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Page:Popular Science Monthly Volume 19.djvu/549

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SCHOOL-ROOM VENTILATION.

bonic oxide passes up the chimney along with the other gases of combustion. As the products of combustion are much lighter than the surrounding atmosphere—volume for volume—on account of their much higher temperature, and as the expansibility of gases is very great, they exert a pressure upon the sides of the pipe or flue through which they ascend. This being the case, these gases will escape through chinks, holes, or defective joints, along their course, like steam through a leaky conduit. Downward air-currents in the flue, and lateral currents from open windows, etc., occasionally blow large quantities of the gases of combustion through the open door of the stove, or through seams or cracks therein; and in these two ways—through stove and flue—sulphur, carbonic oxide, and carbonic-acid gas, may find their way into the room. It is claimed by some physicists that carbonic oxide will make its way through heated iron, and thus escape through the sides of the stove, but the quantity given out in this way—if, indeed, any is so given out, of which there is a reasonable doubt—must be so small that it is practically of no account, while quantities large enough to be decidedly injurious may issue through the door and other openings. Of course, these remarks apply only to schools heated by stoves; but it must not be forgotten that in rural districts, and many cities, all the schools are still heated in this way.

Carbonic oxide is a deadly poison, fixing itself in the blood-corpuscles and paralyzing them so that they can not carry on the function of respiration. To the inhalation of this gas is chiefly due the pale color of those who spend much time in apartments heated by stoves and poorly ventilated. Its presence can not be recognized by the senses, as it is tasteless, colorless, and inodorous.

Carbonic acid is produced in two ways, as before explained—by combustion and by breathing. The quantity thrown off in breathing is very much increased—often nearly doubled—during active digestion. As the fullest meal is taken at dinner, and digestion is most active soon after, it follows that the exhalation of carbonic-acid gas is greatest during the early part of the afternoon, and therefore during this time ventilation needs more attention. Of all the impurities found in the school-room, this is vastly the largest in amount, and popularly considered the most important. It is once and a half as heavy as air. At first sight, it might be supposed that, being heavier than air, it would sink to the floor and settle there in a layer of uniform height and density, like so much water. But this is not the case, for it is even more expansible than air. (Coefficient of expansion of air ·00366; of CO2 ·00371.) Now, the law which governs the mixtures of gases is this:

The mixture of gases in free communication, whatever their density, takes place rapidly, and is homogeneous—that is, the mixture contains the gases in the same proportion; so that the percentage of carbonic-acid gas is about the same in all parts of the room.