188 Mr. C. Barus on Maxwell's Theory of the 



theory have been given by many others, indeed by almost all 

 the chief writers on solid viscosity ; but Maxwell carries them 

 through consistently to a complete theory. 



I have stated that Maxwell's theory of viscosity is the ana- 

 logon of Clausius's theory of electrolysis. Where the latter 

 uses " Theilmolekule " and electromotive forces to effect 

 chemical decomposition, Maxwell has unstable configurations 

 and stress available to produce permanent deformation. In 

 Clausius's case the number of decomposable molecules (i. e. 

 unstable configurations as regards the action of electromotive 

 force) in any given case of actual electrolysis is practically 

 infinite. This corresponds to Maxwell's viscous fluid, hard or 

 soft. In a viscous solid molecular configurations are present 

 in all degrees of stability, with a sufficient preponderance of 

 stable configurations to constitute a solid framework. The 

 relative number of unstable configurations varies with the 

 viscosity of the material. If, therefore, I conceive the case of 

 an electrolyte exhausting itself in respect to electrical conduc- 

 tivity by the chemical decomposition induced by current, until 

 conduction cease, I have the analogon of a solid which is 

 reaching the limit of viscous deformation. 



From this analogy it follows that a solid (?) electrolyte is 

 necessarily viscous ; whereas a viscous solid is only an elec- 

 trolyte when the configurations break up into parts oppositely 

 charged. Again, a viscous solid (?) is probably more viscous 

 when undergoing electrolytic decomposition than when no 

 current passes through it. Experiments to the same effect 

 can, however, be made with greater facility if the solid operated 

 on is such that special instability of molecular configuration 

 can be superinduced by heat instead of electrical action. Such 

 a solid is hard steel, in which, in addition to the ordinary thermal 

 instability, what may be called a carburation instability of mo- 

 lecular configuration asserts itself, even at mean atmospheric 

 temperatures. Inasmuch, therefore, as the gist of Maxwell's 

 theory is stability of molecular configuration, it follows that 

 the evidence which can be derived with reference to it from 

 hard steel must be unique in character: for, despite the extreme 

 hardness and rigidity of tempered steel, instability of molecular 

 configuration demonstrably exists* and is distributed uniformly 

 throughout the metal ; moreover the number of unstable mo- 

 lecular groups can be made to vary over an enormous range 

 at pleasure. 



4. Perhaps the experiments already made on the viscosity 



* Barus and Stroukal, American Journ. [3] xxxii. p. 276, 1886. 



