MS Mr. W. Sutherland on the 



velocity of light through vacuum, or through air nearlv, 

 and r is the period of vibration of the cube. But 

 T = 2~R/{N/ P )i, so that \ = 2cJi/(N/p)t 



It is convenient for the mathematical analysis to bring in 

 the electric properties of the atom, though we shall not finally 

 use them, as will appear immediately. Let the electric 

 moment of the atom be denoted by es to be taken as a single 

 symbol, and let K be its dielectric capacity. Then in " The 

 Electric Origin of Rigidity and Consequences" (Phil. Mag. 

 [6] vii. 1904, p. 417) it is shown that N == 2tt* V/3KB,«, 

 K being introduced to preserve generality. But in " The 

 Mature of Dielectric Capacity" (Phil. Mag. [6] xix. 1910, 

 p. 1) it was found that as regards the relations of the pairs 

 of electrons forming the atom K = l. It has also been found 

 in my papers on the electric origin of cohesion that between 

 atom and atom K = l. I have taken this to be evidence that 

 cohesion is due to electric attraction between the electrised 

 molecule and its immediate neighbours, the attraction acting 

 entirely through the aether in which K = l. Within the 

 atom it appears that the constitutive pairs of electrons act 

 only upon their immediate neighbours through the a?ther 

 with K=l. It further appears that when we have taken 

 account of the pairs of electrons forming matter as a cause 

 of dielectric capacity different from 1. we have not to consider 

 any other similar agency in the sether. In the formula given 

 above for N then we put K=l. In a recent Phil. Mag. 

 article on Molecular and Electronic Potential Energy I have 

 shown that the cohesional potential energy of unit mass of a 

 substance may be written lp 2 (the Kp 2 of Laplace or the a/v* 

 of van der Waals) where Z = 4eV/m 2 , the values and laws 

 of I having been investigated under M 2 Z and M/i in various 

 papers of mine on molecular attraction. If then in the 

 formula for N we put ??i 2 //4 in place of e 2 s 2 , we get N 

 expressed in terms of purely mechanical properties of the 

 atom, the electrical moment es having been eliminated and 

 K reduced to 1 . Thus for the velocity of propagation of a 

 shear through the atom we obtain the expression (jrlp/6)^ and 



X=2cR/( 7 rZ / o/6)i=6x 10 10 x (1617 X 10- 27 )i(M/p)i/(7rlp/^ 



= 973-6 (M/»V(W*. 



In the following table are gathered all the requisite data 

 for computing the wave-lengths of the fundamental mechanical 

 vibrations electrically communicated to the sether by the 

 atoms of the combined alkali metals and the combined 



