necessary for hospitals, or 106.25 cubic meters, or 106,250 liters, per head per hour.

The amount of carbon dioxid given off by school children may be assumed to be 10 liters, for a candle 15, for a petroleum lamp 60, and for a gas flame 100 liters; consequently in artificially lighted rooms additional space must be provided to prevent the accumulation of an excess of carbon dioxid. The amount of additional ventilation required for each form of illumination can be calculated in the same manner as already indicated by substituting the corresponding amounts of carbon dioxid yielded by each; for instance, a gas flame would require =d, where e 100, instead of 17, as in the first example

r

given.

=

According to Hueppe, the degree of pollution of the air through different causes may be determined by taking into consideration the amount of carbon dioxid, heat, and watery vapor given off by a person or by any of the more common sources of illumination. The necessary

data are contained in the following table:

Cubic Space.-The amount of cubic space provided for each person depends to some extent on the nature of the occupation, and on the ease with which the contained air can be replenished. If 85 cubic meters of fresh air are to be supplied per head per hour, it is obvious that this can be more readily effected in a room of 25 cubic meters capacity than in one of only half the capacity

without producing disagreeable draughts. The velocity of the incoming air, the position and size of the inlet openings, as well as the temperature of the incoming air, must be so regulated as to prevent the sensation of draught.

It has been found that in temperate climates the air can be changed satisfactorily only about three times an hour unless it is introduced at a temperature above 18° C. In order that 85 cubic meters of air may be supplied, the cubic space for each person should be about one-third as large, or 28.3 cubic meters. For hospitals and sick-rooms the cubic space must be increased in the same proportion as in the fresh air-supply by about one-fourth, so as to provide a space of 35 to 37 cubic meters per head. For cases of infectious diseases a still larger space should be provided. In schools, as a rule, the cubic space provided is very small in proportion to the space required theoretically. In the schools of France and England the cubic space per head ranges from 2.83 to 4.675 cubic meters. In the modern school buildings of Philadelphia only about 5.6 cubic meters are provided for each pupil, and the air is changed about seven times an hour. With the modern systems of ventilation now in use this amount of space is possibly not much too low to meet the desired results. According to Morin, we require—

It will be noted that Morin allows for a complete. change of air only twice each hour. Where more frequent changes of air can be definitely secured a smaller

amount of cubic space per person may give satisfactory results.

/ General Rules for Ventilation.-The quality of the incoming air is of equal importance with the quantity; therefore, care must be exercised in selecting the source of the air-supply. In large towns it may be necessary to wash or filter the air before it is distributed.

The current of incoming air should be imperceptible. This is of special importance when, as is generally the case, the temperature of the outer air is lower than that of the air of the building. When cold draughts are produced the system of ventilation is faulty. The larger the area of the inlet and the outlet openings the slower the velocity of the air current, but obviously these openings cannot be enlarged indefinitely.

The fresh air must not only be supplied to a space, but it must also be diffused equably throughout the space, so as not to pass directly from the point of entrance to the point of exit. It is very difficult practically to attain this end, but unless it is attained we fail to secure the proper displacement and renewal of the vitiated air.

Ventilation is effected either by natural means or by the aid of mechanical contrivances. The former is called natural ventilation and the latter artificial ventilation.

Natural Ventilation.-In all buildings there is an interchange between the inside and outside air by diffusion through the substance of the walls and floors themselves, but this interchange is insufficient to replenish the contained air, and provision must be made to supply the necessary amount of fresh air through openings in the walls, as doors, windows, etc., or through special openings into ventilating shafts; the latter method being the preferable one, especially for large assembly-halls and school-rooms.

The forces which are continually acting in nature and produce natural ventilation are diffusion, the action of the wind, and the difference in density of masses of air of different temperatures; the latter being the most important.

Diffusion. All gases, including the mixture of oxygen and nitrogen which constitutes atmospheric air, diffuse through space, the force of the diffusion being inversely as the square roots of the densities of the gases. The diffusion of carbon dioxid, and the other gaseous impurities in the air of an enclosed space, into the fresh air takes place not only through the natural openings of rooms, as doors, etc., but also through the walls, floor, and ceiling, because the materials of which these are constructed are always more or less porous and permeable. The amount of ventilation through walls varies with the porosity of the materials of which they are formed; the temperature of the inside and outside air; the force and direction of the wind, etc. Damp walls are less porous than dry walls, and this is partly the cause of the unhealthfulness of damp houses.

The Action of the Wind.-This is exerted in two ways: (1) By perflation—that is, blowing through an air space and thus changing the air contained therein; and (2) by aspiration-that is, sucking up masses of air in consequence of a partial vacuum that is produced on either side of a moving mass of air. Perflation takes place through doors and windows, as well as through walls and ceilings. In the wards of a hospital, where thorough ventilation is of especial importance, the windows should be placed on both sides of the room, so as to allow full sway to the perflating action of the wind. Aspiration is provided when the wind blows over the top of a chimney or ventilating shaft and causes an upward current at right angles to its course. A strong wind may impede the movement of the air up the chimney. Down draught may be produced and smoke forced into the rooms.

Difference in Temperature.-The movement produced by the difference in weight of masses of air of different temperatures is the chief force acting in natural ventilation. When a mass of air is heated it expands, and proportionate volumes of it become lighter; consequently it rises to a higher plane and is displaced by colder and

heavier air. The greater the difference in the temperature of masses of air the more rapid the movement that is produced. The rate of movement may be calculated according to either of the following rules:

1. The velocity of falling bodies is equal to the square root of the space or height through which they have fallen, multiplied by the square root of twice the accelerating force of gravity. V=V 2 gr.

2. Rule of Montgolfier: Fluids pass through an orifice in a partition with a velocity equal to that which a body would acquire in falling through a space or height equal to the difference in depth of the fluids on the two sides of the partition.

=

=

All gases under constant pressure expand equally. If V the volume at o° C., then the volume at t° VX (1 + a. t), where a = 0.003665. The effect of heat on air, therefore, is to increase its volume and to lessen its density directly in proportion to the increase in temperature.

Arrangements in Natural Ventilation.-In cold and temperate climates the openings that are usually present in inhabited rooms are doors and windows. Chimneys are also generally present. Ventilation is not the primary object of these openings, but nevertheless they act as ventilators, and in very many instances they afford the only means for ventilation. Diffusion takes place through all these openings, as well as through the walls, floors, and ceilings, and generally no special arrangements are needed to assist it in ordinary dwellings.

The natural ventilation through the pores of the walls is of slight significance. It occurs constantly in a vertical direction through the floor and ceiling. In winter, when the house is heated, it occurs from below upward, and in summer in the opposite direction, because the house is colder than the outside air. Along the side walls the excess of pressure diminishes from the floor to the ceiling, between which points there is a neutral zone where it is zero. In winter, in consequence of warming the room air, there is an outward movement above this zone,