quently occurring within the tropics, are the means which Nature resorts to in order to dilute or entirely dissipate the exhalations proceeding from, and accumulating in, the places similar to those noticed and if hese more violent commotions of the atmosphere were not to supervene frequently, very many districts of country would be rendered uninhabitable, both by man and by many of the higher animals. Indeed, it has been often observed, that when the air has continued long undisturbed by winds or thunder-storms, and have been long hot and moist, disease assumes a pestilential character. Of this fact medical histories furnish numerous examples. The very remarkable epidemic fever which ravaged the West Indies and the coast of America was ushered in and accompanied, according to the testimony of Rush, Chisholm, Clark, Devèze, and others, by long continued droughts, a still and humid state of the air, and a most unusual absence of thunder-storms and hurricanes. Various other instances might be adduced of a similar relation existing between the states of the air and the prevailing diseases, both within and without the tropics. Indeed, every practitioner who has enjoyed any experience in warm climates, has had numerous opportunities of observing the manner in which the character of the prevailing diseases, and their propagation, have been influenced by the prevalence or absence of thunder-storms and tornadoes. It would even seem that the frequency of these commotions of the atmosphere, as well as their violence, have a very intimate relation to the unhealthiness of a country. Experience has long convinced me that this relation actually subsists; and that it should exist as a general result, is not to be wondered at, since the same conditions and circumstances on which the unhealthiness of a climate depends are also those which give rise to thunderstorms and hurricanes, and which render their frequent occurrence necessary to the health of those by whom the country is inhabited. The western coast of Africa could not be endured, even for a season, by the European inhabitant, and perhaps scarcely long by its natives, did not the Harmattan wind occasionally blow, and tornadoes of most surprising violence occur frequently during the most unhealthy seasons of the year. A similar remark may be made respecting the hurricanes of the West Indies, and the tornadoes and storms which frequently blow in the East. The dense, moist, hazy, and close atmosphere, loaded with the exhalations of putrid insects and reptiles, and of the soil and its vegetable productions, after remaining for a time still and suffocating, enervating those who are destined to breathe it, and infecting their circulating

fluids, suddenly becomes kindled into the most vivid commotionsweeping before it whatever opposes its progress, and blazing out in one ocean of flame, which seems momentarily extinguished by the torrents of rain which rush furiously to the earth, and is immediately again lighted up to its greatest brilliancy and widest extent; so that the atmosphere presents the most extensive and the most sublime conflict between fire and water which the imagination can paint, whilst the irresistible force of the winds seems to sweep both combatants from the field.

The general result of these frightful commotions is to destroy those exhalations which have accumulated in the depths of forests and ravines, and in the bottom of valleys, to purify the air, and to refresh the entire offspring of nature. Their beneficial effects can. not fail of being recognised by the observing practitioner, whilst the unwholesome tendency of that climate or season on which their frequency depends, becomes equally apparent to his notice. But the circumstances which give rise to them would be productive of greater mischief if they did not supervene; and therefore their occurrence is to be looked upon as a benefit, or at least as a lesser evil, calculated to counteract a much greater one, which would inevitably occur if the other did not prevent it.

Besides the influence of humidity in the generation and accumulation of malaria, the operation of the electrical fluids should be taken into account. But to what extent, or in what manner these fluids act, we have no precise information. That they have some action, however, is most probable, inasmuch as their active agency in the animal economy is evident, and as unhealthy states of the air have been observed to have been related to derangements in the electrical conditions, or in the equilibrium of the electric fluids. It is also most probable that the electricity proceeding from the earth at particular parts may so influence the condition and extrication of the gases and the effluvia which the soil yields in those places, and may so combine with them, as to form the efficient principle or cause of disease. Of this, however, we have no positive knowledge; but the review of various states of the air and of the seasons, in connexion with the circumstances of the soil, which have been noticed as productive of disease, and both considered in relation to what information has been furnished respecting the electrical conditions at the time, seem to point out some connexion beyond mere coincidence: but in what that connexion consists, or whether it is to be considered as one of cause and effect, it is not easy to decide. Much requires to be observed

with respect to this important subject, particularly within the tropics; and much of the credit which is to be placed on observations respecting it will depend upon the talent and acquirements of the observer, and upon the means of observation which he shall employ.

SECT. II. On the Nature, Properties, and Effects of Miasmata.

The intrinsic nature of that matter usually denominated marsh miasm and malaria, and which has so powerful an effect upon the human constitution, that more than two-thirds of those who die in warm climates are cut off by its influence, has never been satisfactorily shown, although repeated attempts have been made to ascertain it. Dr. W. Currie supposed that marshy exhalations consist of hydrogen and ammoniacal gases, and that the unhealthiness of low and marshy places arises from a deficiency of oxygen gas in the atmosphere; but eudiometric experiments have proved that the air over those situations contains its due proportion of oxygen, and that it is unwholesome owing to the presence of some foreign substance. Subsequent experiments, performed by Gattoni, Moscati, Brocchi, and Julia, have shown that marshy soils give out carburetted hydrogen; but their effects cannot be imputed to this gas, as it has been found incapable of producing them. From the numerous experiments which have been made on the continent, in order to ascertain this point, it must be inferred, that we have no knowledge of the intrinsic nature of the substance which, vitiating the air in the vicinity of places which have been described, produces most baneful effects upon the human constitution. But a careful comparison of the effects which it produces, and a review of its phenomena, and of the laws which it seems to observe, may render the following remarks deserving of attention.

