Viscosity encountered on passing from Fluid to Solid. 343 



9. From these Tables the mean rate of motion of marine 

 glue appears as 2 centim. per minute, whereas the corre- 

 sponding mean motion for hard steel is (say*) *4 centim. per 

 minute, for the three cases. The couple acting on the cement 

 is 1*4 #, while the couple in the other case is 5*7#. Since the 

 cement is a fluid, the rate of motion for identical couples 

 would be 8 centim. 



The principle given in § 1 may now be applied, whereby 

 viscosities are inversely as the fourth power of the radii, 

 cat. par. Let r/ and y be the viscosities of marine glue and 

 hard steel respectively. Then 



From Table I., ?/ = 200xl0 6 ; hence the mean viscosity of 

 originally untwisted glass-hard steel, during three consecutive 

 alternate twists of 12 minutes each, just within the limits of 

 rupture is at least 



rj = 25 xlO 15 g/sc (3) 



10. The present results subserve their chief purpose in 

 furnishing an estimate of the order of viscosity of glass-hard 

 steel ; for the unavoidable errors encountered in a comparative 

 method like the present are not insignificant. In the first 

 place the thread of marine glue observed for transpiration in 

 § 6 during half a year is not identical with the rod of this 

 material which is twisted (§ 8), neither as to temperature nor 

 composition (cf. remarks end of § 6). Possibly better results 

 might be obtained with old pitch. Again, the rod of marine 

 glue, while it is being twisted, is also being appreciably 

 stretched by its own weight. This complicates the angular 

 measurement. Finally, the sectional error of an opaque rod, 

 not necessarily free from air-bubbles, is considerable. Equa- 

 tion (3) must therefore be looked on as assigning an order to 

 the viscosity of hard steel above 10 16 g/cs. 



11. Before giving the results of the following direct method, 

 it is expedient to insert a paragraph on the viscosity of solids 

 generally. In § 8, while the steel wire is strained by the 

 couple almost as far as the limits of elasticity permit, the rod 

 of marine glue is initially scarcely strained at all. In the 

 latter case, however, the unstable configurations essential to 

 viscous motion are supplied in relatively great numbers by 

 the ever changing distribution of the heat-agitation within the 

 body. In a homogeneous solid free from strain (soft steel) 



* The viscosity of solids being a function of time, strain, and tempera- 

 ture. This distinction is beyond the purposes of the above paragraph, 

 but will be fully carried out below, in §§11 et seq. 



2 D 2 



