oherence was long unexplained.

It was shown by W. von Beetz that the coherence is apparent only, and
that the place where the jet breaks into drops is not perceptibly
shifted by the electricity. By screening the various parts with metallic
plates in connexion with earth, Beetz further proved that, contrary to
the opinion of earlier observers, the seat of sensitiveness is not at
the root of the jet where it leaves the orifice, but at the place of
resolution into drops. An easy way of testing this conclusion is to
excite the extreme tip of a glass rod, which is then held in succession
to the root of the jet, and to the place of resolution. An effect is
observed in the latter, and not in the former position.

The normal scattering of a nearly vertical jet is due to the _rebound_
of the drops when they come into collision with one another. Such
collisions are inevitable in consequence of the different velocities
acquired by the drops under the action of the capillary force, as they
break away irregularly from the continuous portion of the jet. Even when
the resolution is regularized by the action of external vibrations of
suitable frequency, as in the beautiful experiments of Savart and
Plateau, the drops must still come into contact before they reach the
summit of their parabolic path. In the case of a continuous jet, the
equation of continuity shows that as the jet loses velocity in
ascending, it must increase in section. When the stream consists of
drops following one another in single file, no such increase of section
is possible; and then the constancy of the total stream requires a
gradual approximation of the drops, which in the case of a nearly
vertical direction of motion cannot stop short of actual contact.
Regular vibration has, however, the effect of postponing the collisions
and consequent scattering of the drops, and in the case of a direction
of motion less nearly vertical, may prevent them altogether.

Under moderate electrical influence there is no material change in the
resolution into drops, nor in the subsequent motion of the drops up to
the moment of collision. The difference begins here. Instead of
rebounding after collision, as the unelectrified drops of clean water
generally, or always, do, the electrified drops _coalesce_, and then the
jet is no longer scattered about. When the electrical influence is more
powerful, the repulsion between the drops is sufficient to prevent
actual contact, and then