216 Dr. H. Ohatley on Cohesion. 



It would appear that in " amorphous " solids crystalline 

 grains are cemented together by Ci solid " films of the mother 

 liquor from which the crystals hare formed (Desch, Institute 

 of Metals, Sept. 11th, 1919), and in such thin films pre- 

 sumably the principal crystal axes are perpendicular to the 

 faces of the films. 



(3) Cohesion being vectorized, no " central - force " 

 formula can correctly express it, but the writer suggests an 

 empiric rule for mean cohesions of hypothetical atom spheres 



t 2 = Gm 2 /d 



-I 



where n is between 4 and 5 ; t 2 is the cohesion bond ; 



k is ratio (>1) of mean atomic interval to that at 



absolute zero ; 

 G, m, and d as in the Newtonian formula. 



This formula has three possible advantages : — 



(a) It is continuous with the Newtonian formula, i. e. 



when k = co , t = Gm 2 jd 2 . 



(b) It is dimensionally fairly correct for many cases of 



cohesion. 



(c) At absolute zero (i. e. atomic " contact ") it 



practically agrees with chemical affinity. 



(d) Its space rate of decrease is not inconsistent with 



known strengths of materials. Near absolute zero 



© 



the index of the distance is the inverse sixth. 



(e) It is polarized (possibly not correctly), since for all 



powers other than the inverse square the force is 

 not centralized. 



\_Note. — The inter-atomic repulsion appears to be of the 

 order 



_RT 1 



'"Ndo" 3(£-l)' 



where d — atomic spacing at absolute zero; R, T, and N as 

 in kinetic theory of gases.] 



(4) The writer is especially interested in this subject in 

 connexion with problems of accretion, erosion, and stability 

 of river-mud, and lias obtained certain indications of interest 

 from conditions in the Huangpu River: — 



(a) Fine mud resists erosion until the hydrodynamic 

 tangential drag on a surface particle equals about 10 _(? dyne, 

 i. e. each particle, containing some millions of molecules, is 

 retained by at least one inter-molecular " cohesion " bond. 



