Oct. 1, 1885] 
NATURE Sas 
vibration to which there is all the resistance consequent on the 
radiation, and in all probability but little communication. 
The same answer applies to difficulties raised as to the dis- 
tribution of motion. The assumed distributions leave out of 
consideration all resistances, and resistance, however slight, 
would cut off the extreme velocities. 
Mr. H. B. Dixon said that, by a series of observations made 
on a mixture of oxygen and hydrogen at intervals of 1000 hours, 
he had obtained evidence of combination at temperatures below 
that of dissociation. 
Constant Gravitational Instruments. — Sir W. Tinomson 
showed and explained constant gravitational instruments for 
measuring electric currents and potentials. In one instru- 
ment for measuring currents he employs the principle that 
a mass of soft iron of dimensions and shape not differing 
too much from a sphere, experiences, in a field of magnetic 
force, a pull from a place of weaker to a place of stronger force. 
The variation of the field is produced by variation in the 
dimensions of the conductor through which the current passes. 
In an instrument for measuring high potentials he used one pair 
of opposite quadrants placed vertically. The quadrants are con- 
nected to one pole of the instrument whose potential is required, 
and the needle, the lower end of which can be weighted, is 
joined to the other pole. 
On the Dilatancy of Media composed of Rigid Particles in 
Contact, by Prof. Osborne Reynolds.—In the account which 
Prof. Reynolds gave of his paper, he did not submit a complete 
dynamical theory, but discussed a very fundamental property of 
granular masses. To this property he gives the name of 
dilatancy. It is exhibited in any arrangement of particles where 
change of bulk is dependent upon change of shape. In the case 
of fluid matter, as we know it, change of shape and volume are 
independent. In solids they are sometimes not separable. 
With granular masses the result is different—change of shape 
always produces change of volume. And further, in every case, 
if change of volume is prevented any change of form is im- 
possible. 
If we suppose the component granules to be spherical, no gran- 
ule can change its position without disturbing the adjacent ones 
—for the granules are all supposed to be perfectly rigid, and to 
be absolutely in contact—and the internal particles are fixed if 
the external ones are. In illustration Prof. Reynolds showed a 
model of connected spherical bodies arranged in crystalline 
form. This model showed the arrangement of the particles 
corresponding to (say) the condition of least possible density of 
the whole mass (about one-half the density of the separate 
spheres). The shape could then be altered to that which cor- 
responds to maximum density—the change taking place by 
sliding of the particles one upon another. Between the extreme 
states there are intermediate stages of equilibrium corresponding 
to maximum-minimum positions, where alteration in one direc- 
tion produces decrease of density, and in the other increase of 
density. 
In a complete treatment of the problem, friction must be 
closely considered; but in the experiment shown it is not of 
consequence, the result being independent. The above state- 
ments will be true of any continuous mass of granules if we hold 
the boundaries. 
This principle of the dilatancy of such granular media 
explains many phenomena ofcommon occurrence. For example, 
take a sack of corn ; if set on end, it remains perfectly flexible, 
. but if placed on its side it becomes hard, and its shape will not 
alter. Now take an indiarubber sack, fill it with corn—it 
remains perfectly flexible in all positions. The reason for this 
difference of behaviour is that in the former case the boundary 
of the granular mass is inextensible, while in the latter it allows 
increase of internal volume. So if it be possible with an 
extensible envelope, to impose a maximum volume upon the 
contents, effects similar to those obtained with the inextensible 
boundary may be expected: and this can be done. If we place 
some shot (No. 6 was used in the experiment) in a thin india- 
rubber bag, and add a certain amount of water, we obtain the 
result wished. For if the amount of water added be such that 
the spaces between the granules when in close arrangement are 
all filled by it, while with a wide arrangement the amount is not 
enough, a point will be reached in passing from the first to the 
second arrangement such that any further change of shape, and 
consequently of volume, would produce a vacuum. When this 
stage is reached the whole mass becomes perfectly hard. Prof, 
Reynolds illustrated this in a very beautiful manner by means of 
a ball of shot to which a glass tube open at the end was fitted. 
