145 



may be. Therefore, it will be certainly necessary to postpone a final 

 judgement of the various results obtained, until full certainty has 

 been obtained about the real value of the conclusions drawn from 

 the experimental results of the newly discovered methods. 



There is, however, one point in which all these different investigators 

 agree : it is the conviction of the soundness of the contention that 

 constitutive atoms of a molecule preserve their individuality, and 

 to some extent also their autonomy, as the component particles 

 of a crystalline structure. The theory of Sohncke, that a crystal 

 may be regarded as an interpenetration of regular point-systems, 

 and the view maintained by Groth and by Barlow and Pope, 

 that the structural units of these systems and space-lattices are the 

 separate atoms of the chemical molecule, have been supported by 

 modern experience, and their correctness seems to become more 

 and more certain. It will, therefore, be of interest to consider here 

 these new methods of research more in detail. 



20. In the preceding paragraphs we have learned to consider 

 the crystal as a discontinuous system of atoms and molecules regu- 

 larly distributed in space, and separated from each other by very 

 small but definite distances. A long time ago physicists made some 

 evaluations of the order of magnitude of these interatomic and 

 intermolecular dimensions, which they found to be about 10 8 

 or 10 9 cm. 



Now it will be clear that an aggregation of particles of this kind 

 will behave as a continuous body towards most physical agencies, 

 because the dimensions which come into play in such physical pheno- 

 mena, are commonly of an order of magnitude incomparably greater 

 than the extremely small interatomic distances mentioned above. 

 Thus, if for instance a pencil of visible light-rays travel through 

 such a crystalline body, the latter will behave towards these vibra- 

 tions like an anisotropous, but continuous medium, because the 

 wave-lengths of the luminous vibrations vary from 0,00004 

 to 0,00007 cm., this being about thousand to ten thousand times 

 as great as the mutual distance between the consecutive particles 

 of the assemblage. 



From special phenomena observed with Ron t gen-rays, suspicion 

 had arisen among physicists, that the wave-length of these vibrations 

 which seemed to have a close analogy to ordinary light- waves, should 

 be extremely small, much smaller than those of the visible light. 

 Diffraction-phenomena studied by Hag a and Wind, afterwards 



10 



