226 CHEMICAL DISCOVERY AND INVENTION 



experiment due to Professor von Laue of Munich, who showed 

 that the ordered array of atoms in a crystal can behave to X-rays 

 in a manner analogous to the action of a diffraction grating on a 

 ray of light. The results were not at first completely understood, 

 but an interpretation by Mr. W. L. Bragg, followed by further 

 experimental researches in association with his father, Professor 

 W. H. Bragg, has led to extremely interesting and important 

 conclusions. Their work has, indeed, been considered of such 

 fundamental importance that the Nobel Prize for last year was 

 awarded to them. Unfortunately it is impossible in a few words 

 to convey a sufficient account of the method. This has been 

 described with some detail in a lecture delivered before the 

 Chemical Society of London on February 3rd, 1916, and printed 

 with illustrations in the Transactions of the Society for 1916, 

 p. 252. The experiments show that in a crystal the atoms are 

 independent of each other, and are arranged in regular order at 

 distances apart, which can be measured. Thus in a crystal of 

 rock-salt, which is cubic, the atoms of sodium and chlorine are 

 stationed alternately in planes which lie at right angles to each 

 other, so that each atom of sodium is not associated with only 

 one atom of chlorine but stands in the same relation of distance 

 to six atoms of chlorine. Each atom of chlorine is similarly 

 placed with regard to six atoms of sodium, and apparently with- 

 out preference for any one of them. Hence in the solid crystal 

 the idea of molecule disappears, for the entire crystal, small or 

 large, is one molecule. It is only when melted or dissolved that 

 the atoms of sodium and chlorine select partners and move off. 

 These experiments have also revealed the fact that the diffract- 

 ing power of an atom for X-rays is directly proportional to its 

 atomic weight, so that one result is obtained with common salt 

 in which the atoms of sodium (atomic weight 23) and of chlorine 

 (atomic weight 35-5) are very different in mass, while another 

 result is obtained with potassium chloride in which the atomic 

 weights 39 and 35-5 are more nearly equal. Similarly the differ- 

 ences between potassium chloride, bromide (Br=80), and iodide 

 (1 = 127) correspond to different X-ray spectra. One of the most 

 obvious and important results is to confirm the accuracy of the 

 conclusion involved in the theory of Barlow and Pope which has 

 already been mentioned, namely that a crystalline substance is 

 an assemblage of atoms in which the molecular unit does not 

 necessarily exist until the crystal is dissolved or melted. But 



