Page:Popular Science Monthly Volume 36.djvu/639

Rh pure Castile soap, and allowed to settle and clarify, or else filtered, answer very well for a series of simple and beautiful experiments in the forms assumed by soap films in order to fulfill this law of their union. There is a glyceric fluid made, which by various means has all the impurities of soap and water removed, and is toughened by the addition of pure glycerin; and this is of course better, because much more persistent. A bubble made from this glyceric fluid, and carefully protected by a tumbler or bell-glass, will last for hours, and in some cases for days.

For these experiments in form, common suds last long enough to show the forms very satisfactorily, but for experiment in color the more lasting fluid is necessary, so a recipe for it is given here.

When frames made in various forms, by bending fine copper wire, are dipped into the fluid, it is found that the films take on the most wonderful and beautiful shapes in order to fulfill the law of their union. The material of which the films are made does not at all affect their form.

With fine, well-straightened copper wire, outline a cube; this may be done with the fingers or a pair of ordinary pliers, and the figure need not be very exact. The wires can be double along any of the lines; let one end project from some corner for a handle, to be used in dipping the frame into the fluid.

Immerse this cube in the suds, and as you lift the frame out observe the films forming and shaping themselves. They usually take for a moment the form in Fig. 1 and slowly change to Fig. 2. Sometimes they retain the first form; in this case, the central drop with a glass of low power is seen to be not really a drop of fluid, but a tiny cube of films, each meeting the film from the wire edge by a curvature of its faces at exactly the required angle, 120°. The films have constructed in their midst this tiny cube, because the twelve films could not otherwise meet in the center at the proper angle. This cube is formed and kept where a tiny bubble has been entrapped in the system of films. If no such bubble of air has been caught in with the films, they