436 CMROKGIZ, 19, IGG CKOIRIE 
One may regard the flow surfaces as mathematical planes (like 
the plane of the meridian) which occupy different positions rela- 
tively to the material particles as the mass undergoes increasing 
deformation.*. That set of particles which at any instant coin- 
cides in direction with the flow planes undergoes deformation 
and no other particles are subject to gliding at that instant. The 
Fic. 3. Flow distribution in pure shear. 
flow planes range through successive portions of the material 
cube and in pure shear they range through equal portions of the 
cube on each side of the line of pressure. It will be seen later 
that this is not the case in scission or in any strain into which 
scission enters as a component. In the illustration the range R 
or y in the unstrained solid is 18° and the corresponding surfaces 
in the strained mass make angles of nearly 13°. 
The process outlined above must go on in any homogeneous 
solid substance which is not infinitely brittle when subjected to 
pure shear under conditions which preclude rupture; whether 
the mass is of lead or of quartz makes no difference in this 
respect. 
It seems clear that flow in a solid must in some manner affect 
its resistance to rupture. It is conceivable that a body which 
had been strained beyond its elastic limit should split with sore 
difficulty along the lines of flow or approximately at 45° to the 
line of force, than in any other direction. Experiment, however, 
shows that it splits with less difficulty in this direction. The 
"Whatever the amount of a pure shear may be at any instant, the flow lines are 
parallel to lines passing through the intersections of the undisturbed cube with the 
distorted cube. 
