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



amount of free motion. Viscosity in solids is the result of 

 changes of molecular configuration brought about by inter- 

 change of atoms of contiguous configurations, in the manner 

 explained by the Clausius-Maxwell principle*. In solids 

 under stress such changes take place gradually, through 

 infinite time, because the conditions favourable to inter- 

 transfer of atoms are of rare occurrence, and because the 

 number of molecules for which such transfer is possible is 

 limited. This limited interchange of atoms between mole- 

 cules under stress must be a property common to solids, if, 

 according to Maxwell's conception, solids are made up of 

 configurations in all degrees of molecular stability. 



This premised, farther distinction may be made. Questions 

 arise as to whether such action can be indefinitely repeated 

 without rupture, as in plastic solids or in viscous fluids ; or 

 whether it cannot be indefinitely repeated, as in brittle solids, 

 &c. This indefinite repetition of the phenomenon is equivalent 

 to a passage of molecules over or across each other, the 

 phraseology above used in reference to hardness")". 



15. The observations made in the above paragraphs relative 

 to the visible viscous subsidence of a mechanical strain im- 

 posed on a steel rod apply for the complete explanation of the 

 phenomenon of temper. With this purpose in view, it is 

 merely necessary to conceive of hardening or quenching 

 (sudden cooling of steel) as an operation by which a strain of 

 dilatation is imparted to steel. This strain, once applied, is 

 locked up in the metal in virtue of viscosity %. 



The strained structure of hard steel is proved by the ten- 

 dency to rupture during quenching exhibited by the metal, 

 and by the fact that massive pieces of hard steel often explode 

 spontaneously §. The temper-strain may be studied, optically 

 and in other ways, in glass, and at low temperatures even in 



^ * " Betrachten wir ferner das Verhalten der Gesanirnt-moleciile unter 

 einander, so glaube ich dass es auch hier zuweilen geschieht, dass das 

 positive Theilmoleciil eines Gesammt-ruoleciils zu dem negativen eines 

 anderen in eine giinstigere Lage kommt, als jedes dieser beiden Theil- 

 moleciile im Augenblicke gerade zu dem anderen Theilmoleciil seines 

 eignen Gesammt-moleciils hat &c.''" {Median. Warmtheorie, Band ii., 

 2 Ann. 1879, p. 163.) 



Again : — " Thus we may suppose that in a certain number of groups 

 the ordinary agitation of the molecules is liable to accumulate so much 

 that every now and then the configuration of one of the groups breaks up, 

 and this whether it is in a state of strain or not." .... 



" But if a solid also contains groups of the first kind which 



break up of themselves " (Maxwell, I. c.) 



t Cf. Am. Journ. xxxiv. pp. 1, 18, 1887. 



t Cf. Bull. U. S. Geolog. Survey, no. 14, p. 88. 



§ Batchelder, Journ. Frank. Inst. [3] viii. p. 133, 1844. 





