250 REPORTS ON THE STATE OF SCIENCE, ETC. 
Society, loc. cit.). The oil quickly coats every bead and provides the 
minute discs and meniscuses at every point of contact. Oil is preferable 
to water, which dries up too quickly. By varying the size of the beads 
and the nature of the liquid the properties can be varied. "The writer has 
experimented with this material, but has made very few mechanical tests 
on it. ‘The mechanical properties of moist granular material await 
investigation. 
2. Wet Granular Material_—Only extremely fine granular materials form 
stable masses when saturated with water, and it is only with these that we 
are here concerned. Saturated gravel or sand (except the finest) slumps 
down and the water drains off ; we are not concerned with such substances. 
Coarse granular material under water behaves like dry material (see the 
writer’s Institution of Civil Engineers paper, Joc. cit.). 
Very fine granular material, such as China clay powder, when stirred up 
with water only settles very slowly. If the sediment is removed from the 
water, more water drains off, but the mass remains saturated throughout. 
If the sediment is put into a filter press more water may be extruded, and 
the material remaining is a more or less plastic mass held together by the 
negative water pressure or ‘ suction’ of the water in the ‘ voids. It 
possesses resistance to deformation (shear strength), which is due to the 
friction between the grains held together by the suction.? The suction or 
negative water pressure would draw in air but for the layer of water on the 
surface of the mass ; the suction draws this surface layer of water into each 
space between the surface grains, forming innumerable minute meniscuses 
which support the suction. 
Compactability—Wet granular material, as defined above, is almost 
incompressible, since it is made up of solids and water ; but if the water is 
allowed to escape (as, for instance, in a filter press), it is just as compactable 
as dry granular material, and any type of ‘ working,’ as before, facilitates 
the packing of the grains. A slight alternating torsion produced by rotating 
the piston of the press backwards and forwards through a small angle is 
effective, but how to produce the closest packing’ is not known. The 
unwanted extrusion of water from clay due to unexpected compaction is 
liable to interfere with all tests or methods of preparing test-pieces which 
are carried out in closed vessels. ‘The sudden appearance of drops of water 
oozing out through the joints of the apparatus is a most familiar sight, and 
sets a limit to the range of many tests. Extrusion of water from a free 
surface never takes place. 
The permeability of ultra fine-grained material is very small, so that the 
water can only escape slowly, and time is required for compaction. 
Dilatancy occurs in wet granular material, as defined above, just as in dry 
granular material, but its results are different because the volume of wet 
material cannot expand (the minute elastic expansion and the minute 
expansion permitted by the increased depth of the surface meniscuses may 
be. neglected at present). 
When wet granular material is sheared the incipient dilatation causes a 
rapid rise in the suction. The rise in the suction involves an increasing 
compressive stress which has two effects : firstly, it increases the friction 
between the grains and consequently the resistance to shear ; secondly, it 
begins to compact the mass. ‘The combined result of the dilatation and 
this compaction is that the volume remains almost constant while the suction 
and shear strength rise. This action continues till one or other of two 4 
3 Molecular forces probably also have an appreciable effect. 
4 A third condition appears to limit the strain in tensile tests on clay, but 
too few tests have been made to determine what happens with certainty, 
