We do know, however, to a certainty that air shut up in a room, or house, with people, rapidly becomes poisonous and unwholesome. As we breathe on an average about eighteen or twenty times to the minute when we are grown up, and twenty-five to thirty times a minute when we are children, you can readily see how quickly the air in an ordinary-sized room will be used up, and how foul and unfit for further breathing it will become from being loaded with these bad-smelling lighter gases, with the carbon "smoke," with heat, and with moisture. The only way in which a room can be kept fit for human beings to breathe in is to have a draught, or current of air, pouring into it through open windows, or open doors, or ventilating shafts, at least as rapidly as it is being breathed by the persons who occupy that room. By hundreds of tests this has now been found to be on an average about four bushels a minute for each person, and any system of proper ventilation must supply this amount of air in order to make a room fit to sit in. If a man, for instance, accidentally gets shut into a bank-vault, or other air-tight box or chamber, it will be only a few minutes before he begins to feel suffocated; and in a few hours he will be dead, unless some one opens the door. A century ago, when the voyage from Europe to America was made in sailing vessels, whenever a violent storm came up, in the smaller and poorer ships the hatches were closed and nailed down to keep the great waves which swept over the decks from pouring down the cabin-stairs and swamping the ship. If they were kept closed for more than two days, it was no uncommon thing to find two or three children or invalids among the unfortunate emigrants dead of slow suffocation; and many of those who were alive would later have pneumonia and other inflammations of the lungs. On one or two horrible occasions, when the crew had had a hard fight to save the ship and were afraid to open the hatches even for a moment, nearly one-third of the passengers were found dead when the storm subsided. So it is well to remember that we are fearfully poisonous to ourselves, unless we give nature full chance to ventilate us. There are also other ways in which the air in houses may be made impure besides by our own bodies, but none of them is half so serious or important. All the lights that we burn in a house, except electric ones, are eating up oxygen and giving off carbon dioxid. In fact, a burning gas jet will do almost as much toward fouling the air of a room as a grown man or woman, and should be counted as a person when arranging for ventilation. If gas pipes should leak, so that the gas escapes into a room, it is very injurious and unwholesome--indeed, in sufficient amounts, it will suffocate. Or, if the sewer pipes in the walls of the house, or in the ground under the cellar, are not properly trapped and guarded, _sewer gas_ may escape into the house from them, and this also is most unwholesome, and even dangerous.