1922] on The Structure of Organic Crystals 585 



We now have an elementary body of a dumb-bell shape which, when 

 forming part of the crystal structure, will naturally cause a deviation 

 from a simple cube. 



Again, there are principles which are barely established as 

 yet, though it seems probable that they will be found of material 

 assistance in analysis. The greater expansion of some crystals in 

 certain directions than in others seems to depend upon the nature of 

 the bonds. Bismuth expands more along the axis than across it, as 

 we might expect from the fact that in the one expansion the weak 

 bonds alone can be operative. In the same way diamond has an 

 extremely small expansion co-efficient because all the bonds are of 

 the strongest kind, but in graphite, on the other hand, the expansion 

 along the axis may be described as enormous. Mr. Backhurst finds 

 an increase in length of 3 per cent, for a rise of 900° C. At the 



Naphthalene. Anthracene. 



Fig. 1. — Unit cells of naphthalene and anthracene drawn 

 to the same scale. 



OA = a. OB = b. OC = c. 



Naphthalene . . . 8*34 6-05 8-69 



Anthracene . . . 8*7 6*1 11 "6 



Naphthalene a = BOC = 90°, = COA = 122° 49', y = AOB = 90° 

 Anthracene a = BOC = 90°, = COA = 124° 24', y = AOB = 90° 



same time, so far as can be inferred, the expansion across the axis is 

 still quite small. In one case weak bonds only are concerned, in the 

 other strong bonds of the same kind as in the diamond. 



It is when all these considerations are taken into account that it 

 seems possible to make an attempt upon the structure of the organic 

 crystals. They are, of course, very complex ; naphthalene contains 

 ten atoms of carbon and eight atoms of hydrogen, and our ability to 

 interpret X-ray evidence, that is to say, the relative intensities of 

 reflection by the different planes in different orders, is not sufficiently 

 advanced to place so many atoms in their proper position in the cell 

 from this evidence alone. We can readily find the size of the unit 

 cell, show that there are two molecules in it, and that the points, each 

 of which represents a whole molecule, are to be placed as is shown in 

 Fig. 1, but without some further help we can frame no hypothesis 

 on which to proceed. 



2 T 2 