With a close arrangement of the shot, the water, which was 
coloured, stood high in the tube ; but when pressure was applied 
to the bag, the level was lowered. This was shown also by the 
lecturer with a ball containing sand instead of shot. The water 
level sank till the whole was at maximum density, and, still 
more pressure being applied, the level again rose, the maximum 
having been passed. In these experiments about 6 per cent. of 
the water was free at the top of the ball with the close arrange- 
ment of granules. When another ball containing 20 per cent. 
of free water was used, the hard condition could only be 
approximated to by pressure, and then passed. So Jong as the 
maximum is not passed in this case the ball springs back to its 
original state when the pressure is released. But if the maximum 
be passed, it will not spring back. If some of the water benow 
let out, the maximum cannot be passed, except by shaking, and, 
if the flattened ball be then turned on edge, it will bear a 
pressure of a hundredweight without change of shape. 
When the dilatant material, such as shot or sand, is bounded 
by smooth surfaces, the layer of grains adjacent to the surface is 
in a condition differing from that of the grains within the mass. 
This layer can slide between the one succeeding it and the sur- 
face, so that its displacement will cause much less dilatation 
than would be caused by the sliding of a layer within the mass. 
Hence, if two parts of the mass are connected by such.a surface, 
certain conditions of strain may be accommodated by a stream- 
ing motion of the grains next the surface. Thus, if into a glass 
funnel partially filled with shot and held in a vertical position 
more shot be furced from below, the particles will flow up all 
around the sides—not rising in the centre as might have been 
thought. 
As the foot presses upon the sand, when the falling tide leaves 
it firm, that portion of it immediately surrounding the foot 
becomes momentarily dry. When this happens the sand is 
filled, completely wp to its surface, with water raised by capillary 
attraction. The pressure of the foot causes dilatation of the 
sand, and so more water is required. This has to be obtained 
either by depressing its level against the attraction or by drawing 
it through the interstices of tlhe surrounding sand. As this latter 
requires time, for the moment the capillary forces are overcome, 
and the surface of the water is lowered below that of the sand, 
leaving it dry until a sufficient supply has been obtained irom 
below, when it again becomes wet. On raising the foot we 
generally see that the sand under and around it becomes wet for 
alittle time. This is because the sand contracts when the dis- 
torting forces are removed, and the excess of water escapes at 
the surface. 
In referring to the re-ults which might be expected to follow 
from a recognition of the property of dilatancy the author said 
that it places a hitherto unknown mechanical contrivance at the 
command of those who would explain the fundamental arrange- 
ment of the universe, and one which seems to promise great 
things besides possessing the inherent advantage of great sim- 
plicity. He then proceeded to explain, in a general way, how 
bodies in such a medium would—in virtue of the dilation caused 
in the medium—attract each other at a distance, with a force 
depending on the distance, which might well correspond with 
the force of gravitation. Further, owing to the existence of a 
region close to the body in which the density varies several 
times from maximum to minimum, the mutual force might under- 
go a change from attraction to repulsion, and this more than 
once as the bodies approach—a condition which seems to account 
for cohesion and observed molecular force far better than any 
previous hypothesis. 
The transmission of distortional waves becomes possible if 
the medium be composed of small grains with large grains inter- 
spersed. ‘The separation of two such sefs of grains leads to 
phenomena closely resembling the phenomena of statical elec- 
tricity. The susceptibility of such a medium for a state in which 
the two sets of grains are in conditions of opposite distortions 
may explain electrodynamic and magnetic phenomena, while 
the observed conducting power of a continuous surface for the 
grains of a simple dilatant medium closely resembles the con- 
duction of electricity. 
In remarking upon Prof. Reynolds’s paper Sir W. Thomson 
pointed out an interesting question. Take a cube of spheres in 
the condition of maximum volume, and let every sphere touching 
the boundary be glued to it to prevent slipping. Other states 
are possible in the interior, but can we pass continuously to 
