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Roscoe's Chemistry, Vol. I., pp. 434-457; Johnston's The Chemistry of Common Life; Angus Smith's The Atmosphere; Pettenkoffer's The Air in its Relation to our Clothing, Dwellings and Soil; Hartwig's The Aerial World; Parke's Hygiene; Fox's Sanitary Examination of Water, Air and Food; Kingzett's Nature's Hygiene; Becquerel's Traité Elémentaire Privée et Publique.



We have all watched a fragment of ice melt and disappear in a glass of water. The earthy matter that had been caught within its meshes during the process. of freezing, on being freed sinks to the bottom. No longer ice, it has become indistinguishable from the rest of the water present. Such a piece of ice is a perfect illustration of our own origin and destiny. We are enveloped in a vast fluid ocean that encompasses the earth. We live and move in it and are as truly a part of it as the ice is of the water. Generation after generation of men and animals, trees and herbs, crystallize out of it as the ice on freezing crystallizes out of the water. Every succeeding generation melts back into it again, as each winter's crop of ice disappears before the heat of summer. All living things, animals and plants alike, are neither more nor less than masses of crystallized atmosphere in the meshes of which a little mineral matter has become entangled. Out of the transparency of the atmosphere we grew, as ice crystals grow in water, and into it again we are destined to sink, disappearing in this common invisible ocean. An insignificant part of our composition is from the earth. When a body is burnt the handful of ashes that remains represents all there was of it that mother earth can lay claim to. These goings and comings of the matter that forms organisms constitute the facts of the most remarkable.

chapter in the genesis of things. They have made the atmosphere what it is and it, in turn, has both directly and indirectly conspired to mould living forms. into the modern fauna and flora of the earth.

Calculations based on observations of luminous me teors lead us to believe that the atmosphere is not less than one hundred miles deep and may be two hundred. The laws governing the pressure of gases and the action of gravity tell us that it must in shape be an oblate spheroid with the poles markedly flattened, much more so indeed than is the case with the earth itself. Since it is able to support a column of mercury or water weighing about fourteen and three quarters pounds in a tube with a diameter representing one square inch, it must press upon the surface of the earth at every point with about this weight. When two very true and smooth surfaces are rubbed together, they adhere because of this pressure. When a boy presses a piece of wet leather on a stone and is able by an attached string to lift and carry that stone, it is due to this atmospheric pressure. The school experiment with the hollow hemispheres that after exhaustion by an air pump cannot be pulled asunder except with great effort, is another illustration of the same kind. An ordinary sized man bears constantly upon his body at pressure of about fourteen tons; but as this pressure is in all directions and from within outwards as well as from without inwards, the compensation is perfect and reduces the actual pressure to practically nothing. It takes 8131⁄2 cubic feet of air to weigh as much as one cubic foot of water. If 8131⁄2 cubic feet were compressed into the space of one cubic foot its weight would be about the same as an equal volume of water. Anything that volume for volume weighs less than air will be buoyed up and float in it. A balloon, to ascend, must have its total size represent less weight than the weight of air it displaces. Sir John Herschel calculated the total weight of the atmosphere at eleven and two-thirds trillions of pounds; and yet with this inconceivably vast weight it is only one one-million two hundred thousandth of the weight of the earth itself. The pressure of this vast mass of gaseous material varies quite considerably at different parts of the earth.

The higher one ascends, the less it becomes. It varies, too, at the same height at different times of day and night, and before and after storms. By keeping track of this pressure the mariner is often able to foresee and prepare for a storm, and the mountain climber can calculate his altitude at any moment. Wind is the flowing of air from regions of high to regions of low pressure. Where the difference in pressure is very marked the wind is fierce, and where but little, it is mild. Thus arise cyclones, blizzards, and hurricanes as well as the gentlest zephyrs. By a study and comparison of regions of high with regions of low pressure our weather officers are able to predict changes with a good deal of accuracy hours in advance of their occurrence. The time of year has a great influence on these variations of pressure. In April and October it is more even than at any other periods of the year. When water is evaporated and its volume thereby increased almost one thousand fold, it ascends just as a balloon would; but besides its light specific gravity when hot it is also subject to a law of diffusion that would carry it upward even though heavier than air. Since water is always evaporating and since diffusion is always at work, the air is constantly supplied with moisture, sometimes to the point of saturation. To this moisture in the air we are indebted for the maintenance of an even degree of temperature. But for it night would be colder than Greenland, even at the tropics. It is the water in the air that holds the sun's heat and keeps the earth warm where direct sunlight fails to fall upon bodies. The main constituents of air are the two gases, nitrogen and oxygen. In one hundred parts there are about 76 9 10 parts of nitrogen and 23 1-10 parts of oxygen. These gases are merely mechanically mixed together and not held by chemical bonds. Without the oxygen we could not live nor breathe, nor could any ordinary fire burn. The nitrogen seems to serve only as a diluent of the oxygen. In every ten thousand parts of pure air there are from four to six parts of carbon dioxide or carbonic acid gas. This is the same as the poisonous choke damp of the miner, and is the product of our breathing and of coal and gas fires. In still minuter amounts

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