In other respects the physical properties of the air seem to have little influence: the warmest and longest summers are often healthy: the coldest winters, with the exception of accidental inflammatory complaints, are the same: the warmest weather, with the dampest fogs, have been followed by no peculiar epidemic. It is what Hippocrates long since called the something divine or inexplicable, that produces fevers and similar diseases; but, before we notice the "divinity that stirs within us,"we must add a few remarks on situation, as connected with the physical properties of the air.

A dry elevated spot, on a gravelly soil, is said to be most wholesome, especially if sheltered from the east wind. Elevation is however relative; light clouds float in the atmosphere, about 1600 feet above the level of the sea; and the healthiest spot is said to be some way above this elevation. This appears, however, to be fanciful; and it has not been proved that atmospheric moisture alone is injurious. In dry gravelly elevated spots, experience has fixed the most salutary residence for consumptive cases; yet, in these, oxygen seems to abound, which is peculiarly injurious in such complaints; and air of a lower quality, as it has been styled, is seemingly as good; in the opinion of some, preferable. In asthmatic cases elevated spots are manifestly injurious. In fact, theorists may declaim, but facts give the lie to the most plausible declamations. A change is often necessary; and from the effects of that change, the conduct proper for each individual must be ascertained.

It is observed by some authors, that vaults, corn-magazines, apple-garrets, etc. should open to the north; for that point is invariably proper: but the south and west are constantly improper. The most healthy exposure, if a house is to be built, is said to be found by cutting one of the trees that grow there transversely with a saw, observing the rings: the side of the tree on which the distances between each ring are widest is the most healthy exposure, and the windows of the house, all other circumstances being the same, should ever face that way.

We have mentioned the effects of the east wind in general, and we shall now notice them more particularly, though it cannot be yet determined whether they belong to the chemical or physical properties of the air. The atmosphere, while the east winds prevail, is lurid; and, even when clear, the sun has not its brilliant hue. The strength is not equal to the usual exertions; the respiration is not free; the spirits not lively. Asthmatics and hypochondriacs feel it severely; yet it is often dry, and, when it rains during a southeast wind, its fall is frequently periodical, extending only to twelve or twenty-four hours; while the clouds constantly display a promise of fair weather: there is seemingly a perpetual contest between the causes of rain and their antagonists, whatever they may be.

As we have now instruments by which the quality of the air may be measured, it might be presumed, that these would inform us of the cause of this singular state of the atmosphere. The east wind is not peculiar to any situation, so that it is not injurious from passing over a baleful desert, or a successive series.of marshes; nor does the eudiometer show any particular ingredient which may impair health or induce disease. We have not mentioned this instrument in our disquisitions respecting air, as it chiefly informs us of its chemical qualities. As we now approach this subject, we may remark that, in all its forms, the assistance it affords is inconsiderable to the. medical chemist. In crowded cities, and the most apparently healthy situations, remote from "the busy hum of men,"its results are nearly the same. Chemists must decide whether this similarity in the appearances is owing to the imperfection of the instrument, or whether the injurious qualities of the air are not cognizable by it. We have now mentioned this instrument to excuse our future silence respecting it. Its forms, however, we shall afterwards describe, as future enquirers may be more successful. See Eudiometer.

We have said that air consists of oxygen and azote in a gaseous state. To this, when we speak more critically, we must add carbonic acid gas. It has been disputed, whether the principal ingredients are chemically combined, or only mixed mechanically. Neither is true. We cannot indeed mix oxygenous and azotic gas, so as to form a gaseous fluid like our atmosphere; yet they are not chemically united, so as to form a tertium quid; nor even in the more general sense of the word, so as to produce a substance partaking of their united properties; as, when we mix spirit with water, or dissolve sugar in any fluid, it seems that the particles are united in their nascent state, and adhere together rather than form a compound. It appears at first sight singular, that the oxygen which supports life should be in so small a proportion; but the singularity will soon vanish when we reflect, that oxygen alone would be as fatal in the lungs as arsenic in the stomach. It is, literally, like fire which warms; but in excess, will burn. This we chiefly mention to explain the inconveniencies arising in hectic and in asthmatic cases, from air too pure; in the latter it stimulates the weak lungs too violently; in the former it adds to the tone and the irritability of the vascular system, already too great. The mountaineer and farmer, who breathe air highly oxygenated, are strong, robust, and active, but scarcely ever fat. Oxygen makes no part of this animal fluid; and hydrogen and carbone, of which it chiefly consists, do not abound in these regions. Hydrogen, indeed, has been discovered by Saussure on the highest mountains; but its levity carries it beyond human habitations; it is an extraneous body, found in air, but not a component part of it. As its elasticity-is inconsiderable, it certainly contributes to the languor experienced in highly elevated situations.