202 C. Bancs — Viscosity of Solids. 



a property common to solids, whether soft or hard, plastic or 

 brittle, these ulterior distinctions softness, hardness, plasticity 

 (permanent set), brittleness, etc., separate solids by very broad 

 lines. Hence it is improbable that the whole mechanism in 

 virtue of which viscous deformations are possible in viscous 

 fluids, is fully of the same nature as that by which viscous 

 motion takes place in solids at ordinary temperatures. Vis- 

 cosit} 7 in liquids is the mean result of divers superposed phe- 

 nomena, 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 viscosity 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 mole- 

 cules as involves the least amount of free motion. Viscosity 

 in solids, is the result of changes of configuration, resulting 

 from localized thermal agitation, and often superinduced by 

 the atomic attraction of contiguous configurations, in the man- 

 ner explained by the Clausius-Maxwell principle.* 



This premised, further distinctions may be made. Ques- 

 tions arise as to whether such action can be indefinitely 

 repeated without rupture, as in plastic solids, or in viscous 

 fluids ; or whether it can not be indefinitely repeated as in 

 brittle solids, etc. The indefinite repetition of the phenom- 

 enon is equivalent to a passage of molecules over or across 

 each other, the phraseology above used in reference to hard- 

 ness, f § 18. 



The ideas underlying this paragraph may be summarized 

 thus : In passing from the fluid to the solid state of matter, 

 the residual or extramolecular affinities acquire an increased 

 importance relatively to the intermolecular affinities. At the 

 same time the conditions of action have gradually become ex- 

 ceedingly unfavorable. In a liquid, under impressed favorable 

 conditions chemical reaction between molecules is demonstra- 

 ble (electrolysis). In a solid, under impressed favorable condi- 

 tions (strain of dilatation) it is also demonstrable, for instance, 



* "Betrachten wir ferner das Verhalten der Gesammtmolecule unter emander, 

 so glaube ich dass es auch hier zuweilen geschieht, dass das positive Theilmole- 

 ciil ernes Gesammtmoleciils zu dem negative!! eines anderen in eine giinstigre 

 Lage kommt, als jedes dieser beiden Theitmolecule in Augenblicke gerade zu 

 dem anderen Theilmolecul seines eigeneu Gesammtmoleciils hat, etc." Mech. 

 Warmeth.. ii, 2 Aim, p. 168, 1879. •• Again, following Maxwell: "... . Thus 

 we may suppose that in a certain number of groups the ordinary agitation of the 

 molecules is liable to accumulate so much that every now and then the configura- 

 tion of one of the groups breaks up, and this whether it is in a state of strain or 

 not. . . . ." 



"But if a solid also contains .... groups of the first kind which break up of 

 themselves , . . ." Maxwell, 1. c. 



f Cf. this Journal, xxxiv, pp. 1,18, 1887. 



