at which malaria may arrive, so as to be productive of disease, will depend upon the general temperature of the climate and of particular seasons, and upon the humidity of the atmosphere, and the prevalence and force of certain winds, and very materially upon the quantity of malaria which any particular part yields.

It should, however, be mentioned, as an exception to the position which the preceding facts seem to prove, that there are circumstances which render some elevated situations even more unwholesome than places near the margins of marshes. This is owing to the attraction which fogs and vapours have, as they rise from low valleys and marshes, to the ridges and sides of hills which overhang them. This is shown by the heights which rise out of the marshes at Port d'Espagne, in Trinidad. Similar instances are afforded by the island of Dominica, Jamaica, &c. in the West Indies, and by several places in the East. Other situations, also, both considerably elevated above and retired from the sources of malaria, are affected by it to a very great extent; but this is entirely owing to the prevailing winds passing over them, particularly those which blow during the night, without having been broken in their course by any abrupt ascent or ridge of rocks, and without having been deprived of their malaria by passing over woods or forests, by the foliage of which it is evidently attracted.

Malaria, as already observed, seems to be attracted by screens of trees, which often serve as one of the most efficient means of circumscribing limits to the baneful effects of this agent, when its source cannot be destroyed. This fact was turned to advantage in former times. It should also be known, that this noxious exhalation seems to be absorbed in its passage over water. This has been proved in numerous instances. Places situated on the healthy and dry banks of lakes or large rivers have experienced but little inconvenience from the marshes bordering their opposite banks, whilst districts much further removed by land from these sources of disease suffered from their vicinity. Nor was this marked difference in the effects owing to the prevailing winds; for whatever advantage this circumstance was capable of producing, was in favour of the latter place, and against the former. That malaria, or marsh-miasm, is actually absorbed during its passage over water, is further proved by the exemption of crews of ships anchored on coasts abounding with this poison, and lying in rivers sufficiently wide to allow them some distance from their marshy banks. This, viewed in connexion with another circumstance which should be recollected in an examination into the phenomena which this agent

of disease evinces, and which will come under immediate consideration, may furnish some useful hints to those who may have, as all practitioners in warm climates will have, opportunities of turning their knowledge of this part of the subject to a practical advantage.

With respect to distance from its source, at which the marshpoison affects the human constitution, no precise opinion can be formed: but there seems no reason to doubt that it is much diluted, or weakened in its effects, according to the distance to which it is transported. Thus it has been frequently demonstrated, that fevers have been continued or remittent, of the worst type, in low situations and near the sources of malaria, whilst in places removed from, or elevated above, these sources, fevers were either of a mild remittent or intermittent form, the disease being of a milder grade in exact proportion to the distance and elevation of the place from the source of malaria. Instances in proof of this position are recorded by Sir John Pringle and many other writers, and are of frequent occurrence, during the summer and autumn, in Holland, in Hungary, and in Italy. In the East and West Indies, proofs of this may be daily obtained; and they have constantly presented themselves in the Eastern hemisphere.

As the severity of endemic and epidemic disease appears to be chiefly owing to the concentration and activity of the cause which produces it, relatively to the predisposition, strength, and particular circumstances proper to the patient; so it may be said that, in a general average, the severity of the disease affecting many individuals is to be considered as an index to the concentration or intensity of the causes inducing it; and hence, that the mildness of a disease occurring amongst great numbers of men similarly circumstanced, will indicate a dilution or weakness of the cause whence it springs. From these results, therefore, it is chiefly to be inferred, that the malaria is diluted or weakened as it becomes diffused in the atmosphere, or transported from its source; and that it is so weakened in proportion as the dilution is promoted by the vertical currents induced in the air by means of the sun's rays, the dilution becoming still greater as its admixture in the air is farther facilitated by free ventilation, until its bad effects entirely disappear. It must be evident that this general result-the distance to which malaria may be transported from its source, and yet be productive of disease-will vary much, according to the vicissitudes of temperature, the general character of the season, the prevalence of certain winds, and the absence of others, and most materially as the quantity of vapour

existing in the atmosphere may change, the source of malaria be more or less productive, and the obstacles in its course more or less easy to be surmounted. As a general inference, however, it may be mentioned, that malaria cannot be productive of severe disease at a considerable distance from its source.

