Prof. W. Thomson on the Elasticity and Viscosity of Metals, 63 



4. Though in the solid and liquid food infinitesimal quantities of 

 lithium may enter the body, usually no proof of their presence in 

 the organs or secretions can be obtained. 



May 18. — Major-General Sabine, President, in the Chair. 



The following communications were read : — 



"On the Elasticity and Viscosity of Metals." By Prof. W. 

 Thomson, LL.D., F.R.S., F.R.S.E. 



Among the experimental exercises performed by students in the 

 physical laboratory of the University of Glasgow, observations on 

 the elasticity of metals have been continued during many years. 

 Numerous questions of great interest, requiring more thorough 

 and accurate investigation, have been suggested by these observa- 

 tions ; and recently they have brought to light some very unex- 

 pected properties of metallic wires. The results stated in the pre- 

 sent communication are, however, with one or two exceptions, due to 

 the careful experimenting of Mr. Donald Macfarlane, official assis- 

 tant to the Professor of Natural Philosophy, whose interested and 

 skilful cooperation have been most valuable in almost everything I 

 have been able to attempt in the way of experimental investigation. 



The subject has naturally fallen into two divisions, Viscosity, 

 and Moduli of Elasticity. 



Viscosity. — By induction from a great variety of observed phe- 

 nomena, we are compelled to conclude that no change of volume 

 or of shape can be produced in any kind of matter without dissipa- 

 tion of energy. Even in dealing with the absolutely perfect elas- 

 ticity of volume presented by every fluid, and possibly by some 

 solids, as for instance homogeneous crystals, dissipation of energy 

 is an inevitable result of every change of volume, because of the 

 accompanying change of temperature, and consequent dissipation 

 of heat by conduction or radiation. The same cause gives rise 

 necessarily to some degree of dissipation in connexion with every 

 change of shape of an elastic solid. But estimates founded on the 

 thermodynamic theory of elastic solids, which I have given else- 

 where*, have sufficed to prove that the loss of energy due to this 

 cause is small in comparison with the whole loss of energy which I 

 have observed in many cases of vibration. I have also found, by 

 vibrating a spring alternately in air of ordinary pressure, and in the 

 exhausted receiver of an air-pump, that there is an internal resist- 

 ance to its motions immensely greater than the resistance of the air. 

 The same conclusion is to be drawn from the observation made by 

 Kupffer in his great work on the elasticity of metals, that his vibra- 

 ting springs subsided much more rapidly in their vibrations than 

 rigid pendulums supported on knife-edges. The subsidence of vi- 

 brations is probably more rapid in glass than in some of the most 

 elastic metals, as copper, iron, silver, aluminium f ; but it is much 



* " On the Thermo -elastic Properties of Solids," Quarterly Journal of Mathe- 

 matics, April 1857. 



t We have no evidence that the precious metals are more elastic than copper, 

 iron, or brass. One of the new bronze pennies gives quite as clear a ring as a 

 two-shilling silver piece tested in the usual manner. 



