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"In other words, as instances from our table, a bottle holding eight and a half ounces of air, with a half ounce of lime-water shaken in it, would show no precipitate or turbidity if the amount of carbonic acid was not more than .08, i. e. eight parts of carbonic acid in 10,000 of the air in the room and bottle.

"If a six-ounce bottle is used, with the same amount of lime-water, there might be .11 parts of carbonic acid to 10,000 of air, and yet there would be no turbidity; or if a two-ounce bottle was used there might be forty parts of carbonic acid to 10,000 of the air, and yet no turbidity ensue.

"Now, if a bottle of eight ounces, with a half ounce of clear limewater, gives turbidity, you know that there is more carbonic acid in the air than is regarded as desirable for a school-room.

"If a six-ounce bottle, with a half ounce of lime-water, gives turbidity, you will know that there is more than eleven parts of carbonic acid to 10,000, which is an excess.

"If a two-ounce bottle should give turbidity, you then know that there are over forty parts of carbonic acid to 10,000, while there should be not much over eight parts to 10,000.

"By testing with different sized bottles, after you have once found turbidity, you will be able to find out nearly the proportion of carbonic acid."

The lime-water is prepared by dissolving a piece of common caustic lime, about the size of a black walnut, in a quart of water, and then allowing it to settle.

"The sensation of uneasiness produced by breathing impure air is an indication of the injurious effects that result from it, which is too often neglected. When the air is not sufficiently pure to effect the

complete decarbonization of the blood, we have already seen that the result is the circulation of venous blood through the brain; the respiration then becomes impeded, and the nervous system deranged; the extent of these effects, of course, varying with the amount of the exciting cause, and with the peculiar constitutions of the individuals exposed to their influence. Dr. Harwood remarks on this subject, "The want of wholesome air, however, does not manifest itself on the system so unequivocally, or imperatively; no urgent sensation being produced, like that of hunger, and hence the greater danger of mistaking its indications. The effects of its absence are only slowly and insidiously produced; and thus, too frequently, are overlooked until the constitution is generally impaired, and the body equally enfeebled.”

The amount of air-space needed by each individual depends primarily upon the amount of oxygen that is being burnt up or removed from the air of the room.

Our first data are derived from the amount of air consumed or deteriorated by each person. From 350 to 400 cubic feet of air passes through the lungs of a man of usual activity in the twenty-four hours. If every breath took out only a certain amount of air and its oxygen, and the expiration or outbreathing of the air from the lung did not return to the room, the problem would be a simple one, for new and pure air would take the place of the air extracted by breathing. But a cubic foot of air, as it comes from the lungs in ordinary respiration, has lost most of its oxygen, and contains, instead, upwards of seventy cubic inches of carbonic acid, besides organic matter and fouled watery vapor. This air has not only been devitalized, but infused with injurious particles. If there are fires or lights, every cubic foot of coal gas consumes the oxygen of ten cubic feet of air, and produces two cubic feet of carbonic acid. The combustion of a pound of oil consumes the oxygen of 130 cubic feet of air, and produces about twenty-one feet of carbonic acid.-Huxley. While the latter is not laden like the breath with organic matter, it is a devitalization of the air. The need of air-space or ventilation also depends upon shape of room, height of ceiling, floor-space, etc.

With all these facts in view, those who have most carefully considered them, and have tested by experience also, claim that in a room ordinarily tight, 2,000 cubic feet of air must be admitted each hour for each person in it. This is based upon the conclusion that about 650 feet of air is actually needed each hour for each person, but that as practically we can not move the entire air of a room oftener than

three times an hour without draught, we must introduce three times the amount actually used up.

The amount of cubic space required may be stated as from 250 to 300 feet for dwellings, school-rooms, etc., while for tenements, hospitals, etc., it should be much more.

As height of ceiling over twelve feet is not counted, this would give to each person in a room, or to each scholar in a school, a floor-space of about four feet by five, or five by five.

It is well to consider all the various theoretic needs and modifications, because they help us to attain to accuracy. What is called experience needs to be tested by scientific facts, just as scientific facts need to be tested by experience. In this case, with the fact that there are so many modifications, and the additional fact that no room is dependent upon any one inlet, since windows, crevices and even bricks, admit much air, the statement of test most relied upon is that of Parkes and De Chaumont, which is, that the amount of air required for any occupied room is the amount needed to keep the room free from any perceptible odor to a person entering it from the outer air, and to keep the percentage of carbonic acid (carbon dioxide) as near as possible to the normal rates of four parts in 10,000, and never beyond seven parts in 10,000.

