Practice has shown that large plants can be operated more economically than small ones. Steam may be carried through underground, insulated pipes to great distances with but small loss of heat. This has lead to the sale of exhaust steam, from the engines of manufacturing plants, for heating purposes and the establishment of community heating plants, where the dwellings of a neighborhood are heated from a central heating plant; each subscriber paying for his heat according to the number of square feet of radiating surface his house contains. In the practice most commonly followed, with small steam heating plants, the steam is generated in a boiler located at any convenient place, but commonly in the basement. The steam is distributed through insulated pipes to the rooms, where it gives up its heat to cast-iron radiators, and from them it is imparted to the air; partly by radiation but most of the heat is transmitted to the air in direct contact with the radiator surface. The heating capacity of a radiator is determined by its outside surface area, and is commonly termed, _radiating surface_ or _heating surface_. Radiators of different styles and sizes are listed by manufacturers, according to the amount of heating surface each possesses. Radiators are sold at a definite amount per square foot, and may be made to contain any amount of heating surface, for different heights from 12 to 45 inches. The widespread use of steam as a means of heating buildings is due to its remarkable heat content. When water is converted into vapor the change is attended by the absorption of a large amount of heat. No matter at what temperature water is evaporated, a definite quantity of heat is required to merely change the water into vapor without changing its temperature. The heat used to vaporize water in a steam boiler is given up in the radiators when the steam is condensed. It is because of this property that steam is such a convenient vehicle for transferring heat from the furnace--where it is generated--to the place to be warmed. This heat of vaporization is really the property which gives to steam its usefulness as a means of heating. =Heat of Vaporization.=--The temperature of the steam is comparatively an unimportant factor in the amount of heat given up by the radiator. It is the heat liberated at the time the steam changes from vapor to water that produces the greatest effect in changing the temperature of the house. This evolution of heat by condensation is sometimes called the latent heat of vaporization. It is the heat that was used up in changing the water to vapor. The following table of the properties of steam shows the temperatures and exact amounts of latent heat that correspond to various pressures.