavage planes. 

 For /3-naphthol very few data beyond the spacings 



« = 5-85A.U. b = 4'2SA.\]. c = 87 A.U. 



