A learned physician also thus wrote to the author of this chapter: "The subject of the ventilation of our dwelling-houses is one of the most important questions of our times. How many thousands are victims to a slow suicide and murder, the chief instrument of which is want of ventilation! How few are aware of the fact that every person, every day, vitiates thirty-three hogsheads of the air, and that each inspiration takes one fifth of the oxygen, and returns as much carbonic acid, from every pair of lungs in a room! How few understand that after air has received ten per cent of this fatal gas, if drawn into the lungs, it can no longer take carbonic acid from the capillaries! No wonder there is so much impaired nervous and muscular energy, so much scrofula, tubercles, catarrhs, dyspepsia, and typhoid diseases. I hope you can do much to remedy the poisonous air of thousands and thousands of stove-heated rooms." In a cold climate and wintry weather, the grand impediment to ventilating rooms by opening doors or windows is the dangerous currents thus produced, which are so injurious to the delicate ones that for their sake it can not be done. Then, also, as a matter of economy, the poor can not afford to practice a method which carries off the heat generated by their stinted store of fuel. Even in a warm season and climate, there are frequent periods when the air without is damp and chilly, and yet at nearly the same temperature as that in the house. At such times, the opening of windows often has little effect in emptying a room of vitiated air. The ventilating-flues, such as are used in mines, have, in such cases, but little influence; for it is only when outside air is colder that a current can be produced within by this method. The most successful mode of ventilating a house is by creating a current of warm air in a flue, into which an opening is made at both the top and the bottom of a room, while a similar opening for outside air is made at the opposite side of the room. This is the mode employed in chemical laboratories for removing smells and injurious gases. The laboratory-closet is closed with glazed doors, and has an opening to receive pure air through a conductor from without. The stove or furnace within has a pipe which joins a larger cast-iron chimney-pipe, which is warmed by the smoke it receives from this and other fires. This cast-iron pipe is surrounded by a brick flue, through which air passes from below to be warmed by the pipe, and thus an upward current of warm air is created. Openings are then made at the top and bottom of the laboratory-closet into the warm-air flue, and the gases and smells are pressed by the colder air into this flue, and are carried off in the current of warm air.