THE LAWS OF FILMS. 621 



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 



* Plateau's mixture. This must be made in a warm room, temperature about 68 Fahr. 

 Let one half ounce of newly made Marseilles or pure Castile soap be dissolved in one pint 

 (twenty ounces) of hot distilled water. When the solution has cooled to about the tem- 

 perature of the room, it is filtered into a bottle. In using the common filter paper (bought 

 at a drug-store for ten cents a dozen sheets), it is better to put only a small quantity of the 

 mixture in at a time, and to support the paper in a funnel or muslin to prevent its break- 

 ing. The paper soon clogs ; it should then be renewed. The process is slow, but not 

 troublesome. When it has all been filtered, add fifteen ounces of pure glycerin either 

 Price's or Scheering and Glatz's the ordinary glycerins are not fit for the purpose. Let the 

 mixture be violently and frequently shaken ; then allow it to stand seven days ; on the 

 eighth cool it to about 37 Fahr., and filter. If the liquid comes through turbid, pour it 

 back and filter over again through very porous paper. 



