19,6 Perkins: Addition to the Double Bond , I 653 
or the reverse, namely, addition of electrons to an incomplete 
shell. The addition of electrons to a complete shell occurs in a 
certain type of reaction, which may be called the direct addition 
type. 
In general, two separated molecules attract each other by a 
force which is usually called cohesion, but which Langmuir 13 
has shown to be so closely related to the forces of primary 
chemical union that it should be considered a “secondary valence” 
force (possibly even a primary valence force in the case of salt- 
like compounds). 
As two molecules approach each other this attraction increases 
to a maximum, and then, becoming neutralized by a repulsion, 
falls off to zero. In this equilibrium position the two molecules 
Distance between cenfepj 
->. 
Fig. 6. The attraction between two non-reacting molecules. 
are usually not sufficiently close for any interpenetration of 
shells. Such a union between molecules is identical with Ke- 
kule’s idea of a “polymolecule,” but many of the compounds 
considered by Kekule to be of this type are actually direct union 
compounds. In the case of direct union compounds the second- 
ary valence forces are usually very weak, a typical example being 
the forces which hold together in a crystal the molecules of a 
paraffine hydrocarbon. The variation of force with distance in 
such a case is shown in fig. 5. It is here assumed that each 
molecule is free to rotate. The attraction, being chiefly elec- 
13 Langmuir, I., Journ. Am. Chem. Soc. 38 (1916) 2221, 39 (1917) 1848. 
