where similar localities are the more active agents of disease. Analogous proofs are furnished throughout Europe, particularly in its southern provinces, and in North America, whenever long and warm summers have succeeded to heavy and continued rains; and diseases possessing more or less of the true characters of intertropical disorders have prevailed to an extent proportionate to the nature of the locality and the concurrent circumstances, and would have continued to prevail had not the approach of winter put an end to the generation of the causes producing them.

A most important circumstance, which goes far to account for the much greater unhealthiness of moist and marshy situations in warm countries, is the quantity of animal matter, in a state of decomposition, which they present. The same circumstances which render vegetation quick and luxuriant, tend also to generate immense swarms of reptiles and insects: the exuvia and dead bodies of which, mingling with vegetable matter in a state of decay and combining with moisture, give rise to miasms of a much more noxious description than those resulting from vegetable decomposition and moisture alone. In the course of my experience in warm climates, I have always been disposed to consider the number of insects and reptiles with which a place abounds, as more indicative of its unhealthiness than any other circumstance; for in it there is a most powerful cause of disease in its worst forms superadded to those already in existence; and, as the one cause is extensive and powerful, so, generally, is the other. The great unhealthiness of low, moist, and marshy places in temperate climates, during warm seasons, particularly in the months of July, August, September, and October, is as much owing to the immense swarms of insects which then abound, and which die during these months. Italy furnishes numerous proofs of this; and every warm country in the globe will verify the axiom, that a place is unhealthy in proportion as it furnishes, with the various causes of disease depending upon locality and temperature, animal remains and animal substances in a state of decomposition, mingled with the products resulting from the decay of vegetable

matter.

Experience has shown that in pits and mines the air is often in such a state as to suffocate, almost instantaneously, those who attempt to breathe it. Some places are infested by peculiar diseases. In the apartments of persons ill of certain maladies, and in prisons and other places where crowds of people are confined together, disease, when once set up, is wont to make dreadful havoc. In all

these cases it is supposed that a certain noxious matter is dissolved by the air, and that it is the action of this matter which produces the mischief. This noxious matter is in many cases readily distinguished by the peculiar disagreeable smell which it communicates to the air; and it is probable this matter differs according to the disease which it communicates, and the substance from which it has originated. Guyton de Morveau attempted to ascertain its nature, but he soon found the chemical tests hitherto discovered altogether insufficient for the purpose; he has, however, put it beyond a doubt that the noxious matter which rises from putrid bodies is of a compound nature; and that it is destroyed altogether by certain agents, particularly those gaseous bodies which readily part with their oxygen. He exposed air infected by putrid bodies to the action of various substances; and he judged of the result by the effect which they had in destroying the foetid odour of the air. From his experiments it appears there are four substances which have the property of destroying contagious matter, and of purifying the air, the acetic, nitric, muriatic and oxymuriatic acids; but acetic acid cannot easily be obtained in sufficient quantity, and in a state of sufficient concentration to be employed with advantage. Nitric acid may be attended with some inconvenience, because it is almost always contaminated with nitrous gas. The muriatic and oxymuriatic acids are not attended with these inconveniences. The last deserves the preference, because it acts with greater energy and rapidity, and all that is necessary is to mix together two parts of common salt, with one part of black oxyde of manganese, to place the mixture in an open vessel in the infected room, and pour upon it two parts of sulphuric acid; the fumes of oxymuriatic acid are immediately produced, fill the chamber, and destroy the contagion. The oxymuriate of lime, mixed with sulphuric acid, will likewise answer this purpose.

The copious extrication of unwholesome effluvia from salt marshes and partial inundations of the sea, has been long admitted, and has only been disputed by one writer of eminence, who instances, in disproof of the position, the salt marshes of one particular district in the western hemisphere. But there, it is probable, some peculiarity existed in the soil and its productions, which rendered the formation of malaria impossible. The soil may have consisted of a deep bed of sand or gravel, but imperfectly covered by vegetation. Under such circumstances, unwholesome effluvia could scarcely be formed; for vegetable decay could neither be so rapid, nor the products from it be so copious, as to generate the

C

principle which is so productive of mischief. The salt water, also, during its passage through the sandy stratum, would become mixed with fresh water only during the prevalence of rains, and would be so filtered in its course as to be deprived of those animal products which are always so abundant in sea water, and which accelerate putrefaction when it is stagnant.

If, in opposition to the opinion thus brought forward, we inquire into the state of our experience of the matter, we shall find, that, in every instance where the soil is deep, of a rich, dark or clayey mould, or in any respects absorbent, and still more so if it be covered by a rich, rank, or succulent vegetation, and not admitting of a speedy drainage of the waters which may inundate it, insalubrious exhalations are copiously formed under the influence of a warm sun and a moist and stagnant state of the air; and that upon all occasions, when such soils have been inundated by the sea, as from the breaking down of embankments, &c., the formation of deleterious effluvia has become most abundant, under the favourable circumstances mentioned above, and has been even the source of a most devastating pestilence. In proof of this, may be mentioned the noxious situations and salt marshes at the mouths of the Ganges, the Irrawaddy, and the Indus, and numerous other places in the East.

