Heat is carried through such metals as iron by molecular action and such heat transference is known as conduction. The fact that heat is transferred through metals by conduction is of vital significance in the question of heat losses and dissipation, as through walls of buildings for example. If heat be applied to any part of an iron bar or piece of metal it will be transferred to all parts of that iron by the molecular action until it is all heated. This can be illustrated nicely by placing the end of the poker in the furnace fire and in a short time the heat will be felt in the other end of the poker. [Illustration: _Undesirable Local Heating_] The =stove= is a good example of the conduction of heating. The heat is conducted through the iron of the stove to the air in the room, and then by convection through the air to all parts of the room. The molecules that are in contact with the fire first have their motion accelerated by the heat and this motion is passed from molecule to molecule until all the molecules in the entire iron are accelerated in their motion and thus the temperature of the metal is increased. Some metals are better conductors of heat than others, owing to the difference in the character of the connection between the molecules. Silver forms the best conductor of heat among the metals and is used as a standard of conductivity. In conduction heat is produced inside a fire pot, as in a stove, and conducted through the iron then radiated from its outer surface. This is also called indirect radiation and is a more satisfactory method than the direct radiation since the material of the stove will retain the heat for a longer period of time and allow for its more equitable distribution. In this way it is possible to heat the room more evenly than with an open fire. One of the objections to this method is that it is local and has all the disadvantages of a local heating system. The combustion takes place in the room and as a result there are certain amounts of impurities that are admitted into the breathing zone. There is the added disadvantage of having dirt and dust from the fuel and from the ashes and refuse from the process of combustion. These disadvantages are not encountered in a central system. In extremely cold weather the stoves are likely to become overheated in an effort to keep the rooms warm, and overheated stoves not only increase the hazard from fires, but tend to scorch the air. Red hot iron consumes oxygen and gives off carbon dioxide which produces an unfit atmosphere for breathing. It is difficult to maintain an even heat in a room that is heated by a stove for the stove requires a great amount of attention. CENTRAL HEATING Convection