usually observed. That malaria is destructive to animal bodies, and overturns that vital affinity which exists between their material elements, is shown by the fact of dead animal matter running on faster into putrefaction, in situations where this poison abounds. Nor is its power of destruction limited to animal matter: there is good reason to suppose that its presence is destructive of all or ganised nature, vegetable as well as animal, the latter most particularly, whenever their vitality sinks so low as not to be capable of withstanding its noxious influence. Whatever influence, therefore, may be imputed to this agent, or in whatever manner it operates its effects upon the living body, there is every proof in favour of considering it as a powerful septic. Thus it may be readily shown that animals killed in a situation abounding in malaria run much more speedily into putrefaction, than in healthy situations; although the temperature, and state of the air as respects moisture and motion, are the same in both; and in many cases this septic operation is evinced by its effects upon injuries, wounds, and sores, even during life. It has also been repeatedly proved, that substances fabricated of silk, wool, and even of cotton and flax, and exposed to marshy exhalations, will rapidly undergo decay; silk and woollen substances becoming putrid; or swarm with maggots and worms; and cotton and linen at first lose their white colour and assume a dingy or yellowish hue, and afterwards are covered by an efflorescence, and soon afterwards their cohesion and organisation are completely destroyed. These effects are generally rapid and complete in proportion to the moisture and warmth of the air; and as the conditions shown to be productive of malaria may be in full force, and its presence consequently more abundant.

Thus far then with respect to the nature and effects of malaria; let us now briefly survey those circumstances which favour its action on the body. All persons exposed to terrestrial exhalations are not equally affected by them; or there are certain conditions which favour their operation on the human frame. These are generally whatever debilitates and increases the susceptibility and excitability of the system,-such as previous disease; previous exposure to excessive heat, and to the direct rays of the sun; great bodily exertion when so exposed, particularly during a close state of the air; poor diet; the use of salted provisions, and of spirituous liquors; want of sleep; excessive evacuations; debauchery, &c. The indirect debility proceeding from the use of stimulating and heating liquors, sauces, and articles of food, frequently predispose the system to the influence of marshy exhalations, and the excite

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ment which they primarily occasion generally creates a disposition to diseases of the liver and spleen, and, in cases where such disposition is already formed, kindles active disease of these viscera.

Excessive indulgence of any of our appetites is always followed by satiety, debility, indolence, and a languid and sometimes an oppressed state of the circulation. At these periods, the nervous influence is as much diminished as the energy of the circulation is oppressed and weakened. Under such circumstances, both the nervous and vascular systems are more easily invaded by those causes of disease which made their immediate impression in these quarters, and are more readily influenced thereby. Impressions, also, made primarily upon the nervous system, are necessarily extended to the vascular system, which in its turn becomes deranged, and thus disorder is extended to all the general tissues and organs of the body to a greater or less extent. But of all the circumstances favouring the impression of morbid causes upon the body, there is none whose influence is more marked than the depressing passions of the mind. This has been evinced on a large scale in many of our campaigns in the East, and in our expeditions for the conquest of colonies within the tropics; and the beneficial effects of hope, excitement, and constant employment, in warding off the impression of the efficient causes of disease, have been fully demonstrated, as well as the baneful consequences of disappointment, anxiety, and the other lowering influences fully shown.

Of the various circumstances favouring the invasion of the causes of diseases, there are few more general in their operation, or influential as respects the resulting effects, whether considered in relation to warm climates, or to temperate countries during hot seasons, than indolence. Inactivity of mind has a most relaxing effect upon the nervous system, exposes it to the impressions of external causes, whilst it leads to inactivity of body; and both combine to generate plethora, diminish secretion and excretion, and thus to derange the functions of the internal organs and to vitiate the whole mass of fluids, and, through them, to disorder all the functions and secretions of the body. Indolence of mind also, and the want of salutary recreations and employments, throws individuals often in the way of many hurtful seductions and indulgences, which otherwise would not have been thought of.

On the other hand, employments and pursuits which interest the mind, and impart a salutary excitement throughout the whole frame, tend in no small degree to ward off the invasion of the usual causes of disease, and to promote all the more important animal functions.

Before concluding these brief remarks, it should be observed, that during sleep the body is more open to the influence of malaria, and, indeed, of all those causes of disease, which, floating in the atmosphere, are received with it into the lungs ; and the predisposition is the greater during the sleep which follows great excitement and exertion. Much previous exertion, particularly under exposure to the sun's rays, exhaust the powers of the body; and in this state of exhaustion and of sleep, noxious emanations of every description make a deeper impression upon the system. Experience of this has sufficiently taught the inhabitants of districts adjoining swamps or low marshy situations, in Italy and Greece, to retire at night to the hills and elevated places; for at this period, malaria is more concentrated, and the body, relaxed and overwhelmed by sleep from the fatigues of the day, more disposed to become affected by it. Soldiers and sailors, whose occupations, habit of body, and modes of living, are such as to render them highly disposed to the invasion of marshy exhalations during sleep, seldom are so exposed to them, particularly when on wooding and watering duties, without becoming the subjects of fever or dysentery. Proofs of this are of daily occurrence within the tropics, more particularly in the East and West Indies, and on the coast of Africa; and the danger of sleeping when travelling at night, during the autumn, in any of the unhealthy districts of Italy, is well known to every traveller in that country.

