d the speed should not exceed one turn in
two seconds.

The effect is still more real if a screen is placed between the disc and
the mirror, which will only allow one of the drops to be seen.

_Water-drops in Paraffin and Bisulphide of Carbon._

All that was said in describing the Plateau experiment applies here.
Perfectly spherical and large drops of water can be formed in a mixture
so made that the lower parts are very little heavier, and the upper
parts very little lighter, than water. The addition of bisulphide of
carbon makes the mixture heavier. This liquid--bisulphide of carbon--is
very dangerous, and has a most dreadful smell, so that it had better not
be brought into the house. The form of a hanging drop, and the way in
which it breaks off, can be seen if water is used in paraffin alone, but
it is much more evident if a little bisulphide of carbon is mixed with
the paraffin, so that water will sink slowly in the mixture. Pieces of
glass tube, open at both ends from half an inch to one inch in diameter,
show the action best. Having poured some water coloured blue into a
glass vessel, and covered it to a depth of several inches with paraffin,
or the paraffin mixture, dip the pipe down into the water, having first
closed the upper end with the thumb or the palm of the hand. On then
removing the hand, the water will rush up inside the tube. Again close
the upper end as before, and raise the tube until the lower end is well
above the water, though still immersed in the paraffin. Then allow air
to enter the pipe very slowly by just rolling the thumb the least bit
to one side. The water will escape slowly and form a large growing drop,
the size of which, before it breaks away, will depend on the density of
the mixture and the size of the tube.

To form a water cylinder in the paraffin the tube must be filled with
water as before, but the upper end must now be left open. Then when all
is quiet the tube is to be rather rapidly withdrawn in the direction of
its own length, when the water which was within it will be left behind
in form of a cylinder, surrounded by the paraffin. It will then break up
into spheres so slowly, in the case of a large tube, that the operation
can be watched. The depth of paraffin should be quite ten times the
diameter of the tube.

To make bubbles of water in the paraffin, the tube must be dipped down
into the water with the upper end open all the time, so that the tube is
mostly filled