196 0. Bar us — Viscosity of Solids. 



Cycles here, fixed or not, are expressions of the fact that the 

 " past histories " (in Maxwell's words) of the molecular config- 

 urations in the " stress positive " and " stress negative " phase 

 of each cycle are not the same. Shifting is brought, about by 

 permanent molecular break op, the amount of which gradually 

 vanishes. In the ultimate and fixed cycle as many configura- 

 tions are broken during the " stress positive " as are recon- 

 structed in the u stress negative " phase, though they need not 

 be the same configurations. 



These considerations suggest a comparison between " accom- 

 modation" and Prof. E wing's* "hysteresis," for the purpose of 

 detecting the extent to which like causes are discernable in 

 each phenomenon. Both exhibit a static character. But such ' 

 comparison would not be fruitful without special and direct 

 experiments; for the instcmtaneous values of stress and of vis- 

 cosity must be coordinated. 



9. Having thus discussed one phase of the results in table 1, 

 I pass to table 2, which is a digest of the mean values of table 

 1, in so far as such a digest can be made. Following the scheme 

 at the end of the preceding paragraph, this comparison should 

 be made after an infinite number of twists have been imparted 

 to each wire. In such a case, however, the original number of 

 unstable configurations has been seriously reduced; so that 

 apart from the annoyance of so time-consuming a method as 

 this, the original properties of the wire are not clearly present 

 in the results. In wires perfectly free from strain, at the out- 

 set, the first twist leads to the best indications of the viscous 

 quality. As this condition could not always be guaranteed for 

 the wires of this paper, I have accejDted the mean viscous be- 

 havior during the first and second twists as the best available 

 index for comparison. It is sufficient, at least for the present 

 purposes. Again taking the mean for rods of the same nomi- 

 nal temper, I obtain data from which a chart is easily con- 

 structed, by representing mean viscous deformation (ip—<p')JT, 

 varying with time, for each of the divers degrees of hardness 

 An. 25°, An. 100°, An. 190°, An. 360°, An. 450°. It so hap- 

 pens that the normal rod, No. 1, is less viscous than the other 

 rods, Xo. 2 and No. 3, of like temper. Hence the negative 

 numbers in table 2, which may be eliminated by increasing the 

 other data. 



Returning to table 2, it is clear, inasmuch as viscous deform- 

 ations are measured differentially, that (<p — <p')lT and d Q —A\ 

 are to be compared It appears that these quantities increase 

 and decrease together. This is more easily discernable when 



*Ewing: Phil. Trans. Roy. Soc, IT, p. 523, 1885; ibid., II, p. 361, 1886. 

 Prof. Evring's earlier papers are there given. Cf. also Cohn, Wied. Ann., vi. p. 

 403, 1879. 