SECT. IV. On the Means of preventing the Generation of Marsh-Miasmata, and of counteracting their Effects upon the Human Body.

In treating of the circumstances productive of malaria, I have already had occasion to refer to several points belonging to this division of the subject; I shall therefore now confine my observations on the means of preventing the generation of malaria, chiefly to the advantages resulting,

1st, From draining all marshy places where this means can possibly be resorted to. The good effects of draining are too obvious to require illustration. It is only requisite to state, that situations requiring to be drained, will be rendered healthy in proportion to the extent and perfection of the measure, and to the clean state in which the drains are kept. In warm climates, the difficulty of draining is greatly increased, as well as of keeping the drains themselves in due order; yet the benefit resulting from the measure

is obvious. It should, however, be recollected, that marshy grounds, reclaimed by draining, are not altogether exempt from the causes of disease, more particularly in certain seasons; for the drains and ditches themselves cannot always be kept in a state altogether incapable of generating malaria. But disease is greatly diminished by the measure, and the soil is thereby rendered susceptible of cultivation, and subservient to the better support of its inhabitants.

2dly. It is in many places obviously impossible to put draining into practice, owing to their low situations, and to the inundations of rivers or of the sea, to which they are liable. Under such circumstances, embankments may be often resorted to with advantage. But the exact conditions requisite to the obtaining of salutary effects from embankments should be considered, and due regard should be paid to the question, whether the embankment will tend to prevent the flowing off of the water after heavy rains, and whether it may not be more injurious in retaining inundations when they occur, than beneficial in preventing them. Every thing in relation to this, will depend upon the exact circumstances of the locality, and the manner in which they are viewed by those who wish to control them.

3dly. In situations where neither draining nor embankments can be resorted to, as in fens, swamps, and bogs, in the depths of valleys and along the banks of rivers, or near æstuaries, then advantage will often be derived from covering them entirely with water: for lakes do not exhale miasmata until the mud and soil of their bottoms and sides appear. It has often occurred in various parts of the East, that little disease, and that of a slight intermittent form, was prevalent whilst the sources of malaria were completely inundated; but that as soon as the water was evaporated, and the soil exposed to the sun, covered by slime and vegetable and animal matter, fevers of a severe form made their appearance. Numerous instances of this have been recorded by writers on the diseases of the West Indies. Dr. Rollo mentions, that in St. Lucie the greater part of the regular and mild intermittents appeared during the rains, when the marshes and pools were filled, and that the dangerous fevers made their appearance when the water covering these places was evaporated, leaving a bare and slimy surface to the sun's rays; and precisely similar facts were observed at Seringapatam. The ditch round the ramparts of Geneva was at one time drained, and sickness became prevalent in the vicinity, and continued so until it was again filled. But it is unnecessary to enume

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rate instances illustrating what is already obvious: for water covering the sources of malaria to any considerable depth, has the effect of keeping them from the action of the sun and of the air, and of absorbing whatever of this deleterious substance may be generated.

4thly. Cutting down woods and forests, and clearing the soil, so as to subject it to assiduous cultivation is another means of preventing the generation of malaria. Yet, as has already been shown, this means of preventing the generation of marsh poison is often productive of it in a more concentrated form. This is more especially the case within the tropics, in low swampy places near the sea coast. Dr. Rush states, that in Pennsylvania intermittents have increased in severity and frequency since that country has been cleared. Fever was more frequent and severe after clearing Penang in 1801 and 1802; and the same has been remarked with respect to the West Indies and other places of America: so that it may be considered as a general approximation to the truth, that low and marshy situations become still more unhealthy when the trees and woods upon or around them are cut down, unless they are subjected to a careful drainage and cultivation; and even for the first two or three years of such culture, they are at particular seasons productive of disease. As respects, however, clearing woods or forests, covering sloping grounds, dry situations, and the sides of hills, there can be no doubt of the salubrity of the measure, and particularly when these places are afterwards subjected to due cultivation.

The measures which should be taken, after having resorted to those which are calculated to prevent the generation of malaria, are those that tend to confine it to the source that generates it. In many places, neither drainage nor embankments can prevent the formation of terrestrial exhalations. Under these circumstances, it will be found advantageous to cover or surround these sources of disease by tall forest trees, which will confine the exhalations which they generate within their own limits. Where towns or cities are built upon soils and in situations which render them unhealthy, very great advantage will be uniformly derived from making large and deeply situated sewers and drains, which should be well built and arched over, and from attention to paving, and to the constant removal of the mud and filth which acccumulate in narrow lanes and streets. It appears also established beyond a doubt, that the narrow winding streets of many of the towns and cities which are built in low situations and in the vicinity of marshes and lagoons, are, especially when the houses are high, actually conducive to