MIASMA is a Greek word (Alcursda)mignifying pollution or corruption generally, and is employed to designate a certain volatile deleterious principle, arising either from the bodies of the Nick, from animal or vegetable substances, or from the earth, and capable of exerting a morbid Influence on those exposed to its action. To the terrestrial emanations the Italians have given the name of malaria (from mala and aria, bad air), and this word has been generally adopted into other languages'. To those emanations which arise from the bodies of the sick, the term contagion is more properly applied. In common parlance, then, miasma is seldom employed to designate the contagious elilovium of disease, but, with the adjunct marsh, is restricted to the sense in which malaria is used : hence we speak of marsh miasma and malaria as one and the mime thing. Though marshes, whether salt or fresh, are prolific sources of malaria, they are by no means the only sources ; the mud which is left by the drying of extensive ponds and lakes, the half-wet ditches of fortifications, and neglected sewers and drains, are capable of furnishing this poison. The decomposition of vegetable matter, in other circumstances than In connection with soil, is likewise Capable of producing It; this has been exemplified in the eicklInelse of ships from the leakage of sugar into a damp bold Having enumerated some of the sources of miasma, the question naturally suggests itself,— What are the conditions essential to its development! The mere name of marsh miasma suggests the idea of stagnant water, and if the pre ceding enumeration of circumstances under which the production of malaria takes place be examined, it will be found that vegetable matter and moisture are present in all the examples, and that animal matter is so occasionally. But how great soever may be the share which moisture has in its production, it Is certain that only a very small proportion is necessary. A marsh, the whole eurface of which is thoroughly wet, is comparatively innocuous ; but if partially or entirely dried by the summer's heat, it becomes extremely pestilential in autumn ; indeed malaria, in its most intense degree, has been met with in low lands which had become as dry as a brick ground, with the vegetation utterly burnt up, and hence a high temperature seems to be another agent necessary, or at least favourable to its development. According to
Dr. Ferguson, the only condition indispensable to the production of marsh miasma on all surfaces capable of absorption, is the paucity of water where It had previously recently abounded, a rule to which he assures us there is no exception in climates of high temperature. Of the chemical and physical properties of malaria nothing is known; even the very obvious question, whether it is always the same kind of poison, or whether a multiplicity of such poisons may not exist, is one which the present state of our knowledge does not enable us to answer. The occasional existence of putrefaction in conjunction with malaria is an accidental concomitant, but by no means essential to its activity as a poison. With regard to the effects of malaria, these manifest them selves in a longer or shorter period after exposure to its influence, and consist chiefly in the production of intermittent, remittent, and yellow fevers, dysentery, and typhus. The long-continued application of the same poison iu a diluted form gives rise to various disorders of minor import, gradually undermines the constitution, and produces premature old age; even the inferior animals and vegetables partake of the general depravation which characterises malarious districts. The most eflicieut means of preventing the generation of the initiations poison, and, by consequence, the diseases to which it gives rise, are, the draining of swampy lands, and preventing the accumulation of putrid or putreacible vegetable or animal matter.