1922] on The Structure of Organic Crystals 



indication of the general character of such structures, adding to it 



ondition that every atom is to be like every other atom - 

 own kind in respect to relative distances and orientations of all its 

 neighbours, it becomes possible to foretell the probable form of 

 structure, using the X-ray methods for subsequent verification. 

 This method of proceeding may be very much easier than ii 

 taken in the : - ly. We might, for example, have gone far to 

 foretell the structure of fluorspar. It is an ionic compound in which 

 the calcium atoms are doubly charged and fluorines are singly cha: _ 

 Each positive is to be surrounded, therefore, by twice as many 

 neighbours as each negative by positive. The fluorspar structure in 

 which the metal atoms are arranged at the corners and the face 

 centres of the cube, while the fluorines he at the centres of the eight 

 small cubes into which the larger ones can be divided, is one of the 

 very few regular ways in which this numerical relation of 2 to 1 can 

 be carried out. So also in ice. the 2 to 1 arrangement is carried out 

 in a second of these ways, the relative numbers being 4 ; : 

 Ii is the lightest and most open of the 2 to 1 structures, and is con- 



nt with the low specific gravity of ice and with the possibility of 

 compressing the substance into denser forms : at the same time it shows 

 the six-pointed arrangement and the f eatheriness of the snow crystal." 



The earlier results at the same time showed that in the diamond 

 we had a construction of very different properties and nature. Here 

 the atoms are electrically neutral and are bound to one another, not 

 by electrical attraction from centre to centre, but by a more in:: 

 process which probably consists in some way of a sharing of structural 

 electrons. The diamond is on this account the hardest of known 

 substance - 



These considerations amount to a recognition that the bonds 

 between the atoms may be of very different characters, though it may 

 be difficult to draw hard-and-fast lines between them. We can say 

 that there is a very strong electron-sharing bond of which the 

 diamond is typical, and that there are ionic bonds in polar com- 

 pounds which in general are of a weaker character, as, for example, 

 in rocksait. though on the other hand they may " sfcn :„_ wh 

 in the ruby, the ionic charges are larg 



fly, there is a third type, which is found in the organic c 

 where it would appear that the separate molecule can be distinguished. 

 The atoms in each molecule are strongly tied together, but the forces 

 that bind molecule to molecule may be described as i b - They 

 would appear to be weak fields concentrated at definite points on the 

 molecule, the positive and negative cha: _ - vhich they are due 



lying within it. 



The second principle, which emerged fairly early in the experi- 

 ments, was described by my son in an address which he gave in this 



* Proc. PL - London, vol. x xx iv. r: _ H 



Vol. XXIII. Xo. 116) 2 t 



