: 
’ 
W. A. Norton—Force of Effective Molecular Action. 851 
(3.) The distance ab, (fig. 1) between the neutral point and 
the point of rupture increases from 0°302r for k=20, to 0-60r 
for k=5°428. This is about its maximum value. From this 
value of & to the ratio 4°934 it decreases from 0°60r to zero. 
Now r is the distance between the centers of attraction and 
contiguous molecules. We should then expect, on theoretical 
grounds, that when a bar suffers rupture under a tensile stress, 
the elongation would be a small fraction of its lengt is ] 
well known to be generally true for the more tenacious mate- 
rials (e. g. the metals and different varieties of wood). India rub- 
ris a striking exception. Its great extensibility is probably 
due to a great expansibility of its molecular envelopes, under 
a tensile stress. The unequal extensibility of different quali- 
ties of wrought iron, also finds its theoretical explanation in an 
unequal expansibility of molecular envelopes, with the attend- 
ant variations in the molecular curve. 
(4.) The ordinates of the portion ra of the molecular curve 
represent the molecular resistance developed by a compressive 
stress. These increase (as they should do) without limit, as 
the distance Oe between the molecules diminishes. en rup- 
ture occurs under a compressive stress, it is because the molec- 
ular actions developed in directions oblique to the line of thrust 
induce a tensile strain at right angles to this line, and a shear- 
Ing strain in oblique directions, the resistance to one or the 
other of which is overcome. The distance Oa is not the limit 
to the possible diminution in the distance between the centers 
of contiguous molecules, since the act of compression will com- 
press their envelopes, and so diminish the size of the effective 
molecules. 
(5.) The ordinates of the portion me of the molecular curve 
represent the effective attractions that come into operation dur- 
and the separation becomes complete. : 
Beyond d the curve represents the force of contact resist- 
ance. To test this portion of the curve, I undertook in 1876, 
to determine experimentally the laws of variation of this force. 
For this purpose the diminutions of contact distance produced 
by varying increments of pressure, under varying condition 
with regard to the nature, condition, and extent of the surfaces 
