Viscosity of Solids and its Physical Verification. 21 1 



impaired. In the ordinary case of scratching the action is 

 often accompanied by physical discontinuity. The interpene- 

 tration of the molecules of a viscous substance is necessarily 

 favoured by temperature. Hence we infer the experimental 

 result that the viscous influence of temperature is marked. 

 If Clausius's theory of electrolysis be correct, then a certain 

 instability or imperfect uniformity in the molecular structure 

 of solids follows at once from the fact that many solids, 

 notably glass *, may be electrolyzed even at moderately high 

 temperatures (300°). 



The intensity of stress by which the above deformations are 

 evoked was nearly constant, and equal to 0*5 kilog. on centi- 

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

 rods (radius = 0'041 centim.), is admirably adapted for the 

 exhibition of a nearly pure viscous phenomenon, the " Nach- 

 unrkung" of Weber and Kohlrausch. 



It is just here that certain cardinal distinctions will have to 

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

 phenomenon in liquids and in solids, and the molecular me- 

 chanism by which it manifests itself quite the same in both 

 cases. There is nothing in Maxwell's theory to induce the 

 reader to limit viscosity in solids to certain special changes of 

 configuration. In solids at high temperatures, and of course 

 in viscous fluids, there is indeed no need of such 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 complex ; and whereas viscosity (" Nachwirkung" ') 

 still appears as a property common to solids, whether soft or 

 hard, plastic or brittle, these ulterior distinctions — softness, 

 hardness, plasticity (permanent set), brittleness, &c. — separate 

 solids by very broad lines. Hence it is improbable that the 

 whole of the mechanism, in virtue of which viscous deforma- 

 tions are possible in viscous fluids, is fully of the same nature 

 as that by which viscous motion takes place in solids at ordi- 

 nary temperatures. Viscosity in liquids is the mean result of 

 divers superposed phenomena, the occurrence of any one of 

 which, in a solid, would give rise to some special physical 

 property of that solid. From this point of view, since vis- 

 cosity is independent of the other physical properties above 

 enumerated, and since viscosity {Nachwirkung) is common 

 to solids without exception, I have ventured to refer it to such 

 action between contiguous molecules as involves the least 



* Warburg, Wied. Ann. xxi. p. 622, 1884. Literary notes are there 

 given. Warburg is able to replace six sevenths of the sodium of glass by 

 sodium of the anode. 



P2 



