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dust does not move till the oil reaches it shows that the contraction takes
place entirely at the edge of the oil in an almost infinitely narrow band.

This phenomenon of surface-contraction is very remarkable, for it would
be inferred from other hydrodynamical phenomena that viscosity would to
some extent resist the action of contraction, and thus tend to distribute this
action over a considerable area, and that the contraction is not so distributed
shows that there is virtually no resistance to contraction, or that the surface-
tension at the points at which the surface is contracting is at least equal to
the tension at those points of the surface which are at rest.

This conclusion implies much more than the tacit assumption, made by
Laplace and subsequent writers, that the forces of cohesion obey the law of
statical fluid pressure equality in all directions. It is well known as regards
other phenomena that this law holds only when fluids are at rest or in uniform
motion; whereas here we have a case in which the same law holds for a
portion of fluiti which is moving with great rapidity relative to the fluid in

its immediate neighbourhood.

/
/

Laplace's theory is founded on an assumed attraction, between the
molecules, which attraction does not extend to sensible distances, and on the
tacit assumption already mentioned, that the pressure, whether impressed or
n/olecular, is equal in all directions. To explain the apparent absence of
Viscosity in the dynamical phenomena some further assumptions are necessary,
[f the force of cohesion is due to molecular attraction these dynamical
}henomena require that the molecules under their mutual attractions should



412 ON SURFACE-TENSION AND CAPILLARY ACTION. [39

not be in a state of equilibrium, except in so far as they are held by the
forces transmitted from one part of the fluid to another.

Such a condition would exist if the range of attraction extended beyond
the distance of a single molecule, that is, if the molecules are spherical or in
such a state of motion that they cannot fit like bricks. But whatever might
be the shape of the molecules, if the forces of cohesion acted between adjacent
molecules only, then they would be in equilibrium in all positions ; there
would be no instability and no rapid contraction, although, according to
Laplace's theory, the force would be sufficient to prevent extension of the
surface, and hence to explain the statical phenomena of capillary tension such
as the suspension of drops. It is therefore argued that these dynamical
phenomena are important, as throwing a certain amount of light on the
character of the forces which cause cohesion between molecules.




40.



ON THE FLOATING OF DROPS ON THE SURFACE OF WATER
DEPENDING ONLY ON THE PURITY OF THE SURFACE.

[From the Twenty-first Volume of the " Proceedings of the Manchester Literary
and Philosophical Society," 1881.]

(Read October 4, 1881.)

IT is well known that under certain circumstances drops of water may be
seen floating on the surface for some seconds before they disappear. Some-
times during a shower of rain these drops are seen on the surface of a pond,
they are also often seen at the bows of a boat when travelling sufficiently fast
to throw up a spray. Attempts have been made to explain this phenomenon,
but I am not aware of any experiments to determine the circumstances
under which these drops are suspended. Having been deeply engaged in
the experimental study of the phenomena of the surface tension of water, and
the effect of the scum formed by oil or other substances, it occurred to me
that the comparative rarity of these floating drops would be explained, if it
could be shown that they required a pure surface, a surface free from scum
of any kind. For, owing to the high surface tension of pure water, its surface
is rarely free from scum. The surface of stagnant water is practically never
free except when the scum is driven off by wind. But almost any disturb-
ance in the water, such as the motion of the point of a stick round and round
in the water, or water splashed on the surface, will serve to drive back the
scum for a certain distance. This may be shown by scattering some flowers
of sulphur on the surface. This powder is insoluble and produces no scum,
and hence it serves admirably to show the motion of the surface and whatever
scurp: there may be upon it. If when the surface is so dusted a splash be
mane by a stick so as to throw drops on to the sulphured surface, at the first
plash no floating drops are produced; but after two or three splashes in



414 ON THE FLOATING OF DROPS ON THE SURFACE OF WATER, ETC. [40

rapid succession it will be seen that the sulphured scum has been driven
back by the falling water, leaving a patch of clear surface, and on this drops
will float in large numbers and of all sizes. These drops are entirely confined
to that portion of the surface which is clear. The drops, either by their
initial motion or by the current of air, glide rapidly over the surface from the
point at which they are formed. When, however, they reach the edge of the
scum they disappear, apparently somewhat gradually. I have this summer
made the experiment on several ponds and on various days, and I have never
found any difference. Any scum, however transparent, prevented the drops,
and they always floated in large numbers when the scum was driven back in
the manner described, by the wind or any other way.

