70 



Royal Society : — 



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 sus- 

 pected that the diminution in the torsional rigidity on the whole 

 length of the wire might be due to inequalities 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 se- 

 parately. 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. 





Length, in 

 centimetres. 



Weight, in 

 grammes. 



Weight per 

 centimetre, 

 in grammes. 



No. 1 



;, 2 



» 3 

 u 4 



109-2 

 66-7 

 63-2 

 69-4 



5-023 

 3-050 

 2-865 

 3-143 



•04600 

 •04573 

 •04533 

 •04517 





308-5 



14-081 





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

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





Length, in 



Weight, in 



Weight per 





centimetres. 



grammes. 



centimetre, 

 in grammes. 



No. 1 



66-3 



3-183 



•04801 



» 2 



66-4 



3-083 



•04643 



„ 3 



66-5 



3-039 



•04570 



„ 4 



66-8 



3-072 



•04599 



„ 5 



43*4 



1-986 



•04576 



By several determinations of observations on the elongations within 

 the limits of elasticity produced by hanging weights on long wires 

 C 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 further investigation. The amount of the 



