Here, let it be remembered, that in ill-ventilated rooms the carbonic acid is not the only cause of disease. Experiments seem to prove that other matter thrown out of the body, through the lungs and skin, is as truly excrement and in a state of decay as that ejected from the bowels, and as poisonous to the animal system. Carbonic acid has no odor; but we are warned by the disagreeable effluvia of close sleeping-rooms of the other poison thus thrown into the air from the skin and lungs. There is one provision of nature that is little understood, which saves the lives of thousands living in unventilated houses; and that is, the passage of pure air inward and impure air outward through the pores of bricks, wood, stone, and mortar. Were such dwellings changed to tin, which is not thus porous, in less than a week thousands and tens of thousands would be in danger of perishing by suffocation. These statements give some idea of the evils to be remedied. But the most difficult point is _how_ to secure the remedy. For often the attempt to secure pure air by one class of persons brings chills, colds, and disease on another class, from mere ignorance or mismanagement. To illustrate this, it must be borne in mind that those who live in warm, close, and unventilated rooms are much more liable to take cold from exposure to draughts and cold air than those of vigorous vitality accustomed to breathe pure air. 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. The heating and management of the air we breathe is one of the most complicated problems of domestic economy, as will be farther illustrated in the succeeding chapter; and yet it is one of which, most American women are profoundly ignorant. IV. SCIENTIFIC DOMESTIC VENTILATION.