=The Relative Value of Artificial and Natural Ventilation= scarcely needs to be discussed. They are both valuable, but under different circumstances. In dwelling-rooms natural ventilation by doors, windows and chimney usually suffices, especially if the products of combustion of gas are removed through a special flue. Natural ventilation is always occurring, and only needs a little aid in domestic life. For large rooms occupied by many persons artificial ventilation is necessary to maintain pure air. Whatever method of ventilation is adopted, the atmosphere will remain to some extent polluted, if the room and its occupants are dirty. In certain experiments made by Carnelley in schools, it was found that dirty children increased the number of micro-organisms per litre of air more rapidly than dirty rooms. Thus:— ┌────────────────────────┬──────┬───────┬──────┐ │DEGREE OF CLEANLINESS OF│CLEAN.│MEDIUM.│DIRTY.│ ├────────────────────────┼──────┼───────┼──────┤ │_Children_ │ 63 │ 99 │ 159 │ │_Rooms_ │ 85 │ 94 │ 139 │ └────────────────────────┴──────┴───────┴──────┘ _Number of micro-organisms per litre of air._ Hence cleanliness of rooms and of their occupants is quite as important as a good system of ventilation. CHAPTER XXIII. VENTILATION BY THE INTRODUCTION OF WARMED AIR. =Ventilation by the Burning of Coal.= In winter and at any time of the year when the out-door temperature is below 50° Fahr., the warming and ventilation of a room are necessarily combined. If air is admitted unwarmed it will produce draughts, unless directed upwards by Tobin’s tubes or otherwise. In dwelling-rooms such contrivances may suffice; but in any larger building, in order to ensure sufficient ventilation, it is necessary to warm the incoming air. =The Open Fire-place= forms the most common means of ventilation by heat (see also page 159). The ascent of warm air up the chimney, causes cold air to rush along the floor to the fire-place from all parts of the room, especially the door. Part of the air thus approaching the fire is carried up the chimney with the smoke, while the remainder, after having been warmed, flows upwards towards the ceiling near the chimney-breast. It passes along the ceiling, and cooling in its progress towards the opposite wall, descends, and is again drawn towards the fire-place. Thus there is a continuous circulation of the air in a room. In the experiments of the Barrack Commissioners (1861), it was found that the amount of air passing up the chimney while a fire was lit, ranged from 5,300 to 16,000 cubic feet per hour, the mean of 25 experiments being 9,904 cubic feet. We may conclude, then, that with an ordinary grate, a chimney provides outlet for impure air sufficient for four or five persons. Its lack of economy as a heat-producer will be considered later. Its efficiency as a ventilator within the above limits is evident.