# Page:Popular Science Monthly Volume 19.djvu/549

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 ${\displaystyle =}$ ·00366; of CO2 ${\displaystyle =}$ ·00371.) Now, the law which governs the mixtures of gases is this: