, of course, there is no opportunity for
amalgamation.

These experiments may be repeated with extreme ease, and with hardly any
apparatus. The diameter of the jet may be about 1/20 in., and it may
issue from a glass nozzle. The pressure may be such as to give a
fountain about 2 ft. high. The change in the sound due to the falling
drops as they strike the bottom of the sink should be noticed, as well
as that in the appearance of the jet.

The actual behaviour of the colliding drops becomes apparent under
instantaneous illumination, e.g. by sparks from a Leyden jar. The jet
should be situated between the sparks and the eye, and the observation
is facilitated by a piece of ground glass held a little beyond the jet,
so as to diffuse the light; or the _shadow_ of the jet may be received
on the ground glass, which is then held as close as possible on the side
towards the observer.

In another form of the experiment, which, though perhaps less striking
to the eye, lends itself better to investigation, the collision takes
place between two still unresolved jets issuing horizontally from glass
nozzles in communication with reservoirs containing water. One at least
of the reservoirs must be insulated. In the absence of dust and greasy
contamination, the obliquely colliding jets may rebound from one another
without coalescence for a considerable time. In this condition there is
complete electrical insulation between the jets, as may be proved by the
inclusion in the circuit of a delicate galvanometer, and a low
electromotive force. But if the difference of potential exceed a small
amount (1 or 2 volts), the jets instantaneously coalesce. There is no
reason to doubt that in the case of the fountain also, coalescence is
due to _differences_ of potential between colliding drops.

If the water be soapy, and especially if it contain a small proportion
of milk, coalescence ensues without the help of electricity. In the case
of the fountain the experiment may be made by leading tap-water through
a Woulfe's bottle in which a little milk has been placed. As the milk is
cleared out, the scattering of the drops is gradually re-established.

In attempting to explain these curious phenomena, it is well to consider
what occurs during a collision. As the liquid masses approach one
another, the intervening air has to be squeezed out. In the earlier
stages of approximation the obstacle thus arising may not be important;
but when the thicknes