Air is a mixture, and not a compound. This distinction, as before intimated, is one of great importance. A cup of coffee is a good illustration of the former; there we have united water, coffee, cream, and sugar, but no new substance is thereby produced, and each of these ingredients may be removed without affecting the others. Gunpowder is a mixture, being composed of sulphur, nitre, and charcoal—a most admirable mixture it is, too, for every particle of it contains these three substances, as may be shown; the sulphur may be removed by heat, and the nitre by washing, leaving the carbon alone; the microscope also would reveal in each grain these three substances. That the air is a mixture can be proven in two ways. First, water will absorb each of its two principal ingredients, and, secondly, they do not exist in air in that definite ratio which always characterizes chemical combination. The principal materials in air are oxygen, nitrogen, carbonic di-oxide, watery vapor, ammonia, and very minute portions of many other materials. Professor Steele says, that if the entire atmosphere were compressed to the density of that immediately surrounding the earth, it would extend above it only about five miles. Now, if the substances entering into its composition were to be arranged in the order of their specific gravity, watery vapor would form a sheet about the earth five inches deep, carbonic di-oxide another just above it, thirteen feet in depth, then a layer of oxygen one mile thick, and nitrogen another layer above that, four miles in thickness.
In short, four-fifths of the air is nitrogen, about one-fifth is oxygen, four ten-thousandths is carbonic acid, and water exists in variable quantities. It will be readily seen that the chemical and physical changes constantly going on in the surface of the earth must be throwing off other materials into the atmosphere. For example, the spectroscope has shown that common salt exists almost everywhere in the air. This arises from the fact that the ocean surrounds all lands, and its yeasty waves are broken into foam which is caught up by the winds and borne over the whole earth. One of the most remarkable facts connected with this subject is the wonderful uniformity of this mixture. Upon the whole, the amount of each ingredient is nearly the same. Some slight variations, such as the following, are observable: More CO₂ is found near cities than in the country, and there is more of the same over the land than over the sea. That the substances which enter into the composition of air do not arrange themselves according to weight, is due to a most interesting law called
THE DIFFUSION OF GASES.
PHOSPHORUS BURNING IN AIR.
Experiment.—Prepare nitrogen as described elsewhere in this article.
By this we mean that gases tend to intermingle, the lighter even descending, and the heavier ascending, until they occupy the same space. This can be shown in the following manner: Fill one bottle with hydrogen, and another with carbonic acid gas, fit into each a cork, perforated so as to admit a tube, connect the two by inserting a tube, placing the bottle of hydrogen above with the top downward; although the carbonic acid is twenty-two times heavier than the hydrogen, in an hour or two it will rise into the bottle above, as can be proved by pouring into it some lime water, which will immediately become milky, showing that the carbonic acid has united with the lime, forming calcium carbonate. That the hydrogen has passed down into the other bottle may be demonstrated by first absorbing such portions of the carbonic acid as still remain by pouring in cream of lime, when there will be found still in the bottle a substance (hydrogen) which will burn with a faint yellowish light. Another pleasing experiment may be performed in the following manner: Take an unglazed porcelain cup, fit to it a brass cap, perforated so as to admit tightly a long glass tube, insert one end of the tube into some colored water contained in a goblet, the inverted cup being supported above on the other end of the tube; now hold over the cup a jar filled with hydrogen; bubbles will soon be seen escaping through the water from the lower end of the tube, showing that the hydrogen has entered and mingled with the air; remove the jar, and the liquid will rise in the tube, proving that the gas has escaped from the cup. This diffusive force in the atmosphere prevents the accumulation of noxious gases by distributing them throughout the whole mass. The constant agitation of the air in gales and storms facilitates this operation, and it is only in certain confined places like caves, such as the Grotto del Cane,[1] mines, and wells, that we find apparent exception.
SILVER COIN DISSOLVING IN NITRIC ACID.
Experiment.—Place a five-cent piece in some nitric acid for two or three hours. Drop into a portion of the liquid a little salt; you show the presence of the silver. Drop into another portion some aqua ammonia; the blue color reports the presence of the copper.