29G 



On the Elasticity and Viscosity of Metals. [May 18, 



Thus it appears that that specific rigidity which is concerned in torsion 

 is very markedly diminished in copper, brass, and iron wire when the wire 

 is elongated permanently by a simple longitudinal traction. "When I first 

 observed indications of this result, I suspected that the diminution in the 

 torsional rigidity on the whole length of the wire might be due to inequali- 

 ties in its normal section produced by the stretching. To test this, I cut 

 the wire into several pieces after each series of experiments, and weighed 

 the pieces separately. The result proved that in no case were there any 

 such inequalities in the gauge of the wire in different parts as could possi- 

 bly account for the diminution in the torsional rigidity of the whole, which 

 was thus proved to be due to a real diminution in the specific rigidity of 

 the substance. The following sets of weighings, for the cases of the wires 

 of the two last series of experiments on copper, may suffice for example : 



Wire of 308-8 centims. long, cut into four pieces. 



Wire of 301 '9 when last vibrated; further elongated by about 8 centi- 

 metres, when it broke ; then cut into five pieces in all. 



By several determinations of observations on the elongations within the 

 limits of elasticity produced by hanging weights on long wires (about 80 

 feet) suspended in the College tower, it seemed that Young's modulus was 

 not nearly so much (if at all sensibly) altered by the change of molecular 

 condition so largely affecting the rigidity ; but this question requires fur- 

 ther investigation. The amount of the Young's modulus thus found was, 

 in grammes weight per square centimetre, 1159 X 10 8 for one copper wire, 

 and 1153x 10 6 for another which had been very differently treated. 



