19,6 Perkins: Addition to the Double Bond, I 647 
certain assumptions which Langmuir and others 10 have made 
concerning the force acting on shell electrons. To these will 
be added several suppositions which seem to the writer reason- 
able from a physical standpoint and necessary from a chemical 
standpoint. 
DEFINITIONS 
Elastic constraints . — When an external influence acts on an 
unshared electron in shell C of a compound, A — B — C, moving it 
from its position of equilibrium, an elastic constraint, or force 
of restitution, is developed, opposing the motion. This force 
Fig. 1. Forces of restitution acting on an unshared shell electron, upon radial displacement. 
does not vary in accordance with Hooke’s law ; in fact, it may be 
somewhat discontinuous, but it shows approximately the varia- 
tion shown in fig. 1. 
At the present time we cannot accurately resolve this total con- 
straint into its components. It is sufficient for the present 
purpose to note that this constraint is made up of forces attribut- 
10 Concepts similar to some of these definitions can probably be found 
in J. Stark’s “Electrizitat im chemischem Atom,” Leipzig, 1915, a book 
not available to the author at present. Stark was the first to propose 
any approximately satisfactory electronic conception of the direct bond, 
but his theory fails to take account of the very important unshared 
electrons. 
