C. Barus — Viscosity of Solids. 201 



to solids in the restricted sense of " elastische Xachwirkung." 

 Thus if a glass-hard and a soft steel rod be subjected alike and 

 at ordinarry temperature to torsional stress of continually in- 

 creasing magnitude, a stress value will be reached for which 

 the viscosity of the hard rod will be equal to, and eventually 

 overtake the viscosity of the soft rod. I was able to exhibit 

 this phenomenon in even a more striking way at 190°, finding 

 that for rates of twist less than r=3°, steel rod (radius =0"04:l cm ) 

 is much less viscous and more susceptible to the influence 

 of temperature in proportion as it is permanently harder;* 

 whereas for rates of twist greater than t=6°, steel ccet. par. is 

 less viscous and more susceptible to the influence of the influ- 

 ence of temperature in proportion as it is softer. 



Here I may profitably advert to certain considerations postu- 

 lated in an earlier paperf relative to the association of hard- 

 ness with resistance against oo-forces acting through zero-time, 

 and the association of viscosity with resistance against zero- 

 forces acting through oo-time, all magnitudes being- regarded 

 from a relative point of view. " We may reasonably conceive," 

 is there further stated, "that in case of viscous motion the 

 molecules slide into each other or even partially through each 

 other, per interchange of atoms, so that the molecular config- 

 uration is being continually reconstructed ; whereas in the 

 other case (hardness) the molecules are urged over and across 

 each other . . . ." In the ordinary case of scratching the 

 action is usually accompanied by physical discontinuity of the 

 parts tangentially strained. 



The intensity of stress by which the above deformations are 

 evoked was nearly constant and equal to 0*5 kg. on centimeter 

 of arm. This couple, when applied to the given steel rods 

 (radius 0*011 cm ), is admirably adapted for the exhibition of the 

 nearly pure viscous phenomenon, the " j^achwirkung" of Weber 

 and Kohlrausch. 



It is just here that certain cardinal distinctions must be 

 made. According to Maxwell's views, viscosity is the same 

 phenomenon in liquids and in solids, and the molecular 

 mechanism by which it manifests itself quite the same in both 

 cases. There is nothing in the theory to induce the reader to 

 limit viscosity in solids to certain special changes of configura- 

 tion. In solids at high temperatures, and of course in viscous 

 fluids, there is indeed no need of distinction; and viscosity 

 appears as the one property into which the other configuration- 

 properties of solid matter eventually merge. In solids, at low 

 temperatures on the other hand, the case is much more com- 

 plex ; and whereas viscosity ("Nachwirkung") still appears as 



* An. 190°. being of course the maximum hardness admissible. 

 f This Journal. Ill, xxxiii. p. 23. 1S87. 



