To make steel from cast iron it is necessary to remove certain substances which exist in it as impurities, the chief of which is carbon. The method of removal is generally by means of annopen-hearth furnace, in which pigs, or bars, of cast iron, are raised to a great heat by burning gas and air fed into the hearth at high pressure. The molten steel is poured into moulds, and the ingots thus made pass between rollers which press them into girders.
Water Supply and Drainage
Water as it comes from most rivers is not pure enough to drink. Also, we cannot depend on a river for supplies at all times of the year. The solutions to these two problems purification and the use of reservoirs, River water, pumn into a reservoir, is taken out as required and placed in tanks containing purifying chemicals. From these, it flows to open tanks in which there are layers ol gravel that filter away impurities. The final stage involves the addition of still more chemicals, and the water is then kept in covered tanks and reservoirs until pumped through the mains to individual
Hot water is produced in the modern house by one of three methods: gas, electricity or solid fuel. In the first method, water is led over powerful burners, usually by means of a spiral tube, so that while the outlet tap is running the water is receiving heat from the burners. Water is warmed electrically
by means of an immersion heater, which is an electric heating element, protected by a tube, placed inside a water tank. Heating water by a solid fuel stove requires an enclosed fire containing a water-jacket or an open fire with a back-boiler device. From the cold-water tank in the loft, water runs down to the hot-water tank (often in a cupboard), and from there down a pipe to the bottom of the water-jacket. As the fire heats the water it rises up a second pipe to the hot-water tank. It is replaced automatically by colder water coming down the first pipe, and the circulation provides a constant supply of hot water in the tank from which yet another pipe leads out, at the top, to supply the hot taps in the kitchen and bathroom.
Our modern drainage system is just as important as our water supply. In the past, refuse was thrown into open drains in the streets. Insects and germs breeding in these drains spread disease. Modern sewers all run below ground. The “U tube under the sink, bath and w.C., in which water always
remains, prevents gases and unpleasant smells from coming up Irom the sewers into which the waste pipes lead. When thne
sewage reaches the sewage farm it is mechanically sieved to remove grit which is later used for concrete and road-works. Then the solids are separated from the liquids in settling tanks. The liquids are agitated by a jet of compressed air, which
makes the bacteria multiply rapidly and breaks down any remaining solids into small particles. The final products are a harmless liquid which can be released into a river or the sea, and mud which, when fermented, gives off a gas that supplies the main source of power of the sewage farm.
There are two typesthermosetting and thermoplastic. The latter can be reshaped by the application of heat, but the former are subjected to heat during manufacture, and once moulded cannot be altered in shape. Thermoplastics include acrylic, vinyl and polystyrene-all in common use for household purposes and in toy-making. Many plastics are formed by treating coal derivatives such as phenol.