carbon monoxid and dioxid, marsh gas, hydrogen sulphid, gaseous organic substances-such as amins, ammonia, and volatile fatty acids. The solid impurities in air are various forms of dust, inorganic and organic; the débris of vegetable and animal organisms, and living micro-organisms.

Sources of the Impurities.—Impurities due to Respiration. The changes that take place in air that has been respired are both chemical and physical. (1) The volume of the expired air is from one-fiftieth to one-fortieth less than that taken in at the corresponding inspiration. (2) The temperature is raised, as a rule, to that of the blood. (3) Owing to this rise in the temperature there is actually an increase in volume of the expired over the inspired air to an extent of about one-ninth of the latter. (4) There is an increase in the amount of carbon dioxid to between 4 and 5 per cent. (5 and 6) There is an increase of ammonia and watery vapor. (7) The nitrogen is generally increased, but sometimes diminished. (8) The oxygen is diminished to about 16 per cent. There is an addition to the air of (9) hydrogen, (10) marsh gas, and (11) organic matter. Of these alterations, the most important are the increase in the amount of carbon dioxid and the corresponding decrease in the amount of oxygen, the increase of the aqueous vapor to saturation, and the addition of organic matter.

Carbon Dioxid.-The amount of carbon dioxid excreted has been variously estimated at from 31.5 to 37.5 grams per hour. According to Pettenkofer and Voit, the total amount excreted in twenty-four hours for male adults is, on an average, 800 grams, or 406 liters; and according to Vierordt, 900 grams, or 455.5 liters. Assuming the tidal air of each respiration to measure 500 cubic centimeters, and to contain 4 per cent. of carbon dioxid, and that 17 respirations are taken every minute, then the carbon dioxid excreted in one hour is 500 X 0.04 X 17 X 60 = 20.4 liters, or 489.6 liters in twenty-four

hours.

Aqueous Vapor.-The expired air is nearly always sat

urated with aqueous vapor. The absolute amount varies with the temperature of the expired air; but this itself varies very slightly, being nearly that of the blood, ranging from 33.8° to 36.1° C. According to Vierordt, 330 grams of water are given off from the lungs in twentyfour hours; and according to Valentine, 640 grams. Pettenkofer and Voit state that, with the temperature of the atmosphere at 15° C. and the relative humidity at 75 per cent., an adult gives off 286 grams of water from the lungs in twenty-four hours.

Organic Matter.-That organic matter is present in the expired air is obvious from its odor, which is often quite fetid, and from the fact that when it is collected by the condensation of the aqueous vapor it is putrescible. It has been supposed by some that this organic matter is derived from the alimentary canal and from the upper portion of the respiratory tract, but it has apparently been found in air drawn directly from the trachea. The greater portion of it, however, arises from decomposing particles of food lodged between the teeth, and only a small portion of it comes directly from the lungs. A small portion of it is also derived from the mucous membrane of the pharynx and larynx and, probably, from the stomach.

The nature of the organic matter is not known with certainty. It decolorizes solutions of potassium permanganate, and is, therefore, capable of being oxidized. When distilled it is broken up and yields ammonia, and is, therefore, nitrogenous. It is molecular rather than gaseous, and is probably in combination with water, because those substances which are most hygroscopic absorb most organic matter. The quantity of organic matter given off with the expired air has been estimated. Cornelly, Haldane, and Anderson found, in ten observations, that the amount of oxygen required to oxidize the organic matter varied from 1.7 to 13.6 milligrams per liter of condensed vapor, giving an average of 7.6 and 8.3 milligrams for two observers, respectively. Lehmann and Jessen found that between 3 and 4 milligrams of

oxygen were required to oxidize the organic matter in a liter of condensed vapor. Ransome's results indicate the exhalation of 20 milligrams of organic matter in twentyfour hours, and Beu's results, 15 milligrams. In my own experiments' I found the quantity of organic matter in the expired air of healthy men to be, on an average, 10.72 milligrams per liter of condensed vapor. I found also that the amount of organic matter is much greater three to four hours after a meal than immediately after eating, and, likewise, that the amount is directly dependent upon the degree of cleanliness of the mouth and teeth of the person from whom the vapor is collected; the average amount four hours after meals was 11.98 milligrams, half an hour after meals it was only 3.86 milligrams when the mouth and teeth had remained uncleaned for several days, while the average, four hours after meals, was only 2.49 milligrams when the teeth had first been brushed and the mouth thoroughly rinsed with warm water. The amount of organic matter is also greater in those having decayed teeth than in those having sound teeth. In vapor condensed from the breath of a man having an opening directly into the trachea, and in whom there was no communication between the trachea and pharynx, I found the average for three observations to be 9.68 milligrams. In a consumptive person I found the average amount to be 17.34 milligrams. I found the average amount of nitrogenous organic matter given off with the expired air to be 17.5 milligrams of free, and 9.0 milligrams of albuminoid, ammonia per liter of condensed vapor, for healthy persons; and 0.3 milligram of free, and 3.4 milligrams of albuminoid, ammonia in that of consumptive persons.

