»82 Sir William Bragg [May 19, 



- known as X-rays 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 



ttern would be inappreciable. Here lies the value of the crystal, 

 which, being an agg _ of some small atomic pattern repeated 

 again and again through space, shows up on a measurable scale the 



rties of the atoms in the single unit. By the combination of 



- and crystal we can examine the very foundations of material 

 construction. It is difficult to set a limit to what may be the conse- 

 quences of the exercise of these powers since we can now examine all 

 physical effects, so t<~» speak, at their source, and must in the end be 

 able to refer all the physical and chemical properties of materials to 



voperties of the individual atoms and their mutual forces. 

 far the new methods have scarcely begun to show their full strength. 

 A few inorganic crystals have been examined with a view of discover- 

 ing their structure, but the new field of research is barely entered. 

 Inviting roads lie before us pointing in numerous directions. 



Very little has yet been done in the way of applying 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 

 complexity. In the naphthalene molecule there are eighteen 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 crystal- 

 lographic 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 symmetry 

 supplemented the knowledge derived from the examination 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 2Tasp that not long ago seemed most difficult of solution. 



Of these principles, one began to appear in consequence 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 

 iated. 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 



