204 C. Barns — Viscosity of Solids. 



stress may increase to an intensity sufficient to rupture the 

 metal explosively. 



In our earlier papers on this subject Dr. Strouhal and I were 

 much puzzled to know whether the temper-strain, and in gen- 

 eral the phenomena of annealing, were to be interpreted 

 physically or chemically ; whether annealing was a case of 

 viscous subsidence of a mechanical temper-strain; or a mere 

 case of decomposition of chemical hardness. In the light of 

 the present advanced conceptions, this distinction is superfluous. 

 It makes no difference whether the configuration breaks up 

 into parts chemically different (as carbon and iron (say) in 

 steel), or into parts chemically though not structurally identical 

 (as in homogeneous metals.) Viscosity is conditioned by the 

 degree of instability. Again it is cleatr that the principles 

 which account for the subsidence of the mechanical strain, will 

 also account at once for such chemical decomposition as is here 

 in question ; the difference in the two cases being vested in 

 mere details of molecular mechanism. §§13, 14. 



16. However complex the nature of the temper-strain in 

 steel ma}^ be, the behavior of hard steel, when subjected to the 

 influence of temperature, offers sufficient proof of its occur- 

 rence. The laws of annealing hard steel* are as follows : 



(1) The annealing effect of any temperature acting on glass- 

 hard steel increases gradually at a rate diminishing through 

 infinite time ; diminishing very slowly in case of low tempera- 

 tures ( < 100°) ; diminishing very rapidly at first and then 

 again slowly at high temperatures ( > 200°) ; so that the high- 

 est and hardest of the states of temper possible at any given 

 temperature is approached asymptotically. 



(2) The ultimate annealing effect of any temperature (time 

 = oo ), decreases at a retarded rate with temperature, and prac- 

 tically reaches the limit of variation below 350°. 



(3) The ultimate annealing effect of any temperature, t°, is 

 independent of the possibly pre-existing effects of the temper- 

 ature f°, and is not influenced by subsequent applications of 

 t'° ) provided t > t' . In case of partial annealing at t° (time 

 finite), this law applies more fully as the ultimate effect of t° 

 is more nearly reached. 



Postulating the strain discussed in § 15, these laws follow at 

 once from Maxwell's theory; and the explanation {mid, mut.) 

 is identical in character with that given in §§9 to 13, with 

 reference to the applied torsion strain. Inasmuch as annealing 

 is accompanied by chemical decomposition, the conditions 

 under which the temper strain is reduced are those of § 13. 



The third law of annealing asserts that the heat effect is 

 analytic, but not in the same degree synthetic. The carbon 



*Phil. Mag., V, viii, p. 341, 1879; Bull. U. S. G. S., No. 14, p. 195, 1885. 



