SOAP BUBBLES, originally a simple ex periment for pleasing children, produced by making water quite soapy and preferably warm and then dipping a clay smoker's pipe or a tube into the water, withdrawing it with a film of soapy solution of water across the aperture or bowl of the pipe. Then by blowing in the tube the bubble is formed and after it attains some size may be shaken off and float in the air as a spherical balloon if the interior air is warmer than the outer air. A little glycerine in the soap makes them much more durable. Bubbles in their scientific aspect have been studied specially by Plateau, who, by add ing glycerine in a certain proportion to the soap solution, obtained remarkably durable films and bubbles. 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. Thus, if two or more soap bubbles are joined, which is an easy matter, the joined surfaces are approximately a flat plane; but in reality the smaller bubble projects or curves slightly into the larger bubble, be cause the smaller the bubble curvature the greater the tension. One bubble may be blown within a larger one and then if gently shaken may chop through so as to hang from the larger bubble in outer contact. It has been demonstrated that this is due to electrification. With a well-soaped wire and thread it is pos sible to open up a soap bubble in the centre and to bring it to a ring form (like a cruller) without breaking. In making this delicate ex periment all dust must be excluded. Consult Boys, C. V., 'Soap Bubbles' (in
numerous plants of va rious families that contain a poisonous principle called sapoin, which lathers in water, and is utilized as a detergent, both medically and for laundry purposes. The common soap-wort is the European Saponaria officinahs, often found in America along roadsides and railways, as a weed escaped from gardens. It is a rankly growing, smooth and shining perennial, with the characteristic, opposite leaves, in this instance broadly oval, and five-merous flowers of the pink family. The obcordate petals are long clawed, with a scale at the base of each blade, ranging in hue from white to bright-rose color, and the dense terminal corymbs would be handsome were they not marred by fading flowers and brown seed-capsules. The thick
roots and leaves abound in saponin, and be sides its employment in the laundry, the plant was formerly used as a cure for itch; it also yields an alterative drug resembling sarsaparilla. The cow-herb (Vaccaria vaccaria) also con tains saponin. Soapbark or quillaia-bark (q.v.) is a stimulant and irritant drug, the brownish white cortex of Quillaia saponaria, a smooth evergreen tree native to Chile, where its inner bark, reduced to powder, is a substitute for soap. Pithecolobium of several species are leguminous trees, one the savonette or shagbark of the West Indies, which yields other soap barks, in demand for cleaning delicate fabrics. There are also several Chinese trees, which serve the natives for soap, especially valuable in washing silks and the hair. The fruit of Pancoria dclaryi of China resembles the soap berry (q.v.), and the pods of Gymnocladus chinensis, and certain species of Gleditschias, give more substitutes for soap. The refuse or "oil cake" of the seeds of the tea-oil tree (Camellia sasanqua), after the oil has been expressed, is not only employed in washing, but for catching fish. When the seeds or the oil cake are bruised and thrown into water the poisonous saponin stupefies the fish, which rise to the surface and are easily captured.
Amole is a Mexican name covering several plants having saponaceous and cleansing proper ties and utilized by the natives of Mexico and the adjacent regions of the United States. Agave heteracantlia is a common example. The California soap-plant, or soap-root (Chloro galum-pomeridianum),is a liliaceous plant, with wavy-edged, linear leaves and a tall branching panicle of many white flowers. The root is a bulb one to four inches in diameter, invested with dark-brown fibres. This bulb is pounded up by the California aborigines, who throw it into pools where fish have no means of escape, stupefying and capturing them. Hot soap-root is used to cleanse and heal old sores as well as in the laundry. There are several other soap plants, as Zygadenus fremontii and Leucocri num montanum in California, and Acacia con cinna, the soap-nut, the pods of which are used in India as medicines and detergents, especially in hair washing.