290 Prof. W. Thomson on the Elasticity [May 18, 



The frictional resistance against change of shape must in every solid be 

 infinitely small when the change of shape is made at an infinitely slow 

 rate, since, if it were finite for an infinitely slow change of shape, there 

 would be infinite rigidity, which we may be sure does not exist in nature*. 

 Hence there is in elastic solids a molecular friction which may be pro- 

 perly called viscosity of solids, because, as being an internal resistance to 

 change of shape depending on the rapidity of the change, it must be 

 classed with fluid molecular friction, which by general consent is called 

 viscosity of fluids. But, at the same time, it ought to be remarked that 

 the word viscosity, as used hitherto by the best writers, when solids or 

 heterogeneous semisolid-semifluid masses are referred to, has not been dis- 

 tinctly applied to molecular friction, especially not to the molecular friction 

 of a highly elastic solid within its limits of high elasticity, but has rather 

 been employed to designate a property of slow continual yielding through 

 very great, or altogether unlimited, extent of change of shape, under the 

 action of continued stress. It is in this sense that Forbes, for instance, has 

 used the word in stating that " Viscous Theory of Glacial Motion" which he 

 demonstrated by his grand observations on glaciers. As, however, he, and 

 many other writers after him, have used the words plasticity and plastic, both 

 with reference to homogeneous solids (such as wax or pitch even though 

 also brittle, soft metals, &c.), and to heterogeneous semisolid-semifluid 

 masses (as mud, moist earth, mortar, glacial ice, &c.), to designate the 

 property f common to all those cases of experiencing, under continued stress, 

 either quite continued and unlimited change of shape, or gradually 

 very great change at a diminishing (asymptotic) rate through infinite 

 time, and as the use of the term plasticity implies no more than does 

 viscosity any physical theory or explanation of the property, the word 

 viscosity is without inconvenience left available for the definition I propose. 



To investigate the viscosity of metals, I have in the first place taken 

 them in the form of round wires, and have chosen torsional vibrations, 

 after the manner of Coulomb, for observation, as being much the easiest 

 way to arrive at definite results. In every case one end of the wire was 

 attached to a rigid vibrator with sufficient firmness (thorough and smooth 

 soldering I find to be always the best plan when the wire is thick enough) ; 



has been found scarcely possible to count more than twenty of them in one case experi- 

 mented on. 



* Those who believe in the existence of indivisible, infinitely strong and infinitely 

 rigid very small bodies (finite atoms !) may deny this. 



t Some confusion of ideas on the part of writers who have professedly objected 

 to Forbes's theory while really objecting only (and I believe groundlessly) to his 

 usage of the word viscosity, might have been avoided if they had paused to consider 

 that no one physical explanation can hold for those several cases, and that Forbes's 

 theory is merely the proof by observation that glaciers have the property that mud 

 (heterogeneous), mortar (heterogeneous), pitch (homogeneous), water (homogeneous), all 

 have of changing shape indefinitely and continuously under the action of continued 

 stress. 



