Air, in a pure state, is a colourless, transparent, compressible and elastic fluid; and one of the most important elements, whether we consider its application to purposes general economy, or its effects on animated nature. It is the medium through which we breathe, and without which we cannot exist. When perfectly freed from all extraneous and noxious particles, it may be denominated vital air, or oxygen ; and in this state is capable of invigorating and supporting the human frame, in a very eminent degree. Mixed with the common ingredients, it is called atmospheric air, or that by which we are usually surrounded.

In diseases of the lungs, and epidemics arising from a confined or vitiated atmosphere, the administration on air, in a pure state, has been attended with singular success- while, in such complaints, the most powerful remedies have been unable to compensate the want of this necessary article.

Air vitiated by the different pro-cesses of respiration, combustion, and putrefaction, or which is suffered to stagnate, becomes prejudicial to the human frame: hence large cities, public assemblies, hospitals, burying-grounds, etc. are injurious to health, and often productive of contagious disorders.

Plants and.vegetables possess the wonderful property of restoring the purity of air. This, however, takes place only in the day-time, and when they are exposed to the light of the sun : for at night they discharge their noxious particles, and corrupt the atmosphere. Nevertheless, the disadvantage arising from their impure exhalation during the night, is far exceeded by the benefits produced in the day-time ; as the former does not amount to a hundredth part of the pure vital air, which is generated by the same plant, in the course of two hours of a fine day. It has been asserted, that the purity of air may be also restored by wetting a cloth in water mixed with quick-lime, hanging it in a room until it become dry, and renewing the operation so long as it appear needful.

Air has been most successfully applied to various purposes of domestic economy, and in many branches of the useful arts ; such as in the construction of wind-mills, air-guns, stoves, etc.

A mode of forwarding the. distil-lation of salt water at sea, has been discovered, and consists simply in blowing Currents of air through the distilled fluid. The same me-thod has also been successfully employed to take off the unpleasant taste which is sometimes found in milk.

Dr. Reich, of Erlang, describes a particular machine for the purpose of extracting air from the intestines, and thus procuring instant relief in a complaint called tympanites, or the dry windy dropsy. A small tube with a cock having a valve on its side, and so constructed as to turn quickly, is affixed to the common clystering machine. Upon each successive introduction, the cock must be turned, in order to admit the air into the tube, and then quickly closed.

Air which is rarefied, ascends. This is particularly exemplified in the periodical sea and land breezes of hot climates; where, in consequence of the reflection of the sun from the earth's unequal surface, the lower land-air becomes highly rarefied, and rises into the upper atmosphere, while the sea-air, being cool and dense, rushes in to supply its place. Upon this principle, M. Van Marum, a Dutch chemist, has discovered a method of purifying assembly-rooms by a tin tube of nine inches diameter, and ten feet length, to the lower surface of. which lamps are suspended, for the purpose of rarefying the air, and urging it to ascend through the cieling of the room.

Dr. has described the useful effects produced in French prisons, by long air-trunks fixed through the cielings of wards in gaols, to carry off the foul vapours which exhale from the prisoners : he declares that it has not only preserved many of their lives, but prevented them from communicating infectious distempers to persons assembled in the courts of judicature..

We are happy to add, that this valuable improvement has also been adopted in this country.

An apparatus invented by Mr. Salmon, of Canterbury, tor the expulsion of noxious air from veils, has been employed with considerable success.

Air-balloons are constructed upon similar principles ; they continue to ascend, so long as the inflammable gas with which they are filled be lighter than the atmosphere with which they are surrounded.

Noxious and mephitic vapours, arising from wells and other sub-terraneous places, may be effectually corrected by simple ventilation, or the admission of such portions of vital air as will render the whole sufficiently respirable.

To ascertain whether the air of a mine, well, cellar, or large cask, be safe, a lighted candle, suspended by a cord, ought to be conveyed to the bottom, before any person venture to approach it. Should a slight explosion take place, or the light burn dimly, or even be extinguished, the air is certainly noxious; but if the flame continue bright, no danger is to be apprehended.

Another easy expedient of purifying foul air may be adopted, by pouring several vessels of boiling water into such receptacles, before any person be suffered to descend.

