Anp, air, (from the Hebrew term aor, light,) called also gas ventosum. From a variety of experiments, atmospheric air is proved to consist of a mixture of about seventy-two parts of azotic gas, to twenty-eight of oxygen, or vital air. Lavoisier says, of about twenty-seven parts of vital air, and seventy-three azotic. But the proportion of these two gases is subject to variation in the mixture which forms the atmosphere; depending upon local causes. From the decomposition of the atmospheric air, these two gases are obtained; and sometimes in their simple state, sometimes in a proportion different from what they hold when forming atmospheric air, are used for medicinal purposes. The oxygen, or vital air, may be considered as a stimulant, and invigorator of the system; whilst the azotic gas is a sedative, and hurtful to the constitution, by destroying its irritability. Before the present aera of chemistry, it was the only gaseous substance known; and, indeed, almost all that has been formerly written on the air relates only to its physical properties. The chief of which are: First, That it is a fluid of extreme rarefaction, obedient to the smallest motion: the slightest agitation deranges its equilibrium, which is continually endeavouring to restore itself. Though very fluid, it passes through those orifices with difficulty, through which grosser fluids can pass with ease. Secondly, It is invisible; it refracts, but does not reflect the rays of light: it is inodorous, through the vehicle of odoriferous particles: it is insipid; and its physical qualities, chiefly, affect us variously. Thirdly, The weight of the air is not perceived but in large quantities; nor is the comparative weight easily, if at all, to be ascertained, as no two portions are ever of the same weight at different heights in the atmosphere. However, from long and repeated observations, the greatest gravity of the air in Europe is found to be equal, in equilibrio, with thirty-inches and half of quicksilver in the barometer, and the least raises it only to twenty-seven and half. The weight of the common air about the surface of the earth, at the time of the middle weight of the atmosphere, and in every temperate season, is to that of water as one to 850. Fourthly, The elasticity of the air is one of the properties upon which natural philosophers have made the greatest number of experiments, and it has ever been applied with considerable advantage in the arts. Fifthly, Air is necessary to animal existence. This is evident from the experiments made with the air-pump; though not without some exceptions, for toads, vipers, eels, insects of all kinds, and fish, live for a time in the exhausted receiver. They cannot indeed live without oxygen, but they expend it slowly, and separate it more perfectly from the injurious part of the atmosphere. Sixthly, The particles of air are said to be too small for any microscope to discover, and yet they are supposed to be larger than those of fire, water, oil, and many other fluids, since fire pervades glass; oil, water, etc. will pass through many compact substances, whilst air is resisted by strong paper. This argument is, however, fallacious. Seventhly, Air is a vehicle of sound, of the objects of taste, of effluvia to the nose, as is evident from observations made on the tops of high mountains, where our senses become duller than when we are nearer the plains. Eighthly, It is a part in the composition of all bodies. Ninthly, It cannot be rendered of itself solid by any known means. Tenthly, By contact and cohesion in the parts of bodies it becomes solid and unelastic; but when separated by heat, fermentation, etc. its elasticity returns. Heat rarities, and cold condenses it.

The physical qualities of the air have occasioned numerous disquisitions. But extensive inquiries, the comparison of the tables of mortality, experience long continued, have allowed us to draw few conclusions which will bear the test of careful examination. In spring, we find inflammatory complaints; in autumn, bilious diseases: in every season, fevers, in the commencement inflammatory, in the conclusion more or less putrid. To be more particular. Continued cold produces that tension of the fibres, that strong and steady action, which we style inflammatory diathesis: high situations, with a pure bracing atmosphere, produce similar effects. These are partly owing to an excess of oxygen, as we shall presently notice; but, in a great measure, to moderate, continued cold. A previous moist, temperate winter, which predisposes to scrofulous complaints, will, at this period, produce the most fatal consequences in hectic cases. The fever will increase, the ulceration proceed with rapidity, and the heat of the ensuing summer close the scene. Those, however, who are moderately healthy and not peculiarly robust, will find a winter, of no extreme cold, healthy; and the opening spring, expanding the fibres, will give a genial glow and new life to every organ. Summer, of course, may produce its own diseases; but, if we peruse the history of epidemics,we shall, with difficulty, trace any particular bad effects of the heat, till the evenings begin to cool, the fruit to be plenty, and the bile to become a conspicuous cause of disease, from its accumulation and excessive discharges. Winter again recurs, and Dr. Heberden has endeavoured to shew, from the bills of mortality, that it is a fatal season. It may be so in general: old people resist cold with difficulty, and the catarrhus suffocativus, asthma, and similar complaints, are often fatal at this period. In our experience, however, it is not the cold, but the early warmth of spring succeeding cold, which is most injurious: the constitution, braced by cold, cannot bear the subsequent relaxation. A long damp summer has had similar effects.

Philosophers have taught us how much pressure we bear from the atmosphere; and of course, from the diminution of that pressure, we shall feel the want of tension or tone which results from the removal of any support. Thus, when the air is lighter we find a languor come on; when heavier, our spirits are more brisk and lively. The whole is not however owing to the absolute weight of the air, but, in part, to its elasticity; or rather our feelings of health and activity are in the compound ratio of both. Thus, at the height of from 1200 to 2000 feet above the level of the sea, the pressure is greatly diminished; but we feel increased activity, as we are in general above the region of clouds, and the air is more elastic; and the languor felt in very high situations is not uniform or constant; so that it cannot depend on a constant cause. During rain, the mercury in the barometer is not depressed half an inch, yet we feel more languor than on the top of mountains, where it has probably fallen from five to ten inches.