The ventilation of dwelling-houses, although as important as that of churches and other buildings in which large numbers of people congregate, has, however, been almost entirely overlooked until lately, or has been allowed to take quite a subordinate place in the architect's plans. Gas illumination having in a great measure superseded that of candles and lamps, it becomes a more urgent necessity to provide some means for carrying off the products of combustion. These products are more complex and more dangerous for respiration than those from candles and lamps; yet modern houses, although generally furnished with gas-fittings, are built without any provision for a supply of fresh air, or for the carrying off of the foul air. It is an established physiological law that no room should afford less than a thousand cubic feet of air for each person in health, under the ordinary conditions of inlet and outlet; for the sickroom not less than one thousand cubic feet should be allowed to each patient. This quantity of space is the least that is allowed for patients in the large hospitals recently erected.

There is one simple principle that should guide all contrivances for ventilation purposes - viz., to secure a constant change of the atmosphere.

Thorough change of the air of a chamber involves the admission of pure air on the one side and the removal of the vitiated air on the other.

In ordinary rooms, this is imperfectly effected by the fireplace, and by the imperfect closure of door and window frames. It is, perhaps, a source of some consolation that the very imperfection of modern domestic architecture offers some degree of immunity from the consequences of breathing air that is vitiated, either by the lungs of human beings, or by the combustion of gas. We have heard modern houses, in this respect, described as patent self-acting ventilators.

This haphazard mode of ventilation is unscientific and insufficient, being effected only by draughts of air which cannot always be borne without risk to the health. To secure the thorough change of air that has been spoken of, a good ventilator should be placed in the chimney, or some part of the wall communicating with the outer air if there be no chimney. As nearly as possible opposite to the ventilator a flow of pure air should be secured at the top of the room as well as near the floor. If only admitted in the latter position, the heated impure air will collect at the top of the room under the ceiling, and gradually intermingle with the air below. By a little skilful management of the cornice, or other ornamentation along the top of the wall of the room, concealed perforations may be made that shall admit a flow of pure air which will mingle with that which may also be admitted in like manner by concealed perforations in or behind the skirting-board.

If there be a fireplace in the room, a very free admixture of these two air-supplies will take place before they reach the fireplace, which they will surely do by virtue of the suction power of the heated air in the chimney.

It may here be stated that in the summer time, when fires are not being used, there is a draught up the chimney, as may be seen by holding a lighted taper at its mouth. A downward current may in the winter time be obtained by the same means, from a chimney in which there is no fire.

We have said that a good ventilator is an essential part of the system for complete ventilation of a room. A ventilator to be perfect should be self-acting with the current of air. Arnott's are hung on an erroneous plan, the reverse of what they should be. Being fixed on a hinge at the lower border, the valve requires a counterbalancing weight to make it swing with the current of air, whereas if it be suspended from its upper edge at a proper angle, and be made of some light material, as talc, it will open with the slightest upward flow, and be effectually closed by the reverse movement, the downward current of the air. The valve by this arrangement prevents the back escape of smoke into the room, and secures the exit of the vitiated air. This form of ventilator will act equally well with or without the presence of a fire in the grate. They are also made so as to serve for the ventilation of halls and other public buildings.

A great mistake is often made in warming rooms by gas burners. More particularly is this objectionable in a sick room, where pure air is required more than anywhere. If any one will take the trouble to notice, he will find that, on going from the fresh air into a room in which gas is burning (without sufficient ventilation, or a flue to carry off the vapour), there will be distinctly perceptible a more or less pungent smell of sulphurous and other vapours. Where therefore gas is used for the purpose of warming, the warmth is obtained at the cost of the purity of the air. An open fireplace is not only a source of warmth, but it is also a means of ventilation and purification of the air.