1886.] Stress and Strain on the Properties of Matter. 241 



internal friction of metals differs from the viscosity of fluids, for in 

 cases of damping by the latter the logarithmic decrement is inversely 

 as the length of the vibration-period. 



Permanent molecular strain resulting from loading not carried to 

 a sufficient extent to produce sensible permanent extension, diminishes 

 the internal friction, and increases the torsional elasticity. 



Considerable permanent longitudinal extension and permanent 

 torsion produce increase of internal friction and diminution of tor- 

 sional elasticity. The effect of torsion is much greater than that of 

 extension, and the increase of internal friction is much greater than 

 the decrease of torsional elasticity. As a consequence wire- drawing, 

 where we have permanent extension and torsion combined, sometimes 

 increases enormously the internal friction ; in fact in the case of six 

 different metals, it was found that by careful annealing the internal 

 friction was decreased from one-half to one-thirtieth of the original 

 amount of friction of the metals in the hard-drawn condition. Almost 

 equally remarkable is the effect of rapid fluctuations of temperature, 

 even through ranges of only one or two degrees centigrade, in in- 

 creasing the internal friction. 



The internal friction of a metal wire, whether in the hard-drawn 

 or annealed condition, is temporarily decreased, and the torsional 

 elasticity is temporarily increased by loading not carried beyond a 

 certain limit, beyond this limit both the friction and the elasticity 

 become independent of the load. 



The "fatigue of elasticity," discovered by Sir William Thomson 

 in metal wires when vibrating torsionally, is not felt, provided the 

 deformations produced do not exceed a certain limit, depending upon 

 the nature of the metal. The above-mentioned limit is extraordi- 

 narily low for nickel, so low, indeed, that it is difficult to avoid 

 " elastic fatigue " with this metal. This last consideration, and others 

 founded on the results of experiments on the effects of stress on the 

 physical properties of nickel, tend to show that the molecules of this 

 metal are comparatively easily rotated about their axes. 



The author agrees with Prof. Gr. Wiedemann, that the loss of 

 energy due to internal friction in a torsionally vibrating wire is mainly 

 due to the to-and-fro rotation of the molecules about their axes ; any 

 cause, therefore, which increases the mo]ecular rotatory elasticity 

 diminishes the internal friction, and conversely. The author has, by 

 various means, succeeded in bringing down the internal friction to 

 such an extent that, in the case of one wire, it would have required 

 upwards of 15,000 vibrations to diminish the amplitude to one-half 

 of its initial value, provided the vibrations had been executed in 

 vacuo. 



The molecules of a metal tend to creep into such positions as 

 will ensure a maximum molecular rotatory elasticity, and they can be 



