200 C. Bo/rus — Viscosity of Solids. 



sive. Steel An. 200° is in a state of incipient annealing at 

 200°. Thermal and carburation instabilities of high degree 

 being superposed, the effects are correspondingly large. 



Finallv above 300° the molecular instability is largely ther- 

 mal. The behavior of hard steel therefore approaches that of 

 other metals more nearly.* The effect of the carburation in- 

 stability ceases to predominate, and finally vanishes altogether 

 in proportion as the march is made from lower to higher tem- 

 peratures of annealing. 



13. I have finally to touch upon the series of phenomena in 

 which pronounced annealing occurs simultaneously with pro- 

 nounced external viscous deformation. If, for instance, a 

 glass-hard rod is twisted and then suddenly heated to 100°, the 

 rod is both annealed and suffers deformation in virtue of the 

 applied twist at the given temperature. Conformably with 

 the excessively greater amount of molecular instability which 

 characterizes these experiments, the observed viscous deforma- 

 tion must be proportionately large. This prediction of Max- 

 well's theory is fully verified by experiment In the case of 

 the twisted rod postulated, the motion of the'image across the 

 field of the telescope is so rapid, that Gauss's method of angular 

 measurement is no longer satisfactorily available. I may say 

 without exaggeration, that during the small interval of time 

 within which appreciable annealing occurs, a glass-hard steel 

 rod suddenly heated to 300°. is a viscous fluid. I have shownf 

 that if a glass-hard and a soft rod (ccet. par.) be identically 

 twisted and heated to 350°, the former will have lost all its 

 strain, whereas in the soft rod only about -J will have vanished. 



Advantage may be taken of two simultaneous causes of 

 molecular instability in other and purely mechanical ways. 

 Thus molecular instability is produced by drawing soft steel 

 wire through a draw-plate, and the instability increases enor- 

 mously with the intensity of the strain. Experiments which I 

 made in some number by counter-twisting soft and hard drawn 

 steel wire at 30° and at 100°, showed results quite comparable 

 in striking interest with the behavior of tempered steel. The 

 character of both phenomena is the same, so that as far as vis- 

 cous comparisons go, the drawn strain replaces the temper 

 strain perfectly. Of. § 17. 



14. Viscosity in the above pages has been considered apart 

 from the stress-intensity under which the deformation takes 

 place. This is liable to lead to confusion, unless the stress 

 relative to which the constants of viscosity are defined, be 

 kept clearly in mind; or unless the terms viscosity be ap])lied 



*Cf. § 17 



+ This" Journal. III. xxxiv. p. 4, 5. 1887. Kxperiments made by annealing 

 twisted systems. 