It has already been stated that malaria is much obstructed, and in many instances almost entirely intercepted in its passage over woods, closely planted rows of trees, or forests, owing to its being attracted by their foliage; or over rivers, lakes, and inlets of the sea, by its being absorbed by them as it is wafted on their surface. The suburbs and the walls of a city or town also serve to intercept this poison in its passage from its source, as is well known to the physicians in Italy, and to those who have had sufficient experience in a warm climate. Even the buildings of one end of a narrow or crooked street have been known frequently to prove the means of exemption to those of its other extremity from the effects of this substance, owing to their intercepting it, as it were, whilst it is being conveyed by night winds from adjoining swamps or rice fields. Numerous examples of this fact are on record in the works on malaria, and similar instances have frequently come under the observation of those who have enjoyed opportunities of experience within the tropics, or even in the south of Europe. The West India Islands, and many places in the East, particularly Batavia, Seringapatam, &c. furnish numerous examples of the fact. There is even good reason to suppose that the straightening and widening of streets, in towns or cities exposed to the influence of malaria, either from swamps in their vicinity, or from the nature of the soil on which they are built, are unfavourable to the health of their inhabitants; for by these measures, the winds which blow over the adjoining sources of malaria, and become saturated with this poison, have a more ready access to all quarters of a city; and the open streets allow the sun to act upon the soil on which they are built, which, if not well protected by a good pavement, and by drains and sewers, will thereby generate exhalations, which, added to those transported from sources in the vicinity, will materially injure the health of the inhabitants. That this effect was actually known to the ancients, seems apparent from the remark of Tacitus on the rebuilding of Rome after its destruction by Nero.

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Ex eâ utilitete acceptâ, decorem quoque urbi attulere. Erant tamen qui crederent veterem illam formam salubritatem magis conduxisse, quoniam angustia

SECT. III.-On the Effects of Malaria upon the Human Constitution.

The effects of malaria upon the human constitution may be considered to be in proportion to the quantity given out by the soil, to its concentration, to the warmth of the season and climate, and to the humidity of the atmosphere. All these circumstances materially influence the production of malaria: they possess also an additional influence, namely, that of favouring its noxious operation upon the frame, by heightening the susceptibility of the subject. These, along with other causes which dispose the system to this invasion of the marsh effluvium, will come under notice when I come to the various causes which co-operate with it in the production of the diseases depending, either altogether or partly, upon this very efficient agent. At present it will be sufficient to point out those diseases, which, occurring within the tropics may be considered as more or less less the effects of terrestrial exhalations.

With respect to intermittents and remittents, there can be no doubt that malaria is their efficient cause, although other causes may have predisposed the system to their supervention, or may have co-operated with malaria in occasioning them. The severity of these diseases is generally in proportion to the warmth of the climate or season in which they occur; and it has been inferred, with much appearance of accuracy, that the more severe forms of these types of fever are owing to greater concentration or activity, in hot climates and seasons, of the malaria producing them relatively to the susceptibility and other peculiar circumstances of the individual affected. It also appears that, in situations which are evidently productive, owing to their peculiar conditions, of a concentrated kind of effluvium, and particularly if animal matter combine with decayed vegetation in its formation; and if this effluvium is generated in a low, moist, deep, and rich soil, which has been subject to inundations, by a powerful sun, and during a moist, hot and stagnant state of the air-circumstances combining to generate a more concentrated and malignant form of malaria-remittent and continued fevers of a most severe form, of a malignant tendency, and most dangerous as regards their issue, are the result. Even yellow fever, in its worst forms, seems to be the consequence of these

itinerum et altitudo non perindè solis vapore, perrumperentur, ac nunc patulam latitudinem et nulla umbra defensam graviore æstu ardescere.-TACIT. Ann. lib. xv. 43.

causes operating, in a state of great activity or concentration, upon highly disposed subjects.

In situations also where the noxious influence of malaria is increased by the free admixture of the miasms proceeding from animal matter in a state of decay, or where the decay of animal or vegetable matter goes on rapidly together, and when they are moreover reinforced by the exhalations elicited from the lower strata of the soil by the action of the sun's rays, then the effects produced upon the human constitution are of the most malignant description. That the worst forms of fever proceed from this cause cannot be denied; for observation has traced their origin thither in almost every instance; and even the plague, when it breaks out in Cairo, seems to depend upon a similar cause for its origin, although others may combine with it for the full production, development, and diffusion of this form of disease. It has been noticed by Prosper Alpinus, and others who have had the best opportunities of observation, that the plague frequently breaks out in Cairo after inundations of the Nile, which have exceeded their usual bounds, and have left a quantity of slime, with vegetable and animal matter in a state of decay, beyond the reach of the drains and canals.

the

The fever proceeding from marshy effluvia, and from the miasms generated by vegetable and animal matter, assumes various forms or types, according to the particular circumstances in which they originate, whether conditions of the locality, the nature and concentration of the miasm, the warmth of the climate or season, state of the air, or the peculiarity and state of predisposition of the individual affected. As these vary, so does the particular character of the disease; and accordingly we have intermittents of various types and grades ;-remittents of every degree of severity, and variously characterised; some assuming the bilious character, others the inflammatory; some being both inflammatory and bilious, and others being malignant and quickly fatal, or assuming a typhoid and putrid form towards their close ;-continued fevers of every form, grade, and complication, mild in the one case, inflammatory in the other; in this case, marked by excitement at its origin, and soon terminating in depression; in the other case, beginning mildly, but insidiously, and terminating in great local derangement and disorganisation; and in a third instance, marked by great vital depression, which never rallies, by a most offensive and unnatural state of all the secretions and excretions, and a tendency to putrefaction, which rapidly advances the moment that respiration ceases: and lastly, as respects fever, we have intermittents of a severe type run