Tar is used for caulking wooden boats and for waterproofing ropes, roofs, tarpaulins, and roads. Asbestos is likewise used for roofs, and asphalt has similar uses for roads. Asphalt varnishes are also used for protecting iron from weathering. Oils are used both as film in protecting machinery from water vapor and in impregnating fabrica, as raincoat cloth, instrument cases, and oiled shoes. Oils are also the solvent in most paints and bind the particles of pigment into a thin, resistant layer. Rubber may be used in sheet form as a cover, but it is usually vulcanized to a fabric, which gives the rubber durability, or it is deposited by electro-chemical means into the pores of the fabric. Any water-insoluble substance may be used in place of rubber, in such connection, providing it can be conveniently precipitated. Such substances are aluminium hydroxide, metallic soaps, waxe», paraffin, resin, asphalt, tar, and varnish. To make fabrics impervious to water and at the same time to allow for ventilation, recourse is had to waterproofing the yarn and then weaving the fabric out of this yarn. An important addition to the family of waterproof materials is cellophane, a cellulose product from which umbrellas, wrappings and raincoats are made.
The essential of all waterproofing is that the pores of the substance in question be so small that water can not enter. Thus it is found that hard, tightly woven fabrics, such as Navajo blankets, shed water very effectively; and water mammals, such as seals, muskrats, otters and polar bears, are able to shed water by virtue of their downy, oily inner coats.
Waterproofing is of the greatest importance. Rot, decay, and mold are all hastened by the infiltration of, or exposure to, water; health is endangered by undue exposure to water; and the corrosion of metals, such as iron, is possible only in the presence of water vapor. It is estimated that one quarter of all iron used for commercial purposes is lost in the form of rust. and the better waterproofing of iron would be an inestimable boon to humanity.