The effects resulting from the inundations of the ocean are not, perhaps, referrible so much to the circumstance of a small quantity of salt proving a septic, as is supposed by many; for it seems evident that the antiseptic properties of salt are in proportion to its quantity, and that a small portion will not have a septic tendency, because a large one has an opposite effect. The subject has not received its deserved attention; and authors have, in respect to it, been more prone to copy the suppositions and admissions of their predecessors than to examine into the grounds upon which the opinion is founded. That sea water mixed with fresh water and vegetable matter in a state of decay, will increase the generation of effluvia, under the influence of a powerful sun, and render them more concentrated, seems to be the fact. But this result seems to be owing to the quantity of animal matter sea water contains, which occasions it to run faster into putrefaction than fresh water, when subjected to a warm temperature and kept at rest. Much is also owing to the lowness of the situations where inundations take place, and the quantity of vegetable and animal matter in a state of decay which such situations contain. The exhalations proceeding from these places, whether within the tropics or in temperate

regions, during warm seasons, are generally more noxious during very moist states of the air-a condition always present in warm climates; and they are still more particularly hurtful when they have been collecting for a considerable time, owing to the continuation of calm weather, and the absence of thunder-storms, or those more violent atmospherical vicissitudes which are so beneficial in sweeping away the exhalations accumulated in low and narrow ravines, and among the thick underwood of intertropical regions.

The next great source of insalubrious exhalations, are dense and low jungles. Places covered by this species of vegetation are so numerous in all countries within the tropics, that it would be quite endless, as it is unnecessary, to enumerate them. Not only are low situations, but also the sides of hills, covered by this, of all the worst, species of vegetation. Its thickness, exuberance, and the succulency of the plants shooting between the dense brushwood and reeds, offer a constant supply of decayed parts as the unremitting vegetation proceeds, and prevent the sun from reaching the soil; consequently, the roots, the creeping and lower plants, and the decayed parts of all of them, and the exuviæ of myriads of insects and reptiles, are immersed in a moist, rich, and absorbent soil, and a moist and stagnant atmosphere, which being seldom renewed, is thereby loaded with the accumulated exhalations given out from these productive sources. If, in addition to these circumstances, others also of powerful influence in the generation of unwholesome air be added, such as a low confined position between hills, &c., the formation of malaria must be necessarily still more accelerated. Places of this description are presented in every district in warm climates, and furnish us with numerous instances of their bad effects upon the human constitution, and upon the health of Europeans particularly, when they are not avoided, or when exposure to them takes place at improper seasons and under predisposing circumstances. Indeed, when the exact relations subsisting between the soil and the vegetation in situations now under consideration are examined into, they will be found nearly the same as those which marshes most usually present. In jungly places, also, there is seldom any complete range of large or majestic forest trees, which, in temperate climates, frequently skirt the margins of marshes and the low banks of rivers, and confine miasmata to the source whence they arise, and screen the adjoining neighbourhood from their effects. On the contrary, the more stately productions of the soil spring up but rarely, and at con

siderable intervals from each other, among the thick and low brushwood constituting the jungles of warm countries.

Nor are the more extensive forests unproductive of those exhalations which are the chief sources of intertropical diseases; for they frequently present nearly the same circumstances upon which the generation of miasmata depends; and whenever these circumstances are favourable, disease is the usual consequence. The quantity of the decayed leaves with which the soil abounds, its moist state, the moist, hot, and stagnant state of the air, particularly after the rains or monsoons, are the conditions upon which the generation of malaria by forests depends, and which are the frequent causes of fevers. Whilst, however, jungles more nearly approach to the condition of a marsh, and permit the transport of the exhalations to some distance from their source, unless circumscribed by screens of tall trees, forests confine the exhalations they generate to their immediate limits, and seldom permit any to rise above the verdure of their highest branches, or to extend beyond their outskirts. The free circulation of air in places on the confines of a wood, or even around the outskirts of the wood itself, renders such situations even healthy, in comparison to the interior of a dense forest. Much will, however, depend upon the locality, and upon the kind of trees forming a wood or forest. In warm climates forests are met with in a great variety of situations,— covering the sides of mountains and the tops of hills, as well as extending into the plains and valleys. It is chiefly in the latter places where woods become productive of malaria; for there the atmosphere is more frequently stagnant, particularly when they are protected from the full force of prevailing winds by intervening hills. In these latter places, also, the soil and air are much more moist; hence the ground intervening between the large trees, forming a wood or forest is often covered by a rank and luxuriant vege. tation; the decayed parts of which, with the leaves fallen from the forest trees, speedily generate in the moist soil very unwholesome emanations, which may be limited to the precincts of the wood wherein they were produced, or wafted to some distance, according to the circumstances of the situation, climate, and season. Forests, also, in warm climates, are always, particularly in low and moist situations, in a state of vegetation, so that the soil is thereby more completely sheltered from the winds, and from the sun, and the air within them is more frequently sultry, moist, and stagnant, than is the case with the woods or forests of temperate or cold climates, where there is at one season in the year a complete denudation of the trees.