In addition to carbonic acid, expired air contains an undue amount of watery vapor. Minute quantities of animal matters are also exhaled with the breath, which in close, ill ventilated apartments form a clammy deposit on the furniture and walls, and, by putre fying, become organic poisons.
A further necessity for the constant renewal of fresh air arises wherever lights are burned. The deteriorated air of a fire goes off by the flue, but lights are generally burned where the product must mingle with the atmosphere of the apartment. Now, a pound of oil in burning consumes the oxygen of 13 ft. of air, and produces a large amount of water in vapor, and also of carbonic acid. Every cubic foot of gas consumes the oxygen of 10 ft. of air, and forms at least 1 foot of carbonic acid, besides watery vapor, sumetimes mixed with sulphurous fumes.
To counteract these various sources of pollution, and keep the air sufficiently fresh and wholesome, in rooms where many persons are breathing, it is found in practice that on an average about 20 cubic ft. of fresh air per minute for each individual must be sup plied.
Ventilation consists of two operations—the removal of the foul air, and the introduc tion of fresh. Though neither operation can go on without the other going on at the same time, it is convenient to consider the two separately.
The agents employed in removing the air from apartments are chiefly two: that by which nature effects the ventilation of the earth on a grand scale, viz., the draught of ascending currents produced by difference of temperature; and mechanical force, such as pumping. The former is the more common, and is the only one applicable to private houses.
The column of air in the chimney of a lighted fire-place being expanded and com paratively light, exerts less than the prevailing pressure on the air immediately under and about its base. The air, therefore, below and around it pushes it up, and flows in to take its place; the velocity of the movement being in proportion to the height of the chimney and the degree of heat. Thus, although it is often convenient to speak of the air being drawn or sucked into the chimney, the force does not lie in the chimney, but in the greater pressure of the air behind.
Wherever, then, there is a heated chimney, there is a means of removing the foul air. And in rooms moderately lofty and spacious, with windows and other fittings not closer than usual, and a chimney mouth of the usual width, there is little risk, when there are only a few inmates, of any serious vitiation of the air. The heated breath that ascends to the ceiling has time to diffuse itself gradually, and be drawn in a diluted state into the currents that are setting from all quarters toward the chimney. These currents,
however, are one great objection to this mode of ventilation, as they consist in great part of cold air that has just entered by the doors and windows, and are strongest where the inmates sit to enjoy the fire.
The ascent of foul air to the top of the room dictates its exit in that direction, rather than low down at the mouth of the chimney. It is conceived by some that the carbonic acid of the breath, from its greater weight, must be chiefly at the bottom of the room; but this is a mistake. The heated breath ascends instantly, because it is, as a whole, lighter than the air around it: and the carbonic acid in it does not tend to separate,from it and fall down by its superior weight, but, by the law of the diffusion of gases, seek!, to itself equally all over the room, and would do so though it were lying at first on 11,,e tloor. It is on the principle of the foul air ascending at first to the top of a room, that Dr. Arnott's ventilating-valve is contrived. The valve may be used to supplement open-tire draught in small and crowded apartments, and is essential where the fire is burned in a close stove or in the smokeless grate. The valve is represented at v, fig. 1. An aperture is cut in the wall over the chimney, as near to the ceiling of the apartment as maybe convenient. In this is suspended a valve, capable of opening inward to the chimney, but not in the other direction, by which means a return of smoke is prevented. The valve is so balanced on its center of motion, that it settles in the closed position, but is easily opened. A flap of 36 sq.in. is sufficient, where there is good chimney-draught, for a full-sized room with company. This simple apparatus may be painted or otherwise made ornamental. It operates by virtue of the draught in the chimney. Whenever that is active from the presence of a fire, the valve is seen to open inward, and a stream of air from the top of the apartment passes through into the chimney, and is carried off. The operation is precisely equivalent to the stream of air always passing into a chimney between the fire and the mantel-piece, but has the great superiority of draining off the most impure air in the room. A wire descends to a screw or peg fixed in the wall, by which the opening of the valve may be limited or altogether prevented. This is a far more efficient plan of ventilation than an open window, or an opening in the wall near the roof, leading merely to. the outer air; where there is an open fire in the room, such openings rather admit a rush of cold air than let out the foul.