A still better method of dispelling the deleterious air from deep wells or pits, is the following: take a leather tube of sufficient length to reach to the bottom of the shaft or cellar; fix the nozle of a pair of large bellows to the top, and work them briskly for a few minutes : thus fresh air will be introduced, the flame of the candle, on trial, will not be extinguished, and we may descend without any danger,

Artisans who are employed over charcoal-fire, such as dyers, gilders, refiners of metals, etc. are exposed to considerable danger from the vitiated state of the air : to avert the injury to which their lungs are thus exposed, it would be advisable to place near them a flat-bottomed vessel filled with lime-water, and to renew it every other day, or so often as a variegated film or pellicle appear floating on such water. This powerfully attracts and absorbs the pernicious exhalations produced from the burning of charcoal.

Likewise, in the construction of chemical laboratories, smelting-mills, and similar offices, proper attention ought to be paid to their free and constant ventilation 5 as the metallic fumes, and other noxious vapours which they generate, are highly detrimental to health.

In chronic diseases, especially these of the lungs, a change of air is strongly recommended. It has sometimes, independently of any other circumstance, proved highly beneficial ; inasmuch as patients have breathed more freely, even though removed to a damp and confined situation.

The following places in the vicinity of London, have been found in the spring season to be most congenial to consumptive persons, viz. Camberwell, Peckham, the lower parts of Clapham, the drier parts of Lambeth, Battersea, Fulham, Chelsea, etc. As the more temperate season advances, the higher situations, such as Paddington, Penton-ville, Hampstead, Highgate, etc. may then be resorted to with advantage.

Air. - In a late volume of the " Annales de Chimie, " we meet with a memoir by M. Deyeux, on the means of purifying infectious air; and which is extracted from a work of M. Guyton de Morveau, who made numerous experiments with different matters, in order to ascertain- those which were best calculated to prevent the diffusion of contagious atoms in the atmosphere. As, however, we cannot enter into an analysis of his experiments, we shall briefly state their results. According to these, common vinegar, possesses the power of decomposing contagious particles ; but it does not operate in a perceptible manner, unless the infected substances be actually immersed in this acid, or be such as admit of being washed with large portions of it. - Radical vinegar, or the acetic acid, produces effects sufficiently rapid and powerful; but, on account of its high price, it can seldom be employed in considerable quantities. - The nitric acid is well calculated to destroy the putrid effluvia; but, as it cannot without great difficulty be divested of nitrous gas, the action of which is always prejudicial to the health of those who respire it, M. MoR-veau remarks, that the use of this remedy is still attended with great inconvenience. He is, therefore, of opinion, that the muriatic acid affords very great advantages in dispelling contagion, by the uncommon expansibility of its vapours ; which thus penetrate every part of the substance on which the operation may be performed. Nevertheless, he conceives the oxygenated muriatic acid to be superior to every other remedy, both for the celerity and facility with which it is diffused, and likewise for the certainty of its action; In consequence of which, it instantly destroys all putrid miasmata, that may either be floating in the air, or be fixed upon bodies.

It is a circumstance well known to persons conversant with chemistry, that wood, during combustion, yields one-sixth part of its weight in coal, while the remaining five-sixths of smoke (which contains a large proportion of inflammable air), are usually dissipated, without being subservient to any useful purpose. Hence, an apparatus has lately been contrived in France, by M. Lbbon, for applying such smoke to the heating and illuminating of rooms at the same time. It is denominated a Thermo-lamp, and consists of a box or vessel, the smoke rising out of which, after being freed from all vapours and soot, is conveyed through very small tubes, that are concealed in the plaster of the walls or ceiling. Such tubes are made of oiled silk, but the orifice is composed of metal, to prevent the silk from burning, when the gas takes fire on coming in contact with the atmospheric air. Thus, the flame may be conducted in a moment from one apartment into another, without depositing either ashes, coals, or soot ; so that, according to M. Lebon's assertion, chimnies become unnecessary. He farther states, that the fire, thus produced, does not require any particular care in supporting it, while it possesses this advantage, that the pure light may be made to represent flowers, festoons, or other ornamental objects ; or, such fire may be so arranged as to emit its light from above, with the purest lustre.