8 Mr. W. Sutherland on the Fundamental 



with 4*2 x 10 7 as the value of J, we get finally with 10 12 

 dynes as unit of force, 



M 2 Z=5-8(M/p)TM 1 / 6 xlO- 4 (11) 



Thus I, and therefore the latent heat of vaporization of 

 nearly all the metals can be found ; but we also require a 

 similar equation for the compounds of the metals. 



The establishment of such an equation is sketched in 

 section 9 of " A Kinetic Theory of Solids/' but in a form which 

 is not correct without a strained interpretation of some of 

 the symbols : the correct equation for a compound whose 

 molecule contains n Y atoms of mass m Y and n 2 of mass m 2 and 

 so on, the diameters of the atoms being E l7 E 2 and so on, and 

 the mean distances from their neighbours (centre to centre) 

 e ly e 2 and so on, is 



v 1 / n^Yf n 2 m 2 Y 2 * \ 1 ^ . > . 



where as before e d is the domain of a molecule, that is, its 

 share of the total space occupied by the solid, and <£(r) is the 

 force between two molecules. The values of E 1? E 2 , e ly e 2 , and 

 so on are unknown, but it is reasonable to suppose that for 

 approximate purposes l — 'E 1 /e 1 and so on can be replaced by 

 a single mean value proportional to bd, where b is the linear 

 coefficient of expansion of the solid compound : thus w^e 

 replace each by abd, corresponding to the Ibd for metals, 

 then we have the sum %(n 1 m 1 Y^ + n 2 m 2 Y 2 ^-{- . . .) of which the 

 value is : l JMc#, M being the molecular mass and c the specific 

 heat of the compound. The only unknown quantity remain- 

 ing is b, which has been found for very few compounds ; in the 

 case of metals we eliminated it by the relation iTM 1 / 6 = , 044, 

 Let ns assume that a similar relation holds for compounds, 

 namely that 6TM 1/6 is constant, then merging this constant 

 and the unknown a into a single coefficient we finally reduce 

 the last equation to the form 



M 2 / = 5-8xl0-^.^(M/ /O )TM 1 /6 ? . . . (13) 



where k is a parameter to be determined for each type of 

 compound. I have found that k = ^ for such binary com- 

 pounds as NaCl, KI, and so on ; and according to the principle 

 of Joule and Kopp that the molecular specific heat of a com- 

 pound is the sum of the atomic specific heats of its atoms, 

 Mc for these binary compounds is 2 X 6*4. Thus for com- 

 pounds of this type the equation (13) simplifies down till it is 



