Soap Bubbles.
The blowing of soap bubbles is of great antiquity, and is to be seen depicted on an Etruscan vase in the Louvre. The beautiful play of colors familiar to all is due to the excessive but variable thinness of the soap films. The spherical form of the ordinary soap bubble is a direct result of the action of surface tension, the geometrical condition being that with given volume the surface must have minimum area.
Entry from Everybody's Cyclopedia, 1912.