Chemical Compoxiliam of the A.—Recent chemical .researches give the following as the mean composition of 100 volumes and of 100 grains of dry air: Volumes. Grairm' Nitrogen 79.02 76.84 Oxygen 20.94 23.10 Carbonic acid ........................ 0.04 0.06 100.00 100.00Besides the substances just named, other gaseous matters occur, but in quantities so small as'not sensibly to increase the bulk of the A., such as ammonia and anononiacal salts, carbureted and sulphureted hydrogen, carbonic oxide, sulphurous and sulphuric acid, nitric acid, and perhaps iodine, the quantity and even the presence of which are affected by local and meteorological causes. Roughly speaking, then, dry air may be said to consist of 4 volumes of nitrogen and 1 of oxygen, with a slight admixture of carbonic acid, and a mere trace of several other substances. As, however, the air of the A. is never found dry, we must add to the constituents already named watery vapor, the amount of which is constantly changing, according to locality, weather, wind, and temperature. It is stated that of 1000 grains of atmospheric air, the proportion due to aqueous vapor varies from a minimum of 4 to a maximum of 16 grains. By far the most active chemical constituent of the A. is oxygen, to the agency of which are owing the existence of animal life, the maintenance of combustion, the rusting of metals, and the occurrence of several other chemical phenomena too numerons to be detailed. A small portion of this oxygen occurs in the form of ozone (q.v.), a modification which, according to recent chemical discoveries, play s an important part in the chemistry of the A. The nitrogen which forms the bulk of the A. possesses few chemical properties of importance, but performs the important part of diluting the oxygen, which, if it occurred alone, would act with too great intensity. The presence of carbonic acid in the air is shown by the production of the white carbonate of lime in lime-water freely exposed to its influence. Carbonic acid is produced in all processes where carbonaceous matter unites itself with the oxygen of the air, such as in annual respiration, in combus tion, in fermentation, in putrefaction, and similar processes. The green leaves of plants, on the other hand, possess, in presence of sunshine, the power of decomposing carbonic acid into its elements, absorbing .the carbon for their tissues, and restoring the oxygen to the A. in its original purity. Between the processes above mentioned, on the one hand, and the action of plants on the other, the quantity of carbonic acid in the air is kept nearly constant. From the table it will be seen that 10,000 volumes of atmos pheric air contain 4 volumes of carbonic acid. If it occurred in a much larger propor tion, be4ng poisonous, it would become dangerous to animal life; and if it occurred in a much less proportion, the vegetable world would lack its requisite nourishment. The other substances, of which a trace is always or only sometimes found in atmospheric air, are ditlicult to detect in the air itself, but are generally found dissolved in rain water, more especially in that which has fallen immediately after a long drought. Of
these, by far the most important and widely diffused are ammonia and ammoniacal salts, which are of essential importance to the vegetable economy, because, dissolved in the rain, they furnish plants with the nitrogen required by them for the produCtion of their 'flowers and fruit. Nitric acid is detected in the air after thunder-storms, sulphureted .hydrogen in the tainted air of sewers and such like places, and sulphurous and sulphuric acid only in the neighborhood of chemical or smelting works. A considerable quantity of carbonic oxide and carbureted hydrogen escapes unconsumed from our furnaces; and although the latter gas is in addition given off to the air in marshy and bituminous districts, the two occur in almost inappreciable quantity in the atmosphere.
- In addition to its gaseous constituents, the A. contains solid substances in.a state of 'exceedingly fine division, the presence of which is revealed in the sunbeam Many of these minute particles, being the seeds or germs of plants and animals, must exert an important influence on the organic substances on which they may filially settle, inducing in many of them the conditions of disease or putrefaction.
When the composite nature of the A. was first discovered, it was supposed to be a chemical combination of nitrogen and oxygen, but further inquiries have rendered this opinion highly improbable. When any two bodies unite with each other chemically, the substance which results from their combination invariably possesses properties which the original constituents did not possess. Now the atmospheric union of oxygen and nitrogen is distinguished by no properties which may not be attributed individually 'to these gases. We have, then, in this respect, no indication that the atmospheric com bination of oxygen and nitrogen is a chemical one. Again, when -any comlosite gas is dissolved in water, the proportion of the ingredients dissolved in it islexactly the same that in which they occur in the compound itself; but this is not the ease with air clis solved in water, which is found to be richer in oxygen than atmospheric air. sow, as oxygen dissolves more readily in water than nitrogen, it is manifest that this larger pro portion of oxygen arises from both gases acting independently of each other in respect to the water, a condition that would be impossible if they were in chemical union. From these and other corroborative facts, the A. is considered to be simply a mechanical combination of the gases contained in it. This, however, does not prevent the A. from having a uniform composition, as might at first sight be supposed; for when gases are mixed with each other, they intermingle thoroughly throughout the whole space occupied- by them. Local causes may temporarily affect the relative proportion of the :atmospheric indredients, but the changes are so minute as to require the most delicate analysis to detect them.