*RECENT ADVANCES IN SCIENCE 371 



Working on these lines, a great number of materials have 

 been examined which could not be obtained in a form suitable 

 for examination by the spectrometer. This work has mainly 

 been done by Hull and his co-workers at Schenectady, where 

 a broad-minded and generous commercial research policy 

 has made possible a development which slender academical 

 resources could hardly have undertaken. The metals and other 

 elements have been and are being systematically examined, and 

 many important facts and relations are emerging from the more 

 extensive knowledge of atomic arrangement. 



In the realm of atomic theory, the results throw considerable 

 light on the size and shape of the atom. The data can only 

 be profitably considered on the basis of a space distribution of 

 electrons moving about equilibrium positions along such lines 

 as the hypothesis developed by Langmuir [Journal of the 

 American Chemical Society, p. 868, 1919.) In this way. 

 Prof. W. L. Bragg [Phil. Mag. August, 1920) has very suc- 

 cessfully interpreted the distances between atomic nuclei and 

 their constancy for each atom. Hull [Phys. Rev.,ix. 84, 191 7) 

 has even tried to interpret the intensities of the lines of some 

 lighter substances, viz. diamond, carborundum, and iron, by 

 fixing the position of the electrons within the atom. Little faith, 

 however, can be put in the photographic intensities obtained, 

 despite all the precautions taken, nor is our knowledge of the 

 mechanism of scattering sufficient to justify calculations 

 even from such definite assumptions as the ** cubical atom " 

 suggests. 



A very powerful means of analysis may be developed from 

 a combination of spectrometer and powder methods, as each 

 supplements the other. The spectrometer can measure more 

 accurately the relative intensities of the lines from any one of 

 the most important planes while the powder photograph 

 tests the existence and compares the intensities on one film of 

 reflections from a great number of planes. Laue photographs 

 give some information about planes of high indices, and, with 

 the knowledge of " atomic diameters " already alluded to, 

 analysis may be extended to crystal systems which are yet 

 untouched. 



A great field of application is possible, of interest both 

 theoretical and practical, in the chemical and structural 

 analysis of substances as they actually occur in nature or 

 industry. Examination has shown many so-called amorphous 

 substances to be really crystalline, notably "amorphous" 

 silicon (Debye and Scherrer, I.e.). 



A. W. Hull has described the use of the method in chemical 

 analysis. Every crystalline chemical compound gives a definite 

 system of lines which can be identified, though the crystal 



