268 Dr. W, 0. M c (l Lewis on the 



By Vassuxa's result, the conductivity after fusion, for 

 steady currents, is 



<7 = a72'l = 2-(M.10- 5 . 



In the case of sodium light, the formula (41) of the other 

 paper for the number, p, of free electrons per atom is 



p 2 = 143VV v«;.10-% 



and if K be the dielectric capacity, we have in addition 



/3 = 15-59V(K + ac 2 -^ 2 ) . 10- 3 , 



If pi is the value furnished by writing K = l, we find with 

 the present values, 



p = 2 # 85, /d 1 = 4 , 56. 



The value of K required to bring these to equality is 

 — 532, so that we may infer the presence of sympathetic 

 vibration in the liquid also. 



In a later paper, it will be shown that there is some reason 

 for the conclusion that tin is a metal in which higher fre- 

 quency of the vibrations may cause the liberation of more 

 electrons, and that in a case of this kind, the true number of 

 free electrons for a steady current is p 2 /pi, combined with a 

 value of K positive and nearly unity. 



For solid tin this becomes 3"54, and for the liquid 1*78. 

 It is, perhaps, significant that one of these values is almost 

 exactly half the other, and a simple dissociation of atomic 

 aggregates in the process of fusion is suggested. 



XXVIII. On the Latent Heat of Vaporization of Liquids. 

 By Wm. 0. M C C Lewis, M.A., D.Sc, Physical Chemistry 

 Laboratory, University College, London *. 



THE quantity of heat added to any system is expressed 

 usually as follows : 



dQ = C v dT + ldv 9 



where C v denotes the specific heat of the system at constant 

 volume and I the ' latent heat of expansion,' i. e. the heat 

 required to keep the temperature constant while the expansion 

 takes place. Also the change in total energy dJJ of the 

 system may be written 



d\J = dQ-pdv, 



where p denotes the external pressure to which the system is 

 subjected. 



* Communicated by the Author. 



