MIASIIA (Gr. pollution; in the plural, Miasmata), or _MALARIA. It IS proved by the experience of all ages that there is an intimate connection between marshy districts and certain diseases, especially the various forms of intermittent and remittent fever; but the exact nature of the noxious agent, and the circumstances on which its formation and extrication depend, are even at the present day not altogether established. It is clearly neither heat nor moisture, for the crews af clean ships, when cruising in the tropics at a distance from land, are usually very healthy; nor is it any known gas extricated fnim the marsh, for the gases collected by stirring up marshes (carbonic acid, nitrogen, oxygen, and earhureted hydrogen) niay be inspired without giving rise to any symptoms resem bling those produced by malaria It may be regarded as an established fact that the noxious agent is a product of vegetable decomposition occurring under certain conditions of heat and moisture. That vegetable decomposition is the source of tile poison is inferred from various circumstances. For example, this special mornifie influence is nowhere so powerful as in the deltas aud along- the banks of large tropical rivers which, in their flood, bring down the -washings of the soil, full of vegetable remains, which, npon the subsidence of the waters, are left reeking in the hot sun. Again, the poison has been traced, in various places in Italy, France, and the Netherlands, to the practice of steeping flax in stag,nant waters, and even in streams, and in India it WaS formerly the cu.stom, after extracting the coloring matter, to throw the remains of the indigo into large heaps which, in the course of three years, became excellent manure; it WAS found, however, that these heaps, alternately soaked by the heavy rains and heated by a tropical sun, decomposed and emitted miasmata precisely similar in their effects to those produced by marshes. Marsh-miasmata are seldom evolved at a temperature under 60°, but at and above 80° they are prevalent and severe; and the nearer we approach the equator, the more violent, as a general rule, do they become. Although moisture is necessary to the evolution of miasmata, an excess of it often acts as a preventive, and by impeding the access of atmospheric air retards or prevents decomposition. This explains the apparent tuaomaly of an uncommonly rainy season producing opposite effects in different localutes, sometimes not far distant from one another. Thus in tile West Indies a very rainy season induces general sickness in the dry and well-cleared island of Barbadoes; whue at Trinidad, whose central portions are a sea of swamp," and where. it rains nine months in the year, the excessive rain is a preservative from sickness; for in the seasons when the rain falls only eight months or less, the swamps become dty and exposed to the sun, and severe remittent fevers are sure to follow.
Chemistry has hitherto failed in detecting any special ingredient to which the air evolved by marshes owes its poisonous qualities. The air collected in the most poisonous districts gives, on analysis, the same gases existing in the same proportions as normal air, nor (if we except the observations of Boussingault, which have not been confirmed by other chemists) does it give evidence of the presence of any organic.body.
The infecting distance of this poison is a subject of great practical importance; and both the altitudinal ranr.e and the horizontal spread have to be noticed. In Italy.it estimated that RD altitude of about 1,500 ft. assures an exemption from marsh-poison: while in the West Indies an elevation of at least 2,000 fc. is necessary. From observa tions made by sir Gilbert Blane during the ill-fated Walcheren extteditiont it appears that in Europe the horizontal spread of marsh-milismata Over fresh water is less than 3,000 ft.; but over salt water—at all events, in the tropics—the horizontal range is greater. The extent to which the poison may spread horizontally over land is a inuchi more complicated question, and depends, to a great extent, upon the nature of the soil. The effect of trees in intercepting miasmata is very remarkable, and is probably due partly to their condensing the vapors. of the marsh, and partly to their altering the direc tion of the current of air. Pope Benedict XIV. caused a wood to be cut down which separated Villatri from the Pontine marshes, and in consequence, for many years, there was a most severe and fatal fever in a district previously healthy; and the same results have in many other cases followed the removal of treee.
In districts where this poison exists it is found by experience that those who go out of their houses only during the day, after the morning fogs have dispersed, and before the evening mists appear, often escape the bad effects; and a full meal, with a few graini of quinine, should be taken before exposure to the morning air by travelers in a malari ous district.
Dr. Wood.of Philadelphia has pointed out the extraordinary and very important fact, that miasmata are neutralized, decomposed, or iu sonic other way rendered innocuous by the air of large cities. Though malarious diseases may rage around a city, and even invade the outskirts, yet they are unable to penetrate into the interior, and individuals who never leave the thickly-built parts almost always escape. What it is in the air of the city which is thus incompatible with malaria, is unknown; but very probably it is connected with the results of combustion, for the fire and smoke of camps are asserted to have had the same effects. See MALARIA.