856 W.<A. Norton—Force of Effective Molecular Action. 
sion for the neutral distance varies only from zero to less 
than }. 
It is important to remark, in this connection, that the com- 
plete molecular theory of the elastic resistance of materials 
cannot be developed from the single conception that each speci- 
men of every material has its specific formula, or curve of 
effective molecular action, which remains constantly the same 
during all the varying conditions and degrees of stress. Thus 
when two specimens of the same material are compared, 
certain facts are recognized which require that account should 
be taken of the varying dimensions and mechanical condition 
of the molecules when under the influence of the force of stress. 
theory the occasion of its achieving one of its most signa 
triumphs. 
Let us now subject the formula to the test of comparison 
with the laws and mechanical properties of vapors and gases. 
Upon the general theory the effective mutual actions of contig- 
uous molecules of a vapor, or gas, must be repulsive. The 
curve of effective action of such a molecule must then lie en- 
tirely below the axis 2; and hence it must answer to a value 
of & less than 4984. Fig. 8 shows a set of theoretical 
curves answering to various values of & in equation 2, 
m 493 to 4. The vertical scale in this figure is 20 
times that in fig. 2, These curves present two distinct 
varieties; one in which a portion of the curve is concave 
to the axis, x, and another in which the curve is everywhere 
