July 22, 1922] 



NA TURE 



The Structure of Organic Crystals. 1 

 By Sir William Bragg, K.B.E., F.R.S. 



IT may be said with truth that modern advances in 

 physical science are due in the main to the 

 acquisition of the power to handle the individual atom. 

 Until the present time we have always attacked the 

 problems of matter by examining the behaviour of 

 atoms or molecules in groups. The new powers arise 

 in two ways : — 



In the first the individual atom is endowed with 

 excessive speed and energy, and is able to make its 

 individuality felt on this account. The a-particle of 

 the radioactive radiations is a helium atom moving 

 with a speed of the order of one-tenth of that of light. 

 While in possession of the relatively tremendous energy 

 which the speed implies it can, unaided, make a visible 

 impression on a fluorescent screen. It can pass 

 through thousands of other atoms without sensible 

 deviation and, if occasionally it suffers violent deflec- 

 tion, it has penetrated to the very core of the atom 

 which has deflected it. Rutherford has shown us what 

 important deductions can be drawn as to the construc- 

 tion of the atom by examining these rare and sharp 

 deviations, and is going even further in examining the 

 shattering effect which the deflecting atom may itself 

 experience. So also, the electron endowed with 

 sufficient speed can traverse matter and bring about 

 its ionisation and other effects of great interest, but if 

 its velocity becomes less than one million metres per 

 second this free existence disappears. It is attached 

 to the first atom it meets. 



The second method of attack upon the individual 

 atom proceeds on very different lines. It is by w-ay of 

 the mutual action of X-rays and crystals. When we are 

 examining things by eyesight we follow the influence 

 of the objects that we look at upon the waves of light. 

 If we wish to penetrate deeper into the minute, we take 

 advantage of the optical effects of lenses and build 

 microscopes : but, even then we cannot attack indi- 

 vidual objects containing less than many thousands 

 of individual atoms. A limit is set by the difficulty 

 that light cannot show us the form of things which 

 are much smaller than the wave length of the light 

 itself. With the aid of the very short waves known 

 as X-ruvs we can make our way down to objects ten 

 thousand times smaller, but by itself this extension of 

 our powers would be inefficient, because the effect due 

 to one atom or one unit of pattern would be inappreci- 

 able. Here lies the value of the crystal, which, being 

 an aggregate of some small atomic pattern repeated 

 again and again through space, shows up on a measur- 

 able scale the properties of the atoms in the single unit. 

 By the combination of X-ray and crystal we can 

 examine the very foundations of material construction. 

 It is difficult to set a limit to what may be the con- 

 sequences of the exercise of these powers since we can 

 now examine all physical effects, so to speak, at their 

 source, and must in the end be able to refer all the 

 physical and chemical properties of materials to the 

 properties of the individual atoms and their mutual 

 forces. So far the new methods have scarcely begun 

 to show their full strength. A few inorganic crystals 

 have been examined with a view of discovering 



1 Discourse delivered at the Royal Institution on Friday, May 19. 

 NO. 2751, VOL. I 10] 



their structure, but the new field of research is barely 

 entered. Inviting roads lie before us pointing 111 

 numerous directions. 



Very little has yet been done in the way of applving 

 the new methods to the structure of organic crystals, 

 although no study could be more tempting. Their vast 

 variety of form, the perfection of their structure, their 

 importance, all urge us forward, and especially the fact 

 that the whole progress of organic chemistry shows 

 that the science depends upon laws of position with 

 which the X-rays are especially qualified to deal. The 

 difficulty at the outset lies in the complexitv. In the 

 naphthalene molecule there are 18 atoms : in what 

 way can we expect by means of X-rays to solve the 

 intricate problem of their relative positions ? Our 

 first attempts to solve inorganic crystals depended for 

 their success upon two facts : — 



The first, the simplicity of the structures which were 

 attacked. 



The second, the guidance derived from the principles 

 of crystallographic symmetry. 



The determination of the structure of rocksalt 

 opened a way to further determinations of such simple 

 crystals as the diamond, zinc blende, fluorspar, and 

 others. In all these the principles of symmetrv 

 supplemented the knowledge derived from the examina- 

 tion of the intensities of X-ray reflection by the various 

 crystal planes. As the work has proceeded in the 

 hands of observers in many countries, other principles 

 have emerged or are emerging which render further 

 and very valuable aid so that problems appear to be 

 coming within our grasp that not long ago seemed most 

 difficult of solution. 



Of these principles, one began to appear in con- 

 sequence of the very earliest results. It was a very 

 striking fact that in crystals of polar substances the 

 molecule seemed to disappear ; it was in fact dis- 

 sociated, and the structure of the crystal depended 

 upon the grouping of the positive ions round the 

 negative and of the negative ions round the positive. 

 In rocksalt each metal atom is surrounded by six atoms 

 of chlorine and vice versa. If we accept this as an 

 indication of the general character of such structures, 

 adding to it the condition that every atom is to be 

 like every other atom of its 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 if it were taken in the reverse way. 

 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 charged. 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 lie 

 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 



