654 
The Philippine Journal of Science 
1921 
trostatic and due to the distribution of charges on each molecule, 
is of course entirely dependent on the orientation of each mole- 
cule in reference to the other. 
If two molecules are capable of reaction, but only when as- 
sisted by thermal impact, the attractive force will be of the 
nature shown in fig. 6. This figure illustrates well the particular 
type of reaction under consideration, namely, that of direct 
addition to an already complete shell. Such reactions are usually 
dependent on thermal impact, and the molecules show a maxi- 
mum repulsion at a certain distance (fig. 6, a). Direct addition 
reactions not dependent on thermal impact do not have a repul- 
sion at this distance, but in all probability have a minimum 
attraction. 
>. 
Fig. 6. The attraction between two reacting molecules. 
Since the sharp rise to the left of point a in fig. 6 is due to 
the interpenetration of electron shells, we may conveniently 
consider that just at this distance of maximum repulsion (or 
minimum attraction) entrance of one or more electrons into a 
shell of the foreign molecule takes place. This definition makes 
possible a concept of shell boundary from the point of view 
of direct addition, which varies however according to the mol- 
ecule added. The extension of this definition of shell boundary 
to the process of addition to an incomplete shell is unnecessary, 
