The aerial pathology has not yet been successfully cultivated. Man can live and enjoy health from the heat of twenty-eight to one hundred and eight degrees of Fahrenheit. He can exist in a constant fog, where the hygrometer proceeds beyond the extreme of humidity; and, in air which supports the mercury only at twenty-two or twenty-three inches, he is robust and active. The sudden changes are indeed injurious; but the injuries are often transitory and inconsiderable; or, if severe, producing only temporary and acute diseases. But that our observations respecting the effects of the different airs may be more distinct, it is necessary to enlarge a little on the chemical properties of the different gases.

Besides the common, or atmospherical air, there are various other sorts, distinguished by their respective characteristics: 1st. Air, fixed or fixable. By Van Helmont, it was called gas sylvestre, from being produced in vast quantities from the burning of charcoal; from its apparent acid properties, aerial acid, cretaceous acid, and carbonic acid; and fixed air, as readily losing its elasticity, and fixing itself in many bodies. It is an invisible and permanently elastic fluid, superior in gravity to the common atmospheric air, and most other aerial fluids. It consists of twenty-eight parts of carbone, and seventy-two of oxygen, with some caloric, forming about one sixty-sixth of the common atmosphere, though, from its gravity, generally falling to the bottom. It is unfit for respiration; easily dissolved in water; exceedingly destructive to animal life, and produced in great quantities naturally from combustible bodies and many chemical processes. It is found at the bottom of pits; it rises from fermenting liquors; it is one and a half heavier than pure common air; water imbibes more than its own bulk of it; flame is extinguished, and animals are destroyed, by its influence: when the fixable air is separated from chalk and other calcareous substances, they become caustic, or, as they are now styled, pure: it is antiseptic, powerfully preventing and recovering from putrefaction, whence lime-kilns, which discharge great quantities of this air, would be useful in the neighbourhood of populous towns; in clysters it hath been very advantageously administered against putrid disorders, and, mixed with the drink, has been thought to conduce to the relief of patients labouring under putrid fevers. In the form of yeast it has also been administered with good effect in these disorders: but though it may be introduced into the stomach and intestines with advantage, if breathed into the lungs, it is mortal. To fixable air the chief property of some mineral waters is attributed: the Pyrmont and Seltzer water owe their brisk acidulous taste and sparkling appearance to it; and it dissolves iron in a small proportion, when it is mixed with water. Fixable air hath been found useful in cancerous, consumptive, scorbutic, and other disorders, where an antiseptic medicine might be expected to afford relief. It has not only been considered as antiputrescent, but also lithontriptic. When the stomach is disordered, carbonic acid air often gives a temporary and an useful stimulus. It is administered united with water by swallowing kali, or soda, in an effervescing slate, or the one immediately after the other, that the effervescence may take place in the stomach.

Air, vital; called also dephlogisticated, empyreal air, and oxygenous gas. From a variety of experiments, modern philosophers have proved, that in respiration a portion of air is lost; that the first effect produced, is the blood assuming a vermillion colour, by combining with pure air. The second is to establish a veal focus of heat in the lungs, maintained and kept up by the air of respiration. See Heat, vital; and Respiration.

Air, inflammable. It is the lightest of all the aeriform fluids: in general about twelve times lighter than atmospheric air. All animal and vegetable substances, which can be burned in the open air, charcoal excepted, will afford inflammable air, if heated in close vessels: though this is usually mixed with air of other kinds, with water, and with oleaginous matters. Charcoal, and several metals, afford inflammable air by heat, if water be present. Some metallic substances, during their solution in acids, afford, or extricate inflammable air, which is of the purest kind. The common process for obtaining it is by dissolving iron filings or shavings in diluted vitriolic acid. It occupies the upper parts of subterraneous caverns; and has been commonly found in mines and coal-pits, where it is called Fire damp, because it is liable to take fire, and explode like gunpowder. When not combined with oxygen it extinguishes fire; kills animals as readily as fixable air; takes fire by the contact of the electric spark, provided vital air be present, or any combustible body already in a state of ignition, and burning with a brilliant flame. If about two parts, by measure, of inflammable air, and one of vital air, arc mixed together, and set on fire in a vessel strongly closed, which may be done by the electric-spark, the air, if pure, will almost totally disappear, and the product be water, and an acid. It holds about half its weight of water in solution, which imparts to it a disagreeable odour; is absorbed by vegetables, and becomes a component part of their oils and resins.

The sulphureous, the muriatic, and some other acids assume the form of air: but as they are neither found in the atmosphere, nor applied to medical purposes, they form no part of the present subject.

Nitrous air, or nitrogenous gas, or azotic gas, forms an object of considerable importance in chemistry and medicine. It is fatal, when alone, to animal life; though, in combination, highly advantageous to it. This gas, we have seen, forms a large proportion of atmospheric air; and the gaseous nitrous oxide produces effects in respiration highly animating and stimulant. It is also the distinguishing ingredient of animal substances; the principle of animalisation.