This result points to the conclusion that whatever may be the cause of
this suspension, it depends only on the surface of the water being pure, and
not at all on the temperature or condition of the air.




INDEX.



Atmosphere, refraction of sound by, 89, 157
Aurora, the, 7

Ball, suspension of by jet of water, 1

Belts, creeping of, 107

Boilers, steam, heating surface of, 81

Bursting of gun, 399

Bursting of trees by lightning, 41

Calming effect of rain on the sea, 86

Centrifugal pumps, 141

Clouds, electrical properties of, 30

Cohesion of water and mercury, 231, 394

Collision of steamers, 192, 204

Colour bands, use of in studying motion

of fluids, 183, 184
Comets, tails of, 7, 15
Condensation of air and steam, 59
Corona, solar, 7, 22

Destruction of sound by fog, 43

Electricity, statical, induction of in a

moving body, 27

Energy transmitted by waves, 198
Equation of transpiration of gases, 351
Equations of steady motion of gases, 340
Equations of steady motion as affected by

discontinuity, 349

Fluid, heterogeneous, inertness of, 43
Fluid motion, use of colour bands, 183,

184
Forces, surface, caused by evaporation and

condensation, 67, 75, 170, 257
Friction in guns, 35
/Triction, rolling, 110



Gases, dimensional properties of, 257

Gun, bursting of, 399

Guns, friction in the grooves of, 35

Hailstones, formation of, 214, 223
Heat, communication of the cause of sur-
face forces, 67, 75, 170, 257
Heat, lateral flow of measured by that of

momentum, 67

Heating surface of boilers, 81
Heterogeneous fluid, inertness of, 43

Immersion of screw propeller, institution of

cavities in the water, 51, 78
Impulsion of gases, 67, 75, 170, 257
Induction of statical electricity, 27
Iron rails, scaling of, 132
Iron wire, effect of acid on, 48

Light mill, 171
Lightning, effects of, 30, 41
Liquids, cohesion of, 231, 394
Logarithmic homologues of curves, 282

Magnetism, terrestrial, 27
Matter, dimensional properties of, 257
Mercury, cohesion of, 231, 394
Motion of gases, equations of, 340
Motion, vortex, 86, 183, 184

Photometer, new, 178
Propellers, screw, 51, 78
Pumps, centrifugal, 141

Racing of screw steamers, 51, 78, 138
Rails, scaling of iron, 132
Raindrops, formation of, 214, 223



416



INDEX.



Rain, its calming effect on the sea, 86

Refraction of sound, 89, 157

Resistance to rolling, 110

Reversed screw, effect on steering of

steamers, 134, 192, 204, 244, 403
Rifled guns, 35
Rings, vortex, 86, 183, 184
Rolling friction, 110

Screw steamers, racing of, 51

effect of depth on resist-

ance of screw, 78

effect of reversing the screw

on the steering of, 134,
192, 204, 244, 403

effect of unequal velocities

of upper and lower cur-
rents, 149

Sea, calming effect of rain on, 86
Slipping of rolling bodies, 110
Snowflakes, formation of, 223
Sound, refraction of by the atmosphere,

89, 157

effect of wind on, 89
effect of temperature on, 100, 157
effect of fog on, 43



Steam boilers, heating surface of, 81

Steam, condensation of, 59

Storms, thunder, 30

Straps, creeping of, 107

Surface forces due to condensation and

evaporation, 67, 75, 170, 257
Surface tension and cohesion, 233

and capillary attraction,

410, 413
Suspension of a ball by a jet of water, 1

Terrestrial magnetism, 27

Thunder storms, 30

Transmission of energy by waves, 198

Transpiration of gases through porous plates,

257

tubes, 342

Trees, bursting of by lightning, 41
Turbines in series, 141

Vortex motion, 86, 183, 184

Waves, groups of, 198

destroyed by oil, 409
Wind, effect of on sound, 89
Wire, effect of acid on iron, 48



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Online LibraryOsborne ReynoldsPapers on mechanical and physical subjects (Volume 1) → online text (page 40 of 40)