﻿66 Breaking Stress of Crystals of Rock- Salt. 



This is obtained by dividing P by p 1/3 , and becomes 



P 415000x981 1 . ' 

 P7* = 2'82xl0? =1*4 dynes per cm. 



If the attraction between tbe charges in a row is not 

 appreciably influenced by the adjacent rows, this value will 

 represent the accurate value of surface tension in the direction 

 coinciding with the vertical axis. If the field is symmetrical 

 in all three directions parallel to the main axis, this figure 

 will characterize the surface tension of rock-salt in all 

 three directions. 



The symmetrical structure indicated by W. H. and W. L. 

 Bragg is in accord with the experiment in this sense, but on 

 the other hand, the above figure is about 1000 times less than 

 one would expect. 



One could expect such a small value if the salt consisted 

 of molecules with a very small polarity situated at large 

 distances from one another. But in such a case the force 

 in the direction coinciding with that of the doublet would 

 have to be twice that in the perpendicular direction. This, 

 however, is not the case." 



In my paper (loc.cit.) it was shown that in a case of small 

 doublets the adjacent rows have practically no influence on 

 the attraction between the doublets. 



However, in a case of charges situated at regular distances 

 from one another, such seems not to be the case. 



If the charges influence one another, one can expect the 

 forces to be weaker in the middle of the substance, and much 

 bigger at the surface where the above effect is only one- 

 sided. 



It is therefore probable that the above figure 1*4 dynes 

 per cm., although quite characteristic for the substance, is 

 not the actual value of the force in the surface layer. The 

 calculation of these effects is not easy, owing to the fact that 

 one has to deal with a very slowly converging series. I 

 satisfied myself that these influences may be appreciable, but 

 I do not see clearly at the present time whether they can 

 account for the weakening of the forces about 1000 times, 

 or even more. 



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