64 Royal Society : — 



more rapid than in glass, marvellously rapid indeed, in some metals 

 (as for instance zinc) *, and in india rubber, and even in homoge- 

 neous jellies. 



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 f. Hence there is in elastic solids a molecular 

 friction which may be properly 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 distinctly ap- 

 plied 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 ob- 

 servations 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 J 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 im- 

 plies no more than does viscosity any physical theory or explana- 

 tion 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 



* Torsional vibrations of a weight hung on a zinc wire subside so rapidly, 

 that it has been found scarcely possible to count more than twenty of them in one 

 case experimented on. 



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

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



| Some confusion of ideas on the part of writers who have professedly 

 objected to Forbes' s theory while really objecting only (and I believe ground- 

 lessly) 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 indefi- 

 nitely and continuously under the action of continued stress. 



