these plants, and where they cause the formation of the so-called root-tubercles. Indirectly nitrogen is also stored in the soil by certain soil bacteria and in turn vegetation. obtains this nitrogen as food in the form of nitrates and ammonia. Argon, neon, erythron, and krypton are likewise inert gases so far as known at the present time.

The proportion of oxygen in the air varies within somewhat wider limits than that of nitrogen, but, under natural conditions, it is fairly constant, because any slight decrease in its proportion in circumscribed localities is readily corrected through the action of the principle of diffusion.

The proportion of carbon dioxid varies usually between 0.03 and 0.05 part per 100, but it is subject to still greater fluctuations at different points on the earth's surface, and at different seasons of the year. The fluctuations in the proportion of carbon dioxid are brought about by the active decomposition and putrefaction of organic matters through the agency of bacteria, and through the combustion of combustible materials in manufacturing centres. The overproduction of carbon dioxid at any point tends to become equalized through the operation of the principle of diffusion and the movements of the atmosphere. The proportion of carbon dioxid is, generally, greatest at the surface of the earth, and decreases as the elevation increases. It is least in winter and greatest in autumn; less during the day than at night; less on the seacoast than inland; and less on windy days than on calm days. It is decidedly diminished by rain, slightly so by snow, and slightly increased during foggy weather.

The relative proportions of oxygen and carbon dioxid are maintained through the combined action of the vegetable and animal world. The animal kingdom absorbs oxygen and gives off carbon dioxid in return as the result of tissue metabolism. On the other hand, those

members of the vegetable kingdom which possess chlorophyl in their organism have the property of absorbing carbon dioxid from the air, assimilating the carbon and giving off the oxygen in return. This most

interesting cycle is an important factor in the maintenance of the relative proportions of these two gases in the air.

The proportion of aqueous vapor in air varies with the temperature-the average amount being about 1 per cent. The amount of moisture in the air may vary from less than O. I per cent. to as much as 4 per cent. The higher the temperature, the greater the amount of aqueous vapor that is taken up by air. The proportion which is most agreeable to the majority of persons, and therefore the most suitable for health, is about 75 per cent. of saturation at any given temperature.

Ozone-condensed or allotropic oxygen-is present in variable amounts in different places on the earth's surface. The average amount present is 1 milligram per 100 cubic meters of air; the maximum amount being about 3.5 milligrams. This gas is usually absent from the air of cities and the air which has passed through localities that are thickly populated. It is found in the atmosphere over fields covered with vegetation, over forests, and over the ocean. Ozone is an active oxidizing agent, and the air of cities is rich in oxidizable organic matter which absorbs it, consequently it is absent from the air of cities. Generally speaking, the healthiest parts of towns are those receiving the purer and fresher air, containing ozone, coming from cultivated fields, forests, or the ocean.

The amounts of ammonia, nitrous and nitric acids, found in ordinary atmospheric air are insufficient to have any biologic significance. They result principally from putrefaction and from various manufacturing industries.

The atmosphere forms a gaseous envelope which surrounds the earth, reaching a height of from 320 to 350 kilometers above the earth's surface, and penetrating into the porous soil, into caves and mines, and into the ocean to a great depth.

Temperature of the Air.-There are three main factors that influence the temperature of the air of any place, viz., latitude, altitude, and the relative proximity of large bodies of water. The temperature is greatest

near the equator and decreases proportionately with the distance traversed in passing from the equator to the north or south pole. The temperature is also higher at the level of the sea than on the top of a mountain in the same latitude. Places near the seacoast also have a more equable climate than those in the interior. The other factors which influence the temperature of a locality are: The conformation of the earth's surface; the nature of the soil; the character and extent of the soil-covering; and the direction of the prevailing winds. Owing to the high specific heat of water (about five times that of earth and rocks) the ocean absorbs heat slowly and gives it off slowly, and, therefore, it acts as a reservoir of the heat, absorbing it during the day and giving it off during the night, also absorbing it during the summer and giving it off during the winter, thus lessening the heat of summer as well as the cold of winter for places along the seacoast.

