Water vapor is another of the four im portant atmospheres. It may vary in amount from a fraction of 1 per cent for the arid re gions to 5 per cent of the weight of the air for the warm humid regions. It is a little over one half as dense as atmospheric air. When the air has taken up all the water vapor that it can contain at a given temperature it is said to be saturated. Saturation seldom occurs in free air, except inside of a dense fog or cloud, or within the thin stratum of air that by contact with a cold surface has had its temperature low ered to the point of condensation. Even during heavy downpours of rain the air near the earth is usually capable of evaporating some of the rain that is falling through it. In fact light precipitation may be entirely taken up by the lower air and never reach the earth. The dew point is the temperature of saturation. The relative humidity is expressed in percentages of the amount necessary to saturate. At a tern perature of 32° F., air resting over a moist sur face may continue to increase the amount of its vapor of water until it contains 2.11 grains per cubic foot, which amount is sufficient to ex ert a pressure in all directions equal to the downward pressure of 0.18 inch of mercury; it will then be saturated and its relative humid ity be 100 per cent. The point to which atten tion is especially directed is that the pressure of 0.18 inch indicates the maximum pressure of water vapor at 32° temperature, and any fur ther evaporation must be accompanied by an equal amount of precipitation. Now, if this air be suddenly raised in temperature to 51° its capacity per cubic foot will be increased to twice what it was at 32°, the 2.11 grains will only be equal to one-half the number necessary to saturate and the relative humidity be ex pressed by 50 per cent instead of 100 per cent.
The absolute humidity is expressed in grain per cubic foot, or by the pressure in inches : mercury. The hygrometer measures the wax of Water vapor.
Ozone has powerful bleaching and isle fecting properties. By reason of the condition of the molecular structure it is much more active oxidizing agent than onus and this fact in part accounts for the amount observed in the air near the grafi and for the almost total absence of ozone inz the air over large cities, where decaying orra.7 matter existsat all times in comparatively- lam quantities, the ozone rapidly entering tr. chemical union with whatever is in the preen of decay. Ozone in the minute found in nature is healthful, but the qvar..7 is not large that will cause the death 3f ar animal confined in the air containing it I may be that the invigorating effects of tie crisp air of a frosty morning, and of the 17 air of the cold waves of winter, are dot part at least, to the ozone and electricity in air; and may not the healthfulness of aim tain air be due to the increase with elevatioe the quantity of ozone and electricity, as n.-2 as to .the less quantity of dust and &serf germs? Sea air also is rich in ozone and ('!.
ficient in dust and noxious germs. The mar mum daily amount greets the early riser. ozone is present in greatest quantity betwen 4 and 9 A.M., and in winter the amount greater than in summer.
Dust in the Air.— Dust rains into the phere from outer space, and meteors that r consumed through the heat generated by stri ing into our air contribute to the supply. sources are volcanoes, combustion, salts it ...! the spray of the ocean and small particles .• matter lifted up by the winds.
A recent research by Humphrey shows each of the important volcanic eruptions sir 1750 have effected a coaling of the earth's ten peratures through the action of dust shot E'r the upper air and wafted about the earth is shown that the ratio subsisting between if size of the dust motes and the wave lengths solar and terrestrial radiations is such as reflect back into space more of in-coming be; than 'there is intercepted out-going heat fre the earth. Each important eruption has fected the temperature of the earth and color of the sky for from one to three sea' Heat of Interstellar Space.—Although ether transmits• through all space the varine forms of solar energy, none of this energt. comes thermal until it is intercepted by it atmosphere of our earth or by the rase= envelope of some other planet, or by the hie of some meteor or comet or by cosmic ds Objects or planets without atmospheres lk our moon are, therefore, nearly devoid of tre perature, approaching absolute zero, theoretically, is —450° F.
Earth's Heat.— It is believed that the it terior of the earth still retains an intensely temperature, and that this hot mass is rounded by a cool crust that is a poor ere& tor; so that only a small amount escape t the atmosphere. The innumerable van though their average temperature probable equal to that of our sun, are too distant have an appreciable effect in heating the e terior of the earth. It is apparent, thereat that the sun, with an absolute temperature over 20 times as high as the absolute tempera ture of the earth, controls the surface tempera ture of our planet and its atmosphere.
Sun's Heat Variable.—The number of gram calories received per minute on a square centi meter of normal surface outside the earth's atmosphere is called the solar constant. In Abbot's refined measurements at Mount Whit ney he obtained values of the solar constant varying from 1.93 to 2.14, the mean for the first twoyears being 2.022.
Variable Quantity of Heat Received Each Day.— The quantity of heat that falls upon a horizontal area at the top of the earth's atmos phere during any consecutive 24 hours depends upon four conditions: (1) The altitude that the sun attains when it crosses the meridian at noon, (2) the length of the day time, (3) the distance of the earth from the sun, and (4) constancy of solar radiation; these are in a perpetual state of variation, except that near the equator the day and the night are always equal.