184 Mr. C. Barus on Maxwell's Theory of the 



Weber and Kohlrausch, and more recently by Nissen. In 

 almost all cases, excepting alone Butcher's work *, which 

 formulates the theory of Maxwell f , the problem has been 

 approached from distinct points of view. 



Despite the diversity of methods of discussion and the 

 elaboration of evidence, the results arrived at do not in any 

 case so fully represent the phenomenon as to lead to general 

 acquiescence in one elementary physical hypothesis. Boltz- 

 mann's theory is perhaps the most powerful, and is elegantly 

 worked out ; but it is purely mathematical in character. 

 Maxwell's theory has the soundest 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 on any asser- 

 tion, that this theory is a version of Williamson's % theory of 

 etherification, and of Clausius's § theory of electrolysis ; for 

 viscosity in Maxwell's conception is a phenomenon evoked by 

 certain changes of molecular structure, the inherent nature of 

 which is ultimately chemical. I say chemical, because if mole- 

 cular break-up occur, cardinal questions at once arise as to the 

 manner of removal of the debris ; arid 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 behaviour of steel when regarded as a viscous solid and 

 in the light of known facts || is convincingly in favour of the 



* Butcher, Proc. Lond. Math. Soc. iii. 1878. 



t Maxwell, " Constitution of Bodies," Encyelop. Brit. 9th ed. p. 310, 

 1876. 



\ Williamson, Ann. der Chem. und Pharm. lxxvii. p. 37, 1851. 



§ Clausius, Pogg. Ann. c. p. 353, 1857 ; ibid. ci. p 338, 1857. 



j| I refer in particular to the work of Dr. Strouhal and myself. Hard- 

 ness (temper), its electrical and other characteristics, cf. Barus, Phil. 

 Mag. [5] viii. p. 341, 1879; Wied. Ann. vii. p. 383, 1879; Strouhal and 

 Barus, Wied. Ann. xi. p. 930, 1880 ; ibid. xx. p. 525, 1883. Hardness 

 and magnetization, cf. fc>. and B., Wied. Ann. xx. pp. 537, 662, 1883. 

 Density and (internal) structure of hard steel and of quenched glass, 

 cf. Barus and Strouhal, American Journ. [3] xxxi. p. 386, 1886; ibid. 

 p. 439 ; Am. Journ. xxxi. p. 181, 1886. Temper and chemical composi- 

 tion, cf. B. and S., Am Journ. [3] xxxii. p. 276, 1886. Temper and vis- 

 cosity, cf. B. and S., Am. Journ. [3] xxxii. p. 444, 1886 ; ibid, xxxiii. p. 20, 

 1887 ; Barus, ibid, xxxiv. p. 1, 1887 ; ibid, xxxiv. p. 175, 1887. These 

 papers, systematically discussed and enlarged, are embodied with new 

 matter in the Bulletins of the United States Geological Survey, viz. : — 

 Bull. no. 14, pp. 1-226, 1885; Bull. no. 27, pp. 30-61, 1886; Bull, 

 no. 35, pp. 11-60, 1886 ; Bull. no. 42, pp. 98-131, 1887. Others are 

 forthcoming. The Bulletins can be obtained by addressing the Director 

 of the U.S. Geological Survev. 



