s of the layer of air is reduced to the point at
which the colours of thin plates are visible, the approximation must be
sensibly resisted by the viscosity of the air which still remains to be
got rid of. No change in the capillary conditions can arise until the
interval is reduced to a small fraction of a wave-length of light; but
such a reduction, unless extremely local, is strongly opposed by the
remaining air. It is true that this opposition is temporary. The
question is whether the air can everywhere be squeezed out during the
short time over which the collision extends.

It would seem that the forces of electrical attraction act with peculiar
advantage. If we suppose that upon the whole the air cannot be removed,
so that the mean distance between the opposed surfaces remains constant,
the electric attractions tend to produce an instability whereby the
smaller intervals are diminished while the larger are increased.
Extremely local contacts of the liquids, while opposed by capillary
tension which tends to keep the surfaces flat, are thus favoured by the
electrical forces, which moreover at the small distances in question act
with exaggerated power.

A question arises as to the mode of action of milk or soap turbidity.
The observation that it is possible for soap to be in excess may here
have significance. It would seem that the surfaces, coming into
collision within a fraction of a second of their birth, would still be
subject to further contamination from the interior. A particle of soap
rising accidentally to the surface would spread itself with rapidity.
Now such an outward movement of the liquid is just what is required to
hasten the removal of intervening air. It is obvious that the effect
would fail if the contamination of the surface had proceeded too far
previously to the collision.

This view is confirmed by experiments in which other gases are
substituted for air as the environment of colliding jets. Oxygen and
coal-gas were found to be without effect. On the other hand, the more
soluble gases, carbon dioxide, nitrous oxide, sulphur dioxide, and
steam, at once caused union.]

_Stability of the Catenoid._--When the internal pressure is equal to the
external, the film forms a surface of which the mean curvature at every
point is zero. The only surface of revolution having this property is
the catenoid formed by the revolution of a catenary about its directrix.
This catenoid, however, is in stable equili