0. Barus — Viscosity of Solids. 199 



hard steel, continues steadily to increase.* Indeed chemical 

 decomposition above 300° is more marked than below 300° ; 

 yet its bearing on Maxwell's theory is now without interest, 

 because in none of the high annealed rods do configurations 

 unstable at mean atmospheric temperature survive after an- 

 nealing. 



11. Having analyzed the phenomena at mean atmospheric 

 temperature, I come next to consider the conditions of mean 

 relative viscosity at 1 00°. The glass-hard state must here be 

 withdrawn, for consideration in § 13 ; because such a rod would 

 undergo annealing during the viscous measurements at 100°. 



Hard steel annealed at 100° bears the same relations to 100° 

 that glass-hard steel does to mean atmospheric temperature. 

 Hence the reasoning of the preceding paragraph, mut. mut., 

 applies at once. It is merely necessary to bear in mind, that 

 100° is now the temperature of incipient annealing, and that 

 therefore the temperatures which produce corresponding vis- 

 cous effects are proportionately higher. Rods An. 200° now 

 occupy about the same relative position that rods An. J 00° did 

 in § 9 ; An. 300° the same relative position as An. 200°, etc. 

 Moreover for equal increments of the temperature of annealing, 

 the increment of viscosity shown by the rod at 100° diminishes 

 rapidly as temperature increases, etc. 



In one respect the present results differ from the above : the 

 phenomena are here spread out over a scale (roughly estimated) 

 about ten times larger. This means, following Maxwell's the- 

 ory, that at 100° the number of unstable molecular configura- 

 tions is relatively much larger than at mean atmospheric tem- 

 perature. The reasons, though not far to seek, are exceedingly 

 significant. In hard steel, at 100° two causes of molecular 

 instability here produce superposed effects. The first is the 

 chemical or carbon instability already discussed ; the second 

 cause is purely thermal. Of. § 17. 



The explanation of the diagram for 100° is now clear. Vis- 

 cous deformation is marked in all the rods examined for An. 

 500° to An. 100° ; but the deformability increases at a rapid 

 pace in proportion as we pass from softer to harder steel, be- 

 cause in such a march the carburation instability, superimposed 

 upon the thermal instability, increases rapidly. Molecular con- 

 figurations on the verge of instability are encountered in con- 

 tinually increasing numbers. 



12. The line of argument followed out for 100°, applies 

 mut. mut. at 200°. Results of this kind I published else- 

 where, f The character of the evidence bearing on all the 

 points in question is here even more pronounced and conclu- 



* This Journal, xxxii, pp. 277, 282, 1886. 

 f This Journ., Ill, xxxiv, p. 14 to 16, 1887. 