In heating a room, the problem with which we are chiefly concerned is, how so to heat it as to maintain a comfortable warmth and a purity of the atmosphere in accord with the conditions we have mentioned. While, technically, ventilation means to restore the air to its outside purity, this is never done; it practically means the keeping of impure air so diluted or mixed with pure air as to secure a standard compatible with health. Fortunately, within certain limits, there are powers of adjustment within the human organism which render it possible to be comfortable in, and not to be injured by, air which approaches to normal purity. But, if we go far beyond these bounds, there are embarrassing or destructive elements which are just as much a part of nature's law, and which cause a decided injury to health.

In the heating and ventilating of any room, the chief point of consideration is, how to warm a room and, at the same time, maintain a proper and uniform purity of air. This leads us to inquire:

(a) How to keep to a minimum the consumption of oxygen by lights, breathing, etc.

(b) How to get rid of all organic matter from the lungs, or from

other sources, which, in the form of decayable particles, contaminates, or is ready to contaminate, the air.

(c) How to prevent or get rid of dust and organic particles, which, if not putrescent, in a mechanical way interfere with the quality of the air.

(d) How to secure such moisture of the indoor air as is favorable to health and comfort.

As the outside air is, as a rule, purer than any inside air, the first question is, how to introduce this so as to avoid draught.

In order to make this a more single and simple question, we assume that the air will be heated after it has come into the room.

To such rooms fresh, unheated air must come in from without, and must come in at a slowness of velocity such as will not be so perceptible as to cause draught. This feeling of draught depends in part on the velocity of introduction, and, in part, on other conditions. "The warmth of the moving air influences the sensation of the persons exposed to it. At a temperature of 55° or 60°, a rate of 11⁄2 feet per second (or about 1 mile per hour) is not perceived; a rate of 2 and 21 feet per second (1.4 and 1.7 miles per hour) is imperceptible to some persons; 3 feet per second (2 miles per hour, nearly,) is perceptible to most; a rate of 3 feet is perceived by all persons; any greater speed than this will give the sensation of draught, especially if the entering air be of a different temperature or moist. If the air be about 70° Fahr., a rather greater velocity is not perceived, while if it be still higher (80° or 90° Fahr.), the movement becomes again more perceptible. This is also the case with the temperature below 40° Fahr.

Our power of introducing air into a room without draught depends upon the size of the room, the number of persons to be supplied with air, the temperature of the air in the room and of that being introduced, the relative temperature of outside and inner and the mode of introduction. In a small room it is more difficult to have the air distributed before reaching the person, and so he may feel a draught. Where there are numbers of persons the air must be introduced more rapidly unless there is adaptation of size of room and modes of introduction thereto. We have already noted variations made by temperature and moisture. If the air comes in through some direct inlet, and nearly all at one or two points more, draught is likely to occur. Smallness of opening may give direction to the current, as where a hole.

in a pane of glass directs a current upon some exposed part of a person near by, and causes a draught which a wide-open window would not. As a rule, we are not so likely to have draughts when the air is introduced at various points in small quantities instead of at two or three points in large quantities. We also do much to prevent the sensation of draught if we introduce it above the heads of persons occupying the room, and in such wise as to secure for it a a slight ascent. Thus, if the lower sash is raised and a tight-fitting strip of board is placed under it, the only inlet will be near the middle of the window, between the lower and the upper sash. The upper part of the lower sash serves to give to the air as it enters and gains a little heat, a slight upward motion. The direct current of the air is intercepted. It is also true that if a wire gauze is put in under a sash, or at the upper part, it cuts the air so as to diminish draught.

Another plan is so to introduce the fresh air from outside as that it shall become heated in the room, but before diffusion through it. Thus the draught is intercepted, and at the same time a proper temperature is imparted. One plan for doing this is illustrated in the Galton grate, where cold air from without is let in around the rear of the grate, and, being warmed, becomes diffused from the sides of the grate into the room. Another plan is, where a stove is surrounded by a metal case, or "jacketed," so that cold air is let in around the stove and, being heated, ascends. above the jacket and is diffused in the room. School Circular XXVIII. (No. 2), page 10, illustrates this method. In each of these cases the air for the draught of the stove or grate is derived from the rooms, and so, to a degree, ventilates the room, while the air that moves about the stove, or grate, inside of the jacket, and so ascends and is diffused through the room, is fresh outside air.

For heating the air already within the room directly, we have the fire-place, the stove, steam or hot-air pipes and radiators. Of these, the fire-place and stove have directly to do with ventilation, as well as heating. We therefore speak of these here, leaving the others for after consideration.

THE FIRE-PLACE.

What even an ordinary stove will do thus to ventilate a room, in the process of heating, is well stated by Prof. Curtman, M.D., of St. Louis:

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