SOAP BUBBLES. Alany important applica tions of the mechanics of liquid surfaces can be studied very conveniently by means of soap bub bles and soap films. By measuring the diameter of a bubble and the pres sure upon the air within the elastic strength of the film can be measured. Naturally the pressure in a small bubble is great er than in a large one, because the curvature of the surface is greater. This is very prettily shown when two bubbles of different sizes are joined as in Fig. 1, when it will be observed that the partition film p is always con vex into the larg er bubble A, be ing pushed to that form by the greater pressure in the smaller bubble B. In air quiet and free from dust two bubbles may be rested against each other as shown in Fig. 2 a, like two elastic balls, but if a stick of sealing wax be rubbed to electrification and brought near the bubbles, they will coalesce as in Fig. 2 b. One bubble may be blown in side of another as shown in Fig. 3 a. Then electrifica tion will cause it to fall through the outer bubble to the, form Fig. 3 b.
Soap-bubble films on wire frames ar range themselves in a manner beautifully to confirm and illustrate the laws of the composition and equilibrium of forces. A wire frame, as Fig. 4 a. with a thread tied upon it, is instructive. if it is dipped in soap solution and a complete film put on it, the thread will move freely about in the film, but if the film on one side of the thread he broken, the film on the other side will pull the thread to the form b. If the thread have a loop in it the form c may be obtained, and the open ring will move freely about in the film. A good so lution is made of fresh oleate of soda with a little glycerin, or Castile soap may be success fully used. Great care must be exercised to keep the solution free front dust, but it must not btr filtered. For complete details, consult : Boys, Soap Bubbles and the Forces Which Mold Them (London, 1895), an interesting volume describ ing many simple and instructive experiments. See CAPILLARITY.