Bad Air), Or Marsh Miasm Malaria (Ital. Mala Aria (Gr.Bad Air Or Marsh Miasm Malaria Ital Mala Aria 110018 , to infect), an emanation which produces in mankind intermitting and remitting diseases. This poison is not cognizable by the senses, nor can it be detected by chemical tests; it is known only by its effects. The concurrence of vegetable matter susceptible of decay, of moisture either on the surface or a short distance below it, and of a certain elevation of temperature, is necessary tor its evolution; of these, long continued heat has the greatest influence in increasing the intensity of the poison. Comparatively harmless in the northern part of the temperate zone, it becomes malignant and deadly in places equally favorable to its production, just in proportion to the increase in the mean annual temperature. Marshes, whether salt or fresh, and wet meadows are especially subject to malaria, particularly when drying under a hot sun. Grounds alternately Hooded and drained are fertde sources of it, and it is this which renders the cultivation of rice so deleterious. Grounds which, from the nature of the subsoil, retain the moisture a short distance beneath the surface, though that may be dry and parched, are favorable to the production of malaria.

The process of clearing a new country of its woods, and thus exposing the soil to the full action of the sun, is commonly followed by the prevalence of fevers; and the same evils often follow the ploughing up of meadow lands. It is not necessary that the amount of the vegetable matter be great or its growth recent, since malarious diseases have often, been caused by the drainage of ponds and lakes: and the fevers that prevailed at Bourg-en-Bresse ceased on rilling in the half wet ditches of the fortifications. The low grounds on the margin of lakes and the alluvial lands bordering rivers in warm countries are always plagued with malaria. In India ground covered with low thick growths of brushwood or of weeds and grass, called jungles, are so well known to produce malarious fevers, that they are there termed jungle fevers; even open woods in tropical climates are productive of malaria. The steeping of hemp and ilax, and the decay of vegetable refuse, pota 5, etc, in confined localities, as cellars or the hold of a vessel, have resulted in fever. - The quantity of water required for the generation of malaria is not large, a marsh completely covered with water being innocuous; it is only when the moisture is drying up under the influence of the sun that it becomes pestilential.

So in tropical climates disease prevails chiefly at the commencement and after the termination of the rainy season, and is less prevalent while the earth is saturated. In some cases the quantity of vegetable matter concerned in the production of malaria must be exceedingly small. Dr. Ferguson, one of the medical ofn-cers in the army of the duke of Wellington, says " In Spain, during the month of May, 1809 which was cold and wet, the army remained healthy; but in June, which was remarkably hot and dry, marching through a singularly dry, rocky country of considerable elevation, several of the regiments bivouacking in the hilly ravines which had lately been watercourses, a number of the men were seized with violent remittent fever (the first which bad shown itself on the inarch) before they could move from the bivouac the next morning; and this portion of the troops exclusively were affected with this disorder for some time In this instance, the half dried ravine having been the stony bed of a torrent, in which soil never could be, the very existence of vegetables, and consequently of their humid decay and putrefaction, was impossible, and the stagnant pools of water still left anion- the rocks by the watercourse were perfectly sweet.

Vet" tl.'.S situation proved as pestiferous as the bed of a fen." ("On the Nature and History of Marsh Poison," Edinburgh, 1821.) Here, however, the total absence of vegetable matter would be difficult to prove, and would he in contradiction with all other experience. - Whatever may be the nature of malaria, it is most concentrated near the surface of the earth, and becomes weaker as we rise above it; it is also most active at night, probably from the influence of the sun in rarefying and producing currents in the atmosphere, and perhaps, too, because it has a peculiar affinity for the fogs that are then apt to prevail. In malarious countries it is well known that exposure to the night air is apt to be followed by fever, and that those who sleep in the upper rooms of a house are safer than those who lodge on the ground floor. While as a general rule low and damp grounds are much more unhealthy than the hills in their neighborhood, yet in numerous instances this rule does not hold good, or is even reversed. The experience of the British army in the East and West Indies is conclusive on this point.

In many cases this can readily be explained by the effect of winds and currents of air carrying the malaria to the higher ground, which had been generated on the lower; thus in Italy the malaria from the borders of Lake Agnano reaches the convent of the Camaldules, situated on a high hill three miles distant. Connected with the propagation of malaria by currents of air is the fact that woods sometimes act as a screen, protecting a place from the malaria which would otherwise be conveyed to it from some neighboring source; in Italy fevers have frequently become prevalent on the cutting down of trees which have thus served as a shelter. It becomes an interesting question how far malaria can be carried by winds. This has been very variously estimated; probably three or four miles is the maximum. - The effects of malaria are by no means confined to the production of fevers and diseases of an intermittent type, but it is only in warm climates and in certain unfavorable localities that its full effects tipon the constitution are observed.

In such places the growth is stunted, the complexion sallow, the limbs slender, the abdomen tumid, the hair lank and scant, and the teeth defective; life is commonly extinguished before 40 years of age, and the population is only kept tip by immigration from healthier localities. Yet it is remarkable that when in such places persons live beyond their 40th year, they frequently recover some measure of health and attain to old age.