Bacteria in Expired Air.-No bacteria are given off with the expired air in ordinary, quiet respiration. In the forcible expirations during speaking, coughing, or sneezing it has been found that small particles of mucus and moisture are thrown off which carry microorganisms. In this manner a person suffering from the 1 Smithsonian Contributions to Knowledge, 989, Washington, D. C., 1895.

1

various infectious diseases may infect the atmosphere and the furniture of the room in which he lives. It must be remembered, however, that only in several diseases which are localized in the respiratory tract would there be danger of the dissemination of the specific disease-producing bacteria. This mode of dissemination may take place in tuberculosis of the respiratory tract, in diphtheria, and in pneumonia. In those diseases in which the specific bacteria are localized in some other part of the body there would be no danger of their dissemination through coughing, speaking, etc.

Impurities due to Perspiration.-The secretions of the skin consist of sweat proper (an acid, watery fluid containing about 1.8 per cent. of solids) and of sebaceous matter, and the quantity varies greatly according to the temperature and humidity of the air, the amount of exertion, etc., but may be taken as ranging from 800 to 1000 cubic centimeters during twenty-four hours. Epithelial cells are constantly disengaged from the skin. Considerable amounts of carbon dioxid are also given. off through the skin.

Impurities due to Combustion.-The principal impurities due to combustion are carbon, carbon monoxid, carbon dioxid, sulphur, sulphur dioxid, sulphuric acid, sometimes hydrogen sulphid, ammonia compounds, and

water.

The impurities arising from illuminating-gas are olefiant gas and other hydrocarbon vapors, hydrogen, carbon monoxid, and marsh gas. Besides these normal constituents of the gas we frequently find present carbon dioxid, hydrogen sulphid, and other sulphur compounds.

Impurities in the Air of Work-rooms and Factories.The air of work-rooms and factories contains the impurities arising from the respiration and perspiration of the occupants, and, in many instances, also the products of combustion arising from the process of heating and lighting. Such rooms are often overcrowded and overheated, and in consequence these impurities are

present in excessive amounts, while from lack of personal cleanliness there is a greater proportion of the organic impurities arising from perspiration.

The special impurities of the air resulting from the manufacturing processes present one or the other of the following conditions: (a) Increase of temperature, as in mines or bake-houses, the "gassing" gassing" rooms of silk mills, and the "sizing" sheds of cotton mills. (b) Excessive humidity, as in some deep mines and in the "sizing" sheds of cotton mills. (c) The presence of deleterious gases; in mines carbon monoxid and dioxid, carburetted hydrogen and hydrogen sulphid may be present; in bleaching works sulphur dioxid is evolved, and also in copper works, though in the latter it at once passes into the outer air. (d) Vapors of hydrochloric, sulphuric, and nitric acids, and of chlorin are given off in certain processes of the manufacture of steel. (e) Carbon disulphid is present in the air of vulcanized India-rubber works. The fumes of phosphorus in match-making, and (g) the fumes of zinc in brass founding; (h) arsenical fumes in copper-smelting, the preparation of wall-paper, the manufacture of artificial flowers, and in the preparation of skins for mounting; and (i) mercurial vapors are given off in bronzing and gilding, and in the manufacture of artificial flowers.

Impurities in the Air of Dwellings.-The impurities in the air of houses are those arising from respiration, perspiration, and combustion, and, in addition, arsenical vapors may be present when the walls are covered with paper containing arsenic. In cities the air of houses may also contain impurities arising from sewers or from cesspools.

Dust in the Air.-The most injurious constituent of the air in certain manufactories and in the air of the streets of cities is dust. Mineral dust is given off in certain manufacturing processes and in mines. This is especially injurious in establishments where cutlery and files are made, also in gun factories. Dust of vegetable