The electrical resistance of soft rubber insulation is well recognized. Gloves, rubber shoes of special construction and pro tective blankets for linemen are serving useful purposes in the electrical industry. Hard rubber compositions are also used ex tensively for electrical purposes. Telephone receivers, many parts of telephone station equipment, insulators, plugs and sockets, storage battery containers, and many other articles are made of hard rubber. The heat conductivity of rubber is only about 30o the conductivity of steel. Therefore, at temperatures not so high as to cause decomposition, it is a good heat insulator. Hard rubber sponge is an excellent heat insulator but its rela tively high cost limits its use for this purpose. Resilience of rubber lends value in connection with some of the other properties already enumerated. Tires, valve discs, uni versal joint equipment, nursing nipples, milking machine cups, golf balls, tennis balls, elastic webbing, and aeroplane shock absorber cord, already mentioned, would be of little value if they were not resilient as well as possessing the other properties under which they are listed. Rubber compositions are used for resisting solvent or chemical action of many substances. Car heating hose for railroad trains and steam hose for many indus trial purposes, steam packing for flanged pipe joints or digestor heads exemplify the value of rubber in resisting the action of steam. Air hose, distillate (petroleum) hose and printing press rolls of rubber illustrate its use for resisting oil. Railroad and
storage tanks lined with rubber are used for muriatic, phos phoric or dilute sulphuric acids and for many corrosive solu tions, such as aluminium sulphate and ferric chloride ; rubber lined tanks are used extensively also to hold acids through which sheet steel is passed in the cleaning ("pickling") operation and to contain solutions of various kinds used for electroplating. Belts are used in canneries for carrying food products of various kinds, and also in creameries where it is necessary that the rub ber covers be resistant to butter fat. Salt solutions of many kinds, which corrode metal, can safely be handled in rubber lined equipment. Rubber-lined pipes and valves are now pro duced, permitting the installation of apparatus rubber-lined throughout. Hard rubber pumps, pipes, and fittings, and metal apparatus lined with hard rubber serve the chemical industry for certain types of service where soft rubber is unsatisfactory. Hard rubber may be used at higher temperatures than soft rubber and will resist greater concentrations of corrosive materials. Ad vantages have been obtained in tank linings by sandwiching a layer of hard rubber between two layers of soft rubber. The ten sile strength of soft rubber materials based on dimensions be fore stresses are applied will vary according to composition up to 4,500 lb. per square inch. Hard rubber compositions vary from i,000 to r o,000 lb. per square inch. As already mentioned, however, rubber may be combined with other materials to secure articles of greater strength.