WARMING AND VENTILATION 453 long, and 6 ft. wide) can be renewed once in 10 minutes without creating any appreciable air currents. Dr. farkes maintains that, with natural ventilation, in the English climate, the air can be safely changed only three or four times an hour, which necessitates a larger initial air space. But the chief practical diffi- culty in the rapid change of air in small rooms arises from the position of the inlets, which have to be so near the person that draughts can scarcely be avoided. Moreover, in rooms of small space with a large supply of fresh air, it is impossible to obtain a uniform diffusion, as direct currents arise between inlets and outlets. Drs. Parkes and De Chaumont main- tain that the space for each healthy adult ought to be at least 1,000 cub. ft., while it is often not half or a quarter that amount. In estimating the quantity of air required for effectual ventilation, it is to be remembered that the air immediately around the person is becoming constantly and rapidly vitiated by exhalations from the skin and the lungs, and the movement of diffusion requires to be so constant and active as to carry it away and keep the body surrounded by fresh air. The supply may become a question of expense as well as of health, when lower standards must be accepted. Those exchanges of air which are spontaneously effected in houses by means of various facilities, but without the artificial application of power, are known as natural ventilation. All inequalities of temperature in the air set it in motion. If the air in a house is warmer than that without, it will escape through windows, doors, cracks, and flues, and the colder outside air will rush in by all chance apertures to maintain the equi- librium. The movements of the external air greatly assist these exchanges. The wind ex- erts an aspirating action through chimneys and air shafts, creating a partial vacuum in them, which produces up currents. Cowls placed on the tops of chimneys and air flues favor the aspirating power of the wind. Op- posite windows afford cross ventilation when opened, if there is much external atmospher- ic movement. Various outlets and inlets are arranged in the tops of windows, or substi- tuted for the glass panes, or by inserting per- forated bricks in the walls near the ceiling, or by valves in chimneys, or passages that can be opened and closed over the doors. Such ar- rangements are irregular in their effects, and require vigilant attention to make them ser- viceable. In artificial arrangements for secu- ring pure air, the agent of warmth becomes the motor of ventilation. The ordinary open fire- place was one of the earliest means adopted to secure both objects. The heat in this case is entirely radiant, being thrown off directly from the burning fuel or reflected from the sides and back of the fireplace. It strikes upon the walls, ceiling, floor, and furniture of the room, which are warmed and gradually impart their heat to the contiguous air, thus producing gen- tle and equalizing currents. As the fireplace is at the side of the apartment, and as radiant heat decreases rapidly in intensity, the warm- ing is very unequal. Near the fire it is hot, and at a distance cold, while a person can be warmed only on one side at a time. The open fireplace is the most wasteful of all arrange- ments for warming, as a copious stream of air passes up the chimney which takes no part in combustion, but carries off with it much heat. This loss is six sevenths, seven eighths, or even more of the heat produced, so that scarcely 12 or 14 per cent, of the heat is utilized. The coal grate is more economical for warming than the larger wood fireplace, chiefly because it lessens the current of air which enters the flue. Like the fireplace, it is closed on three sides, and these should be of some slow-conducting sub- stance and not of iron, which carries away the heat so fast as to deaden combustion. The art of burning fuel to the best advantage in open grates is to maintain the whole mass in a state of bright incandescence by preventing all un- necessary abstraction of heat, either by contact of surrounding metal or currents of cold air flowing over the fire. To be burned with econ- omy, that is, to get from fuel the greatest amount of heat possible, it must consume rap- idly and with vivid combustion. To insure the greatest heating effect, the air which comes in contact with the fuel should part with the whole of its oxygen. Every particle of air passing through the fire which does not aid combustion obstructs it, first by directly car- rying off a portion of the heat, and secondly by cooling the ignited surface so that it at- tracts the oxygen with less energy. Air en- tering below a fire rapidly loses its oxygen and becomes contaminated with carbonic acid, both changes unfitting it for carrying on the process actively in the upper region of the fire. If therefore the mass of burning material be too deep, the upper portions burn with least ad- vantage; or if the pieces of coal be large, scarcely any depth of fuel will be sufficient to decompose the whole of the air which arises through the wide spaces. The modifications of fireplace and grate are innumerable. Some have circular fronts to favor radiation, which is liable to expose the fire to so much air as seriously to obstruct the combustion. An iron plate for a fire-back has been employed to warm an adjacent room behind a fireplace, and for the same purpose grates have been hung upon pivots so as to revolve and thus warm two rooms alternately. In one plan the coal is introduced below the fire, working its way from above downward and consuming the smoke. Grates are often set so low that the radiations pass along parallel with the floor, which is not warmed as it would be if the fire were higher and the radiations struck down- ward. But, though defective and wasteful for heating, the open fireplace secures considerable ventilation. The magnitude of the open space above the fire, though a source of wasted heat,