Pressure of the Atmosphere. The average pressure of the atmosphere varies according to the altitude of the locality, and also in the same locality at different times. At the sea-level this average pressure amounts to a little over a kilogram per square centimeter, and is sufficient to support a column of mercury 760 millimeters in height; hence the total weight supported by an average man is about 18,000 kilograms. This weight or pressure is considerable, but it is unnoticed because it is equalized by the internal pressure of our bodies, which adapt themselves to the normal fluctuations in the atmospheric pressure. Variations in the atmospheric pressure are measured by means of barometers. The mercurial barometer is usually employed in making these observations (Fig. 1). Marked deviations from the normal atmospheric pressure, such as are found in exceedingly high altitudes, in balloon ascensions to great heights, or when descending to great depths in mines, or working in tunnels, are manifested by effects. which are referable to the increased or decreased tension of the atmosphere. Rarefied air as found at great heights induces a condition known as mountain sickness or balloon sickness, and consists in increased heart action,

more rapid respiration, headache, followed by graver symptoms as the rarefaction increases, such as vomiting of food, bile, and blood, with great pain in the stomach, followed by death. There are frequently minute hemorrhages into the spinal cord as the result of inspiring rarefied air. The insufficient supply of oxygen in the rarefied air is perhaps the principal cause of the symptoms manifested.

d

a

The effects of passing from the normal atmospheric pressure to a greater pressure, as in diving-bells, in tunnels under rivers, are different from those seen on ascending to a great height, and here the effects are due to the increased pressure upon the body. Every 10 meters of water adds. the pressure of 1 atmosphere-1 kilogram per square centimeter of body surface. The increased pressure causes the sudden liberation of gases in the tissues and blood, where they interfere with the circulation and stop the heart. The difference in pressure on the tympanic cavity causes vertigo and pain in the ear, and if the difference in pressure is great the drum of the ear may be ruptured when the Eustachian tube is occluded. Ordinarily the difference in pressure is equalized by the act of swallowing. On coming out of a caisson the reverse in internal and external pressure takes place. This is also relieved in the same manner. Man can work at a kilometer below the sea-level without injury, and he can travel to a height of 5 or 6 kilometers without being affected by

FIG. 1.Mercurial barometer: a, cistern containing the mercury; b, screw in movable bottom of cistern, to raise or lower the mercury to the "fiducial" point; c, the vernier; d, the thermometer.

the decrease of pressure. When the pressure is suddenly increased or decreased beyond these points the effects are manifested. The more slowly the change is brought about, and the smaller the amount of exertion accompanying the change of pressure, the less the effect produced. Great variations from the normal atmospheric pressure are highly injurious to all persons suffering from organic disease of the heart and lungs, and to those suffering from an atheromatous condition of the arteries, because this condition prevents the arteries from readily adjusting themselves to the altered pressure, thus leading to hemorrhages. In coming out of the caisson the change in pressure must be brought about slowly. The too rapid change induces spinal hemorrhage. At least six to ten minutes should be allowed for each additional atmosphere of pressure to make the change safely. The air-locks, where the change of pressure is made, should be at the top of the shaft, and not at the bottom, so that the men are not obliged to climb the ladder when they come into the ordinary pressure.

Distribution of Atmospheric Pressure.-The barometer is high (1) when the air is very cold, for then the lower strata are denser and more contracted than when it is warm. The contraction causes the upper layers to sink down, bringing a greater number of airparticles-that is, a greater mass of air-into the column, so that the pressure at its base is greater; (2) when the air is dry, for then it is denser than when it is moist; (3) when in any way an upper current sets in toward a given area, for this compresses the strata underneath.

Conversely, the barometer is low (1) when the lower strata are heated, causing the surfaces of equal pressure to rise, and the upper layers to slide off, consequently the mass of air pressing on the area below is reduced; (2) when the air is damp, for as the density of aqueous vapor at o° C. temperature and 760 millimeters pressure is 0.7721, air being 1, the mixture is lighter the more vapor it contains, and consequently damp air does not