It must not be supposed that the tubercle bacillus is necessarily transmitted directly through the air from the lungs of the sick to be implanted in the lungs of the healthy. The germs may remain for a time in the dust turn and _débris_ of damp, filthy, and overcrowded houses. In this congenial soil they retain their vitality for a long time, and possibly may take on more virulent infective properties than they possessed when expelled from the diseased lungs.[37] Illustration: Fig. 94. Example of a Micro-Organism—Bacillus Tuberculosis in Sputum. (Magnified about 500 diameters.) 221. Ventilation. The question of a practicable and economical system of ventilation for our homes, schoolrooms, workshops, and public places presents many difficult and perplexing problems. It is perhaps due to the complex nature of the subject, that ventilation, as an ordinary condition of daily health, has been so much neglected. The matter is practically ignored in building ordinary houses. The continuous renewal of air receives little if any consideration, compared with the provision made to furnish our homes with heat, light, and water. When the windows are closed we usually depend for ventilation upon mere chance,—on the chimney, the fireplace, and the crevices of doors and windows. The proper ventilation of a house and its surroundings should form as prominent a consideration in the plans of builders and architects as do the grading of the land, the size of the rooms, and the cost of heating. The object of ventilation is twofold: First, to provide for the removal of the impure air; second, for a supply of pure air. This must include a plan to provide fresh air in such a manner that there shall be no draughts or exposure of the occupants of the rooms to undue temperature. Hence, what at first might seem an easy thing to do, is, in fact, one of the most difficult of sanitary problems. 222. Conditions of Efficient Ventilation. To secure proper ventilation certain conditions must be observed. The pure air introduced should not be far below the temperature of the room, or if so, the entering current should be introduced towards the ceiling, that it may mix with the warm air. Draughts must be avoided. If the circuit from entrance to exit is short, draughts are likely to be produced, and impure air has less chance of mixing by diffusion with the pure air. The current of air introduced should be constant, otherwise the balance may occasionally be in favor of vitiated air. If a mode of ventilation prove successful, it should not be interfered with by other means of entrance. Thus, an open door may prevent the incoming air from passing through its proper channels. It is desirable that the inlet be so arranged that it can be diminished in size or closed altogether. For instance, when the outer air is very cold, or the wind blows directly into the inlet, the amount of cold air entering it may lower the temperature of the room to an undesirable degree.