(From to pollute). Miasmata have lately claimed the attention of pathologists, as they are the causes of some of the most fatal fevers to which mankind are subject. In the more strict pathological investigations of modern authors they are distinguished from contagion, which is confined to the effluvia from the human body, when subject to disease; yet this contagion, when it does not proceed immediately from the body, but has been for some time confined in clothes, is sometimes styled miasma. Another kind of miasma as already noticed, (see Contagion,) is putrid vegetable matter, and indeed every thing of this kind which appears in the form of air. Miasma, then, strictly speaking, is an serial fluid, combined with atmospheric air, and not dangerous except the air be loaded with it; for diffusion, as we have seen, renders it harmless. It is not always discoverable by the smell, and scarcely ever by the nicest eudiometrical tests: it is not therefore hydrogen or azote, though there is great reason to suppose that it is a modification of these; for, from marshes hydrogen generally arises, and, from the human body, the chief injurious exhalation is azote. Hydrogen and azote also destroy irritability, or induce sudden debility, effects generally found from the miasmata which produce fever. As we know not the nature of miasmata, therefore, we cannot discover their corrector. Diffusion, however, renders them harmless, and it is sufficient for us that free air will prevent their deleterious effects.
What the variety of miasmata may be we cannot Each infectious disease has its own, diffused round the person which it has attacked, and liable to convey the disease at different distances, according to the nature of the complaint, or to the predisposition of the object exposed to it. This part of the inquiry rather belongs to contagion, and to the particular disease. A patient in the small pox seems to diffuse an infectious atmosphere to the distance of from ten to fourteen feet: measles and scarlatina are less active in this respect, and even the plague seems not to be infectious, except from fomites, but from actual contact.
The miasmata of marshes, those only whose effects we can more distinctly perceive, produce intermittents, and remittents of the worst kind. They produce also dysentery and the epidemic catarrh, of which the infection is usually conveyed by the air. The bilious fever of America we have supposed to be the natural autumnal remittent, and therefore may be referrible to the. same source; nor can we avoid concluding that every endemic disease must have its origin in the peculiar exhalations of the country.
The putrid vegetable matter which has been accused as the cause of many fevers may be truly such, for we know that many parts of vegetables produce azote Yet their effects in this respect have not been traced with accuracy. Continued fevers are chiefly referrible to contagion: but the causes of intermittents in some constitutions seem to produce fevers of the more continued form; and the miasmata of marshes, when they -have excited their peculiar fever, may certainly, through the medium of the human body, produce continued fevers. Human effluvia confined, independent of a morbid state, become undoubtedly the cause of fever the most continued in their form, and are then truly miasmata.