=Objections to Ventilation by Heating Apparatus.=—When _warmed air_ is admitted into a room, it is very apt to be _dry_ and irritating. This can be usually avoided by having water standing in the room, so as to allow evaporation. A more difficult problem is to ensure the complete absence of all products of combustion, particularly of the products of incomplete combustion. CHAPTER XXIV. THE WARMING OF HOUSES. =Physiological and Physical Considerations.=—The warmth of our bodies is naturally kept up by the oxidation changes constantly going on in the system. In Chapter XL., p. 265, are discussed the modes in which heat is lost by the system, and the influence of clothing in controlling the amount of this loss. Artificial warming of houses has a similar action to clothing. It diminishes the demand on the system, and so economises the amount of food required. The degree to which this diminution of loss of heat by clothing and artificial warming of houses may be carried varies with circumstances. There can be no doubt that if food be abundant, exposure to external cold, if not too extreme, is on the whole beneficial, for vigorous people. But for old people and young children, means of artificial warmth require to be more carefully provided. Severe cold is for them often the harbinger of death. The =Degree of Temperature= at which living-rooms should be kept will vary with circumstances. For _healthy adults_, any temperature between 50° and 60° Fahr., will be moderately comfortable; for _delicate children and old people_ it may be 65° with advantage. For _sick rooms and hospitals_ the temperature of 60° is usually adopted, but this is by no means always necessary. A temperature of the room as low as 50°, except for such diseases as whooping cough and bronchitis, suffices if the patient is well covered with warm personal and bed coverings. _Convalescents_ from any acute illness bear low temperatures badly. =The Different Kinds of Heat.=—Heat may be communicated by radiation, conduction, and convection. By =radiation of heat= is meant the process by which heat passes from a fire or other source of heat, through a vacuum, dry air or any other medium, without heating any of the media through which it passes, but only the bodies against which it finally impinges. The solid bodies (including ourselves) which are warmed by radiant heat, by a process of conduction then warm the surrounding air. This method is the nearest imitation of the natural warmth of the sun. =Conduction of Heat= is the passage of heat from one particle to another, whether it be of a gas or solid. It is an extremely slow process when air is concerned, and may be practically ignored.