Nitrogen gas, or the mephitic air of former authors, is very extensively diffused. Its specific gravity is inconsiderable, for it is lighter than atmospheric air, in the proportion of 985 to 1000. Nitrogen, with caloric, forms this gas; and, with different proportions of oxygen, the nitrous acid in its various forms. Willi the full proportion of oxygen, it forms the nitric acid, the aqua fortis of the shops: with a less proportion it becomes nitrous acid; with still less nitrous gas; and with a very small quantity the nitrous oxide. Nitrogenous gas is neither acid nor soluble in water; and the nitrous gas is employed as a test of the purity of air in the eudiometer, q. v. If the air contains oxygen, it thus changes the gas into nitrous acid; and a larger proportion of the acid is formed when the oxygen is more abundant; while with impure air no change is produced. In medicine it has scarcely been employed: it is said to be antiseptic, and to kill worms, but experience has neglected to register its effects, or has disregarded it.

The nitrous oxide is heavier than air, and soluble in double its quantity of water. The taste it imparts is sweet, and the odour agreeable, though slight. Combustible bodies, at a high temperature, decompose this oxide; and it unites with alkalis, though not with acids. In fact, if an acid, it is the lowest in the scale, and to dispute whether it be so, is to contend with air. Its effects on respiration are singular. It is said to animate the person who breathes it to a degree little inferior to phrensy: the sensations produced are highly pleasurable, and no languor follows. Though much must be allowed to the enthusiasm of a discoverer, and to the experience of effects wholly new and unexpected, yet very pleasing sensations have been undoubtedly felt on its being inhaled. To what these are owing has not been ascertained. A slight reflection will shew, that though life is really sustained by oxygen, yet this ait-is not proper for breathing for any continued period. The pleasure excited by fresh air does not arise from the oxygen, for it is not increased, or at least to an inconsiderable extent, in proportion to the quantity contained in the air breathed. Why azote, that is alone fatal to life, should be the necessary ingredient, is not clear. The great principle of distinction of animal substances, chemically considered, is indeed azote: this principle, so copious in these, is found in a small proportion, and only in particular parts, of the vegetable kingdom; and it is the great problem in the function of animalisation, to discover the sources of the azote. May it not then be the air, and may not the animal system feel a peculiar pleasure in the supply of this principle, which must neutralise, or assimilate, the vegetable food ? It is not an improbable supposition, but it has escaped us, if it has been noticed by any former physiologist.

Air, in so many various ways injured, viz. by breathing, by burning bodies, etc. is restored by many means; a few of which only have been discovered. Plants absorb carbonic acid gas, and restore, in their turn, a pure air; and thus, combining with azote, may, imperceptibly to our senses, renovate the atmosphere. We may thus account for the different result of the experiments of philosophers, some of whom have discovered that plants exhale pure air, while others deny it. Inflammable air seeks the upper regions of the atmosphere, and is destroyed in the meteoric explosions, when too copious; while the portion arrested in its progress contributes, as we have said, to the production of the oils and resins of vegetables.

Thus nature very completely restores the various changes in the constitution of our atmosphere, which the different processes constantly going on may, in her regular course, have occasioned. Yet the air is accused as the cause of numerous diseases; and it really is so. Sudden cold checking the perspiration will apparently produce almost every form of the pyrexiae. Partial cold will produce rheumatisms; damp air, catarrhs; and in old people those defluxions which are called humoral asthmas, and catarrhi suffocativi. The continued heat of summer occasions bilious disorders; and the cold of winter a return of the more active inflammations. The air is, however, chiefly a vehicle of injurious effluvia; some of which only can be ascertained. Marsh miasmata, as they are styled by pathologists, are the cause of numerous intermittent and remittent fevers, as well as those apparently of a more continued form. It has been ascertained, that a clayey soil, when moistened, will attract the oxygen of the air, and leave its azotic part not sufficiently guarded to support the vis vitae; and it is found that districts become unhealthy chiefly when the earth begins to appear, in consequence of a diminution of the water. It is singular, that Linnaeus, with a view to prove the cause of in-termittents to be an argillaceous earth, has traced very minutely the prevalence of intermittents in clayey countries; a circumstance which may be explained from the views just assigned. To this diminution of the oxygen must be added a larger and unusual proportion of inflammable air from the parts of marshes still covered by water. To these conjoined causes many epidemics are owing: and when the changes in the physical properties of the air appear to produce fevers, they act only as exciting causes of these miasmata, in a manner to be afterwards explained. Sec Infection and Epidemics.

It is not found that an unusual proportion of fixed air is injurious: it falls to the lowest strata of the atmosphere; and, whatever be the quantity, it is apparently absorbed. The very extensive diffusion of catarrhs and other epidemics, of small pox, measles, etc. is from causes combined with the air, and no part of the atmosphere. The contagion of putrid fevers, viz. the contagion conveyed by the patient, or by the medium of the attendant's clothes, are substances combined with the air which the nicest instruments have not yet been able to detect, though much may be expected from the persevering ardour of modern experimental philosophers.

Hoffman, in his Med. Rat. Syst. artic. De Acre, and Boerhaave on Air, have collected all that is valuable from their predecessors and contemporaries.

Hale's Statical Experiments. Chaptal's and Thomson's Chemistry. Huxham on Air and Epidemic Diseases. Shaw's Abridgment of Boyle's Works, in the article Air. Parkinson's Medical Pocket Book. Dob-son on Fixed Air. Chaptal's Elements of Chemistry; also Lavoisier, Fourcroy, and Nicholson.