436 Mr. W. Sutherland on the 
metals is of the same order of magnitude and in about the 
same succession for different pairs of metals, as if calculated 
from the heats of combination with oxygen, chlorine, or some 
other powerfully electronegative element. Helmholtz’s elec- 
tron theory of chemical valency furnishes a simple explanation 
of this principle, if it is suitably modified by making the 
doublet rather than the electron the main agent. From 
“The Hlectric Origin of Molecular Attraction ” (Phil. Mag. 
[6] iv.), and previous papers on molecular force, it follows 
that the doublet of a compound inorganic molecule such as 
NaCl consists of two parts, one belonging to Na and the other 
to Cl. This means that the doublet of NaCl is made up of 
two doublets, just as a single bar-magnet can be made out 
of two. Reinganum (Ann. d. Ph. |4] x.) has proposed the 
use of doublet in place of electron. Helmholtz’s original 
view of chemical valency can be expressed by writing the 
formula for NaCl in the form Naf#)Cl. But when we assign 
to Na and Cl a single doublet each, in place of the single 
electron, the formula becomes ZNabfClh. The inner pair of 
electrons are so close to one another that their external 
effects are neutralized, and the total external effect is that 
due to the two outer electrons which act as a single doublet. 
The diagram makes clear the 
importance of the substitution Ee b 4 b 
of doublets for electrons as : 
the agents in chemical com- 
bination. The points A, B, &A BC D 
C, D denote the positions of 
the four electrons whose sym- 
bols are placed over them. 
The distance BC being negligible in comparison with AB 
and CD, the electric moment of the whole molecule is e. AD, 
to which Na contributes the moment of its own doublet 
¢. AB, and Cl the moment ¢.CD. The investigation of the 
values of these parts has occupied a considerable portion of 
my papers on molecular attraction. Now the very smallness 
of BC makes it of fundamental importance in the electrical 
theory of thermochemistry, for the electrons ) and $ at B 
and C have their electric energy expressible as e?/BC?. Thus. 
BC, which is neglected in the study of molecular attraction, 
becomes the principal physical quantity in the thermo- 
chemistry of atoms. Potential chemical energy is stored by 
increasing the potential energy of inner doublets like BC. 
The equation for the reaction between Na and Cl must now 
be written 
24Na+hCltyClt=2#NabHtClh . . . (24) 

