44 Mr. H. Tomlinson. The Influence of Stress [Apr. 28, 



The effect of suddenly chilling steel heated to a high temperature 

 was found to be similar to that of excessive permanent extension 

 of iron ; and in general, it was concluded to be highly probable that 

 whether the distance between the molecules be increased by mechanical 

 strain or by the strain caused by sudden cooling, the elasticity in the 

 direction of the line of separation of the molecules diminishes to a 

 minimum as the separation increases, and then begins to increase. 



Moreover, it would appear from the results which will be described 

 in the other parts of this paper, that most, if not all, the physical 

 properties of a substance are affected in a similar manner by stress and 

 strain ; whether these latter be the result of mechanical or of any 

 physical agency. 



Torsional Rigidity. 



The torsional rigidity of the wires was determined by the method of 

 vibrations. The vibrators were similar to those employed by Sir 

 W. Thomson in his experiments on the rigidity and viscosity of metals,* 

 namely, thin cylinders of sheet brass, supported by a thin, flat, rect- 

 angular bar. The wire to be tested passed perpendicularly through a 

 hole in the middle of the bar r and was there soldered ; the other end 

 of the wire was then soldered into a stout iron bar, firmly held in a vice 

 attached to a rigid support. 



Great difficulty was experienced in making gx>od observations of the 

 rigidity of zinc, tin y and lead, in consequence of their great viscosity. 



Several experiments were made to test the effect of permanent tor- 

 sion and permanent extension on the modulus of rigidity. 



From these experiments was concluded : — 



(1.) That the loss of rigidity produced by twisting or stretching a 

 wire beyond the limits of elasticity is partly diminished by rest. 



(2.) That the loss is more sensible with large arcs of vibration than 

 with small ones. 



(3.) That the influence of rest is more apparent in the case of large 

 vibrations than in that of small ones. 



(4.) That continual vibrating through large arcs has a similar effect 

 on the rigidity to that produced on the longitudinal elasticity by heavily 

 loading and unloading. And — 



(5.) That in the case of hard steel the effect of vibrating through a 

 large arc for several minutes makes temporarily the rigidity as deter- 

 mined from such vibrations greater than that determined from smaller 

 vibrations. 



From the values of the torsional rigidity and " Young's modulus " 

 were calculated the ratios of lateral contraction to elongation for the 



different metals, the formula used for this purpose being a=^-— 1, in 



* " Proc. Roy. Soc," vol. 14, p. 289. 



