Thus the strong and healthy husband, feeling the want of pure air in the night, and knowing its importance, keeps windows open, and makes such draughts that the wife, who lives all day in a close room and thus is low in vitality, can not bear the change, has colds, and sometimes perishes a victim to wrong modes of ventilation.

So, even in health-establishments, the patients will pass most of their days and nights in badly-ventilated rooms. But at times the physician, or some earnest patient, insists on a mode of ventilation that brings more evil than good to the delicate inmates.

The grand art of ventilating houses is by some method that will empty rooms of the vitiated air and bring in a supply of pure air by small and imperceptible currents.

But this important duty of a Christian woman is one that demands more science, care, and attention than almost any other; and yet, to prepare her for this duty has never been any part of female education. Young women are taught to draw mathematical diagrams and to solve astronomical problems; but few, if any, of them are taught to solve the problem of a house constructed to secure pure and moist air by day and night for all its inmates by safe methods.

We have seen the process through which the air is rendered unhealthful by close rooms and want of ventilation. Every person inspires air about twenty times each minute, using half a pint each time. At this rate, every pair of lungs vitiates one hogshead of air every hour. The membrane that lines the multitudinous air-cells of the lungs in which the capillaries are, should it be united in one sheet, would cover the floor of a room twelve feet square. Every breath brings a surface of air in contact with this extent of capillaries, by which the air inspired gives up most of its oxygen and receives carbonic acid in its stead. These facts furnish a guide for the proper ventilation of rooms. Just in proportion to the number of persons in a room or a house should be the amount of air brought in and carried out by arrangements for ventilation. But how rarely is this rule regarded in building houses or in the care of families by house-keepers!

As a guide to proportioning the air admitted and discharged to the number of persons, we have the following calculation: On an average, every adult vitiates about half a pint of air at each inspiration, and inspires twenty times a minute. This would amount to one hogshead of air vitiated every hour by every grown person. To keep the air pure, this amount should enter and be carried out every hour for each person. If, then, ten persons assemble in a dining-room, ten hogsheads of air should enter and ten be discharged each hour. By the same rule, a gathering of five hundred persons demands the entrance and discharge of five hundred hogsheads of air every hour, and a thousand persons require a thousand hogsheads of air every hour.

Therefore in calculating the size of registers and conductors, we must have reference to the number of persons who are to abide in a dwelling; while for rooms or halls intended for large gatherings a far greater allowance must be made.

The most successful arrangement for both warming and ventilation, is that employed by Lewis Leeds to ventilate the military hospitals, and also the treasury building at Washington. It is modeled strictly after the mode adopted by the Creator in warming and ventilating the earth, the home of his great earthly family. It aims to have a passage of pure air through every room, as the breezes pass over the hills, and to have a method of warming chiefly by radiation, as the earth is warmed by the sun. In addition to this, the air is to be provided with moisture, as it is supplied outdoors by exhalations from the earth and its trees and plants.

The mode of accomplishing this is by placing coils of steam, or hot-water pipes, under windows, which warm the parlor walls and furniture, partly by radiation, and partly by the air warmed on the heated surfaces of the coils. At the same time, by regulating registers, or by simply opening the lower part of the window, the pure air, guarded from immediate entrance into the room, is admitted directly upon the coils, so that it is partially warmed before it spreads through the room; and thus cold draughts are prevented. Then the vitiated air is drawn off through registers both at the top and bottom of the room, opening into a heated exhausting-flue, through which the constantly ascending current of warm air carries it off. These heated coils are often used for warming houses without any arrangement for carrying off the vitiated air, when, of course, their usefulness is gone.

The moisture may be supplied by a broad vessel placed on or close to the heated coils, giving a large surface for evaporation. When rooms are warmed chiefly by radiated heat, the air can be borne much cooler than in rooms warmed by hot-air furnaces, just as a person in the radiating sun can bear much cooler air than in the shade. A time will come when walls and floors will be contrived to radiate heat instead of absorbing it from the occupants of houses, as is generally the case at the present time, and then all can breathe pure and cool air.

We are now prepared to examine more in detail the modes of warming and ventilation employed in the dwellings planned for this work.

In doing this, it should be remembered that the aim is not to give plans of houses to suit the architectural taste or the domestic convenience of persons who intend to keep several servants, and care little whether they breathe pure or bad air, nor of persons who do not wish to educate their children to manual industry or to habits of close economy.

On the contrary, the aim is, first, to secure a house in which every room shall be perfectly ventilated both day and night, and that too without the watchful care and constant attention and intelligence needful in houses not provided with a proper and successful mode of ventilation.