D-K 



C. JBarus — Viscosity of Solids. 181 



d'dt ttB* 



&t£*w\-» d ">\ 



where D is the moment of the applied couple, // Lamy's con- 

 stant and tp some function of co. 



Replying to Meyer's critique that a theory. of this kind is at 

 variance with the present state of knowledge in atomistics, 

 Boltzmann* disclaims all present purpose to connect his theory 

 with definite physical hypotheses. He points out, however, 

 that the assumed dependence of the existing strain on fore- 

 going deformations, is easily justified when the simultaneous 

 changes of molecular configuration are taken into account : for 

 it is not necessary to suppose that the elastic forces, as such, 

 have any dependence on the preexisting stress. The changes 

 of configuration in question are closely similar to Maxwell's, so 

 that in this respect, Boltzmann's theory may be looked upon as 

 one form of mathematical development of Maxwell's physical 

 hypothesis. I may add that Boltzmann tested his theory with 

 .data of Kohlrausch, Neesen, and Streintz. A special series of 

 experiments subsequently undertaken by Kohlrauschf give 

 additional strength to Boltzmann's deductions. The theory 

 does not, however, predict permanent set. 



A theory similar to Boltzmann's, but atomistic in character, 

 is due to Seesen.J It also assumes the occurrence of solid vis- 

 cosity. Neesen distinguishes the forces producing and retard- 

 ing motion and the final purely elastic forces which obtain 

 when viscous motion has subsided. Neesen practically postu- 

 lates a change in the constants of elasticity. Warburg§ objects 

 to Neesen's deductions because they contain no reference to the 

 form of the molecule. Meyer (1. c.) fails to find in it definite 

 causal relations to the observed viscous motion. 



Braun'sJ research, though largely experimental in character, 

 deserves mention here, because of special light which it throws 

 on the superposition of different viscous deformations. Ex- 

 cepting in glass, he finds that these molecular motions do not 

 take place independently of each other. He concludes that 

 elastic and viscous deformations owe their occurrence to forces 

 of different origin, and he refers viscous motion to the partial 

 rotations postulated by Weber and Kohlrausch. 



Warburg, *f following out the suggestions contained in 

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



* Boltzmann: Wied. Ann., v, p. 430, 1878. 

 f Kohlrausch: Pogg. Ann., clx, p. 225, 1877. 

 i Neesen: Pogg. Ann., clvii, p. 579, 1876. 

 § Warburg: Wied. Ann., iv, p. 233, 1878. 



|f Braun: Pogg. Ann., clix, p. 337, 1876; cf. Kohlrausch: Pogg. Ann., ox, p. 

 227, 1877. 



T Warburg: Wied. Ann., iv, p. 232, 1878. 



