Viscosity of Solids and its Physical Verification. 187 



tional strength to Boltzmann's deductions. The theory, 

 however, does not predict permanent set. 



A theory similar to Boltzmann's, but of an atomistic kind, 

 is due to Neesen *. It also assumes the occurrence of solid 

 viscosity. Neesen distinguishes the forces producing and 

 retarding motion and the final purely elastic forces which 

 obtain after motion has subsided. Neesen practically postu- 

 lates a change of the constants of elasticity. Warburg f 

 objects to Neesen's deductions because they contain no 

 reference to the form of the molecule. Meyer fails to find in 

 it definite causal relations to the observed viscous motion. 



Braun's research!, though largely experimental in character, 

 deserves mention here, because of special light which it 

 throws on the superposition of different viscous deformations. 

 Excepting glass, he finds that these molecular motions do not 

 take place independently of each other. He concludes that 

 elastic and viscous deformation owe their occurrence to forces 

 of different origin, and he refers the viscous deformations to 

 the partial molecular rotation postulated by Weber and by 

 Kohlrausch. 



Warburg §, following out the suggestions contained in 

 Braun's results, formulates a new theory in which viscosity 

 is the result of partial rotations of finite molecules of a form 

 other than spherical. 



Nissen's || theory is unique. He considers the sether in the 

 space surrounding the body, in its relations to the sether 

 within the intermolecular spaces of the body ; and he bases 

 his theory of viscosity on the conditions under which the 

 external aether enters the said intermolecular spaces, when the 

 body is deformed by stress. He thus obtains both a time and 

 a thermal effect. In many respects this curious theory seems 

 to me to anticipate Osborne Reynolds's If in recognizing the 

 importance of the " dilatancy " of a granular medium. 



3. Maxwell's ** theory would require more extended com- 

 ment ; but the terms in which his views are expressed are so 

 precise that it is impossible to abbreviate them. The reference 

 must, therefore, here suffice ; cf. § § 9, 14. Aside from the 

 remarks of the next paragraph, the ideas underlying Maxwell's 



* Neesen, Pogg. Ann. clvii. p. 579, 1876. 

 t Warburg, Wied. Ann. iv. p. 233, 1878. 



\ Braun, Pogg. Ann. clix. p. 337, 1876 ; cf. Kohlrausch, Pogg. Ann. 

 clx. p.227, 1877. 



§ Warburg, Wied. Ann. iv. p. 232, 1878. 



|| Nissen, Inaug. Dissert., Bonn, 1880. (Not accessible in Washington.) 



r Reynolds, Rep. Brit. Assoc. 1885, p. 896 ; Phil. Mag. [5] xx. p. 469, 



•• Maxwell, /. c. p. 311. 



