Obtain a piece of good card-board as large as the figure, and having brushed it all over on one side with thin paste, lay the figure upon it, and press it down evenly. Place it upon a table, and cover it with a few thicknesses of blotting-paper, and lay over all a flat piece of board large enough to cover it. Weights sufficient to keep it all flat may be added. This must be left all night at least, until the card is quite dry, or else it will curl up and be useless. Now with a sharp chisel or knife, but a chisel if possible, cut out the forty-three slits near the edge, accurately following the outline indicated in black and white, and keeping the slits as narrow as possible. Then cut a hole in the middle, so as to fit the projecting part of a sewing-machine cotton-reel, and fasten the cotton-reel on the side away from the figure with glue or small nails. It must be fixed exactly in the middle. The edge should of course be cut down to the outside of the black rim.
Now having found a pencil or other rod on which the cotton-reel will freely turn, use this as an axle, and holding the disc up in front of a looking-glass, and in a good light, slowly and steadily make it turn round. The image of the disc seen through the slit in the looking-glass will then perfectly represent every feature of the growing and falling drop. As the drop grows it will gradually become too heavy to be supported, a waist will then begin to form which will rapidly get narrower, until the drop at last breaks away. It will be seen to continue its fall until it has disappeared in the liquid below, but it has not mixed with this, and so it will presently appear again, having bounced out of the liquid. As it falls it will be seen to vibrate as the result of the sudden release from the one-sided pull. The neck which was drawn out will meanwhile have gathered itself in the form of a little drop, which will then be violently hit by the oscillations of the remaining pendant drop above, and driven down. The pendant drop will be seen to vibrate and grow at the same time, until it again breaks away as before, and so the phenomena are repeated.
In order to perfectly reproduce the experiment, the axle should be firmly held upon a stand, and 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 with paraffin. It must then be closed for a moment above and raised till the end is completely out of the water. Then if air is allowed to enter slowly, and the tube is gently raised, bubbles of water filled with paraffin will be formed which can be made to separate from the pipe, like soap-bubbles from a "churchwarden," by a suitable sudden movement. If a number of water-drops are floating in the paraffin in the pipe, and this can be easily arranged, then the bubbles made will contain possibly a number of other drops, or even other bubbles. A very little bisulphide of carbon poured carefully down a pipe will form a heavy layer above the water, on which these compound bubbles will remain floating.
Cylindrical bubbles of water in paraffin may be made by dipping the pipe down into the water and withdrawing it quickly without ever closing the top at all. These break up into spherical bubbles in the same way that the cylinder of liquid broke up into spheres of liquid.