to a close atmosphere. It is generally agreed that this condition is reached when the proportion of carbonic acid approaches one thousandth. Observation shows, in fact, that the proportion of carbonic acid increases in the same degree as the insalubrity of the air, and may, up to a certain point, afford a measure of it; but the inconvenience we suffer from bad air is in reality attributable rather to the putrescible organic products of respiration and transpiration which it contains. According to Péclet, the air driven out from the ventilating chimneys of crowded rooms exhales an odor so noxious that it can not be borne with safety, even for a short time. According to some chemists, the disagreeable odor that characterizes close air is due to a particular substance possessing an alkaline reaction and the property of giving off ammonia, which escapes from the lungs. The real culprits are these miasms which affect the smell. The carbonic acid, which is comparatively an inoffensive gas, only indicates the change the air has undergone. The experiments of MM. Regnault and Reizet go to show that an animal can live in an atmosphere containing seven hundredths of carbonic acid, provided the proportion of oxygen is maintained at twenty-one hundredths. Animals have been observed to perish in a tight inclosure even when the carbonic acid is eliminated as fast as it is formed, and the lost oxygen is restored; and Mantegazza has shown that if two birds are placed under two different bell-glasses, and the carbonic acid formed by one is absorbed by quicklime, and the organic matter exhaled by the other is taken up by animal charcoal, the latter bird will survive considerably longer than the former. We add that Dr. Pettenkofer has been able to breathe for several hours, without inconvenience, air containing one hundredth of carbonic acid developed, not by respiration, but by a chemical process. These facts indicate that the few thousandths of carbonic acid diffused in it are not the cause of the effects produced by an atmosphere vitiated by respiration. The oxygen content diminishes in nearly the same proportion as carbonic acid is developed; but the effects produced by "close air" can not be explained by the deficiency—say of one per cent—of oxygen; that may be remedied in part by more active breathing.
Carbonic acid has sometimes been wrongfully charged with effects which were really due to a small proportion of carbonic oxide, a product of imperfect combustion and of the reduction of carbonic acid. Carbonic oxide is a deadly poison, and destroys the red globules of the blood. To its disengagement may be attributed the unhealthy effects
- According to M, de Chaumont's observations in English barracks, the odor begins to be perceptible when the proportion reaches 0·0008; and this hygienist is inclined to reduce the admissible proportion to 0·0006; but I believe it sufficient to adopt one thousandth as a limit which we shall be fortunate if we never exceed in practice.
- It blackens sulphuric acid, discolors permanganate of potash, and communicates to water in solution a fetid odor (A. Proust, "Traité d'Hygiène").