bodies receives contamination from those bodies, as it also generally does when it comes in contact with the works of man; and the sewage system and the gaslight system of our time do a great deal to render the atmosphere foul.

But this applies only to the small portion of the atmosphere close to us, and when the air is in motion, other portions, which come from a distance, and are therefore not thus tainted, are supplied; whilst the offensive matters become less and less hurtful or disagreeable as they mix with the larger volume of purer air, until they are no longer perceptible or productive of evil,

Movement of the air is of two kinds-one always quiet, made by the interchange of gases mixing those of different kinds together, which if enclosed become one composition. This natural movement is illustrated by the fact, that a whiff of tobacco-smoke, or the scent of a drop of eau-de-cologne, may be seen, or smelt, for a moment in a certain spot only; but in a short time, as we know, it extends, and impregnates the whole air within certain limits. This quiet movement is constant, and not visible, but is of the utmost value in ventilation.

Besides, it acts in all directions, and goes on wherever there is matter to be distributed, or space in which to distribute it.

Without this influence, the exposure of the body to the many foul and poisonous agents contained in the air immediately around us would render life miserable, and at length destroy it.

People who sleep in bedsteads having what are called "canopies," would, without this movement, soon be killed by an accumulation of poisonous gases.

The second kind of movement of the air in a room is due to influences acting from without, and not arising from any action in the air. The great external influ

ence is the wind, which penetrates wherever the outer air can gain access; but the influence of wind cannot always be the same-it must vary with its force, and the direction from which it proceeds. For instance, an east wind would bring a colder air in this country than a west wind would bring; and the current of wind is of course arrested, and interfered with, by obstacles, such as walls, or trees, or mountains.

Every extensive system of ventilating buildings must be based upon the action of the air; but this action must be controlled and regulated. This directing of the supply of external air, so as to benefit to the greatest possible extent the interiors of buildings, gives much trouble to those who endeavour to carry out some really practicable system of successful ventilation,

The third important cause of motion in the air is heat.

You have often seen a Thermometer. This is a heatmeasurer; it shows on its face, by the position of the mercury in its tube, the different degrees of heat and cold in the atmosphere immediately surrounding the instrument. When the mercury in the thermometer, which we generally use in England, shows 32 degrees (marked 32°), we know the air to be of that degree of coldness at which water will freeze.

Every degree of increase of warmth in the atmosphere will be shown on the thermometer by the position of the mercury; and for each degree of warmth above 32 degrees, air increases its size th part; so that if the air in a room could be heated to 512 degrees, that is 480 degrees above freezing-point (32°), the quantity would be doubled in size. You can easily understand that if a certain space is quite filled with air to begin with, the same space cannot hold twice the quantity of air, and some means of escape must be found, or the room would burst.

No room ought to be, or can possibly be, really air

tight, and some of the heated air will escape through crevices of the door and windows, and up the chimney, even if the windows are shut. You see, therefore, that heat, instead of hindering, really promotes ventilation, because it produces movement in the air.

But movement of the air by means of heat may be carried too far; for if the air is heated above the degree of the heat which is thrown out of the body, there consequently is no power in that air to cool the body when it comes into contact with it. The body must throw out its vapours, and the overheated air holds the moisture about the body, causing it to condense on the skin in the form of perspiration, and rendering the persons in the room liable to catch cold on the first change of temperature.-Adapted from Journal of the Society of Arts, Paper 849.

CHAPTER XLVIII.

VENTILATION.-No. 2.

THE general method of supplying ventilation is to make use of doors, windows, and fireplaces; and upon the opening and shutting of these, the ventilation of rooms in most cases entirely depends.

In small houses, there may be a chance of these being opened or shut in a proper way to secure ventilation enough for the space; but in large buildings, there is seldom any person who has sufficient time at his disposal, or sufficient memory, or sufficient interest in the subject, to give proper attention to this.

In all cases, it is better so to provide for the ventilation of buildings, as to leave nothing dependent upon the will or the care of any person. In many sittingrooms, when a fire is not wanted, the opening into the chimney is kept closed by what is called a "register," and in bedrooms by a fireboard. In cold weather, too,

people carefully shut up all windows and doors, especially at night, when more external air is really required, in consequence of the burning of a lamp or gas in the room. Happily, few windows or doors fit perfectly, and so some outside air can find its way in, in spite of the care taken by the inmates to keep it out.

Ventilating rooms by the windows has been tried in various ways; and of all window ventilation, perhaps the best is, that by windows opening at the top and bottom. The idea involved in this is, that the heated air will rise and escape at the upper opening, and a fresh supply of cold air will at the same time rush in by the lower opening.

Some people believe so entirely in this principle, that they leave the upper sash a little drawn down at night, under the impression that hot or impure air will go out, and no cold air will come in. This is a mistake: wind will enter at any opening, be it high or low; therefore, if the opening be not so far removed from the bed, or sufficiently high above the heads of the inmates, that the current of cold air may pass over them, this means of ventilation is absolutely injurious.

This should, therefore, not be the only means of ventilating low rooms, that is, rooms less than ten feet high. Some windows are made so that the upper portion may fall inwards, with a view to direct the current of air upwards, and it is held in that position by a cord, or by falling into a socket. Such a ventilator admits. too large a volume of air in one place in cold weather, and cannot be safely used unless the room be of great height at least fourteen feet; but even then it causes discomfort, since the current of cold air at last makes its way downwards, and the room becomes cold, unless more means of warmth are provided than can be obtained by open fireplaces.

This plan is in operation in the bedrooms of some houses in windows within three feet of the head of the

sleeper, but in such places the natural remedy is ap plied, namely, shutting them, and thus no pretence at ventilation is kept up.

Other windows have their lower half to fall forward, and the evils just mentioned are increased; a more modern, but scarcely better, invention divides the window into four or more parts, each or all of which may be opened at pleasure; this contrivance fails in effecting proper ventilation, for the same reasons which make the other two ineffectual.

Some of the injurious effects of the above plan have been remedied by putting pieces of perforated zinc in some of the highest panes in the window, and in windows of which the upper sash is lowered, a strip of perforated zinc is fastened across the top of the frame. Both these plans may be used with good effect.

Ventilators not connected with windows are found in great variety. Some are placed in the ceiling, and communicate with the room by openings of various forms and sizes, and with the outer air by external gratings with or without special channels, or simply by accidental openings in the roof. The principle upon which they are used is, that the foul air ascends, and should have the means of leaving the room at the top, whilst fresh air enters at the bottom. The term "foul air" is often only another expression for heated air. If the heat be produced within the room, whether by gas or living beings, the air will be both heated and foul; but if it be produced by fire in a proper stove or grate, or by hot-water pipes, the air may be heated, but it is not foul.

But supposing the air to be both heated and tainted, the foul air will not, as a matter of course, ascend to the top of the room.

Certain hurtful matters-among which is carbonic acid gas the principal acid gas exhaled from the human body-are heavier than air, and, with a still