390 SCIENCE PROGRESS 



The next salient fact is that in anthracene crystals, 



we find that 



a = S'7 A.U. b = 6'i A.U. 



c= II-6 A.U. /3= 124° 24' 



[i Angstrom Unit (A.U.) = lO"* cm.] 



and that there are again two molecules per cell. We notice 

 that a, b, and ^ are practically unaltered, whereas c is length- 

 ened by 2-9 A.U. ; i.e., if we assume that the molecules in both 

 crystals lie end to end along the c-axis in structures that are 

 similar, an extra ring of the benzene dimensions (2*5 A.U.) 

 would account for most of the increase in length of the c-axis. 

 Moreover, considering that the overall lengths of the two 

 molecules, without allowance for the hydrogen atoms at their 

 ends, are 6-41 A.U. and 8-86 A.U. respectively, we have now a 

 vacant space between the ends of two molecules of rather 

 more than 2 A.U. in which two hydrogen atoms have to be 

 fitted ; which agrees very well with what might be expected, 

 even though we have no definite knowledge of the actual 

 distance between the centres of a carbon and a hydrogen 

 when united by a valency bond, nor between two hydrogens 

 when not so united. 



It is difficult, without the aid of such excellent models 

 as Bragg himself uses, to impart or conceive a clear mental 

 picture of the completed structure which he has ascribed to the 

 naphthalene cell. Suffice it here to say that in the final 

 arrangement all the molecules (which may be represented 

 sufficiently for our purpose by the chemical structural formula) 



(a) (a) 



lie practically in planes pai llel to OCGA (the symmetry 

 plane of the cell). The ^-hydrogens of each molecule lie up 

 against the corresponding hydrogen of the next, while the a- 

 hydrogens abut against the carbon atoms of the next molecule. 

 The perfect cleavage of the crystals is parallel to the plane 

 CDEG, and this is sharply marked in the model as passing 



