C. Barns — Viscosity of Solids. 179 



Despite the diversity of methods of discussion and the elab- 

 oration of evidence, the results do not in any case so fully rep- 

 resent the phenomenon as to lead to general acquiescence in one 

 elementary physical hypothesis. Boltzmann's theory is per- 

 haps the most powerful and is elegantly worked out ; but it is 

 purely mathematical in character. Maxwell's theory has the 

 broadest physical basis, although left by its author in shape 

 merely of a terse verbal sketch. 



Now it seems to me, if indeed I may venture any assertion, 

 that Maxwell's theory is a version of Williamson's* theory of 

 etherification, and of Clausius'sf theory of electrolysis. The 

 transition made is from unstable groupings of atoms to unsta- 

 ble groupings of molecules. But preserving minutely all the 

 essentials of Maxwell's argument, the experiments of this 

 paper permit me to go one step further, by which viscosity is 

 a phenomenon evoked by certain changes of molecular struc- 

 ture, the inherent nature of which is ultimately chemical. I 

 say chemical because if molecular break up occur, cardinal 

 questions at once arise as to the manner of removal of the 

 debris ; and the phenomenon thus depends not only on the 

 past history, but on the immediate future history of the typical 

 mean configuration. The analogy of the three theories is very 

 close, so that they admit of generic classification. They are 

 examples of the invasion of statistical method into liquid and 

 solid molecular kinetics. 



The behavior of steel when regarded as a viscous solid and 

 in the light of known facts, J is convincingly in favor of the 

 view to be advocated ; and it was the direct bearing of some of 

 the results on Clausius's theory of electrolysis, that led me to 

 suspect a chemical explanation,§ before I became aware of the 

 existence of Maxwell's article. To show how clearly Maxwell's 

 theory interprets the complex and almost anomalous phenom- 

 ena of viscosity exhibited by steel, is the chief endeavor of the 

 present paper ; but I shall also add other matter. 



2. It is desirable to pass in brief review the divers hypothe- 

 ses on the nature of solid viscosity to which I have referred. 



O. E. Meyer's) theory is the earliest and most direct. It 

 discusses the action of elastic forces in a medium of imperfect 

 elasticity, and develops formulae to express the diminution of 



* Williamson: Ann. d. Cliem. u. Pharm.. lxxvii, p. 37, 1851. 



f Clausius: Pogg. App., c, p. 353, 1857; ibid., ci, p. 338, 1857. 



\ I refer in particular to the work of Dr. Strouhal and myself. These papers, 

 systematically discussed and enlarged, are embodied with much new matter in the 

 Bulletins of the U. S. Geological Survey, viz: Bull. No. 14, pp. 1 to 226, 1885; 

 Bull. No. 27 pp. 30 to 61, 1886; Bull. No. 35, pp. 1 1 to 60, 1886 ; Bull. No. 42, pp. 

 98 to 131, 1887. Other references are given in the text. 



§ This Journal, III, xxxiii, p. 28, 1887. It is much to be regretted that Max- 

 well's theory was published out of the line of a physicist's usual routine reading. 



I Meyer: Pogg. Ann., cli, p. 108, 1874. 



