On the Virial Equation. 33 



Hence — „ __ 2p 



Is 



m 



2K. 



I£ we take Drude's values o£ n and n~K, we obtain the following 

 values of r s " : — 



Metal. n. wK. r s ". 



Silver 0'18 3'67 41 



Gold 0-37 2-82 15 



Sodium 0-005 2*61 1043 



If n were zero, and nK not zero (Lord Kelvin's " ideal 

 silver"), r s " would be infinitely great! If we had a very- 

 thin film of sodium, and a ray of light were repeatedly 

 reflected within the film at the proper angle, its intensity 

 would be very much increased. This would be contrary to 

 the conservation of energy. The explanation of course, is, 

 that in such thin films the geometrical laws of reflexion and 

 refraction do not hold. 



The observations recorded in this and the preceding article 

 were carried out in the Physical Institute of the University 

 of Gottingen at the suggestion, and under the continual 

 advice of Prof. W. Voigt. 



IV. Lord Rayleigh on the Virial Equation. 

 By S. H. Buebury, F.R.S* 



IN his paper on this subject in the Philosophical Magazine 

 for April 1905, Lord Rayleigh considers a system of 

 molecules, elastic spheres, which also exert on each other 

 finite forces. And concerning these forces he assumes that 

 within the effective range of any type sphere there are many 

 others, and that the forces which the type sphere exerts on 

 the more distant molecules within this range are not in- 

 appreciably small compared with those which it exerts on 

 its immediate neighbours. In his own language, the forces 

 are " of the character considered in the theory of capillarity, 

 that is extending to a range which is a large multiple of 

 molecular distances, and not increasing so fast with diminish- 

 ing distance as to make the total effect sensibly dependent upon 

 the positions occupied by neighbours." Then he goes on to 

 say: " Under these restrictions symmetry ensures that the 

 resultant force upon a sphere, situated in the interior and not 

 undergoing collision, is zero ; and the whole effect of such 

 forces is represented (Young, Laplace, Van cler Waals) 



* Communicated bv the Author. 

 Phil. Mag. S. 6. Vol. 10. No. 55." July 1905. D 



