Soap-bubbles. As a form of amusement for children the blowing of soap-bubbles is of great antiquity, and is to be seen depicted on an Etruscan vase in the Louvre. In their scientific aspect soap-bubbles and soap-films have been studied specially by Plateau, who, by adding glycerine in a certain proportion to the soap solution, obtained remarkably durable films and bubbles. The beautiful play of colours familiar to all is due to the excessive but variable thinness of the soap-film. It is in fact an illustration of the interference phenomenon known as Newton's Rings (see INTERFERENCE, NEWTON). If at any part the film becomes thin enough the black spot appears. If this black portion is touched the film is shattered at once, although it may in its thicker portions be pierced by a needle without losing continuity. The spherical form of the ordinary soap-bubble is a direct result of the action of Surface-tension (q.v.), the geometrical condition being that with given volume the surface must have minimum area.
With soap-films formed on frames of wire the same principle holds—for given boundary and given internal volume the area must be a minimum. Thus, by a skilful arrangement of soap-films, we may make visible many highly interesting problems in pure mathematics. See Soap-bubbles, by Professor Boys (S.P.C.K., 1890).