248 



Properties of Vulcanised India-rubber. 



[June 6, 



tude which, the vibration would have if the phase of the vibration was 

 such that a maximum distance from the position of rest occurred at 

 that point. 



Let \ be the logarithmic decrement of the vibration, c 1 and c n the 

 amplitudes of the 1st and %th vibrations respectively, 



1 c 

 then X -= r-log e 10 log— ,* 



and if p be the coefficient of viscosity, 



_ 4R?wl\ 



The symbols with the exception of X having the same meaning as 

 those in Experiment (4). 

 The values found for p were 



p. 



Soft grey 1374 ^ 



Red 2' 678 > pounds per square inch. 



Hard grey 7'725J 



The coefficient p represents the tangential force required to distort a 

 cube of one inch of the material at the rate of one inch per second, 

 independently of that necessary to overcome the elastic reaction, on 

 the assumption that the viscous resistance to distortion varies as the 

 rate of distortion. Part of the apparent viscosity however must be 

 due to the difference of the rates at which the sub-permanent set is 

 produced and removed. 



(8.) The densities of the specimens were 



Soft grey 1'289 



Red 1-407 



Hard grey 2'340 



(9.) Chemical Composition. 



I had no means at my disposal here of making a good analysis, 

 but a rough determination of the percentage of sulphur was obtained 

 by decomposing a known weight of each kind with caustic soda and 

 nitre, and observing the quantity of barium chloride required in each 

 case to precipitate the sulphate formed. The results were 



Sulphur, 



per cent. 



Soft grey 5*7 



Red 2-1 



Hard grey 3*8 



* Maxwell's ' Electricity and Magnetism,' vol. 2, 239. 