The sum of our knowledge of the nature of this poison seems to be, that it proceeds from those elements which exist in a rich soil and nourish the vegetable and animal kingdoms; and that these elements, when subjected to the action of the sun, together with the influence of the air and moisture, form new combinations, which are volatilised by the sun's heat, and readily combine with the moisture present in the lower strata of the atmosphere. This conclusion seems to be supported by the physical and medical history of terrestrial exhalations, as far as they have been investigated. That they combine with aqueous vapours either during the period of their formation, or when they rise into the atmosphere, is proved by various circumstances. Whilst the vapours exhaled by the heat

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of the sun, and carrying with them the miasms given out by the soil, are kept in a state of rarefaction by the sun's influence, and conveyed to the higher regions of the atmosphere, then the usual effects of these miasms are not produced. But as soon as the heat diminishes so far as to permit the vapours to descend and to become condensed towards the earth's surface, then the effects resulting from terrestrial effluvia become manifest. In the former case the effluvia are rarefied, along with the aqueous vapour, so that they are incapable of making a hurtful impression on the system when received into the lungs in the latter case they are condensed with the vapour, and precipitated to the lowest stratum of the atmosphere, and are inhaled into the body in a larger quantity, and sufficient, in many instances, to produce effects according to the circumstances of the individual. Hence it is that the evening and night dews are much more to be feared in all districts and countries subject to terrestrial effluvia than any other period. This precipitation of the aqueous vapour and malaria forming mists and dews, particularly in low situations, is especially to be dreaded during still or stagnant states of the atmosphere. Within the tropics the earth retains its heat, after the sun has gone down, longer than the air. This circumstance tends to the accumulation of effluvia upon the surface; for, as their extrication from the soil proceeds to a considerable extent after the sun has gone down, they are no longer rarefied into the higher regions of the atmosphere, but are mixed with the dews and vapours descending at the same time; so that the sources whence exhalations are formed, being generally the lowest, are also the reservoirs wherein they accumulate. In warm climates their accumulation is in some degree prevented during the land winds which blow through the night. But these winds are frequently too light to sweep away the exhalations, which are collected in ravines and sheltered valleys; and when they are sufficiently strong for the purpose, they only serve to convey them to other places. It is chiefly during the calms which precede and follow the night winds that the accumulation of effluvia in low places takes place; and as these calms, with their attendant vapours, are usually about sunset and sunrise, exposure at those times should be avoided.

Malaria has been considered, in addition to its possessing the property of combining with aqueous vapour, as being specifically heavier than atmospheric air; and its presence and accumulation near the surface of the soil which produced it, has been attributed as much to this property as to its combination with moisture. Of

this, however, there is no direct proof, although the circumstance in question seems very probable from the following facts. It is well known to the inhabitants of Italy, and to all who have travelled in that country, or who have been any time within the tropics, that elevation above the sources whence malaria proceeds furnishes exemption from its influence, and that the exemption is in proportion to the height of the elevation. In many districts of Italy and Greece the villages are built upon elevated rocks, or hills rising abruptly above the surrounding low grounds, in order to avoid the miasmata which they generate. Sezza, which is beyond the reach of malaria, is about three hundred yards above the Pontine Marshes; and Tivoli, which is elevated about a hundred yards above Rome, is much healthier than this city. According to Humboldt, Encero, situated above Vera Cruz, is not affected by the diseases which render this port and its adjoining coast so much dreaded. This writer states that 920 yards, the elevation of Encero above the sea, is the highest limit of the yellow fever, the product of the most concentrated state of malaria. Dr. Hunter has stated that even the elevation of one floor is sufficient to afford a very great exemption from the diseases arising from terrestrial exhalations. He found that the number of cases of fever occurring on the ground floor of the barracks near Kingston, Jamaica, were, in relation to those affected in the first floor, as three to one. Similar facts have come under my own observation in various parts of the East. Whether these circumstances are to be explained by the affinity which the effluvium has to aqueous vapour, or by allowing it to possess a great specific weight, does not materially influence the facts which have been adduced, nor the much greater number more which might have been mentioned; nor are the inferer.ces which may be drawn from them materially affected by adopting either opinion. Dr. Ferguson has supported a third opinion, different from the two now referred to, and supposes that the marsh effluvium has a strong attraction to the surface of the earth, which tends to keep it near the situation whence it proceeds; but although places are generally more exposed to the effects of malaria in proportion to their lowness, yet the prevalence of certain winds, and other circumstances, occasion many exceptions to this as a general inference.

It would be interesting to ascertain the greatest height at which marsh exhalations produce their effects. My observations, added to those recorded by various writers, have furnished results so different, as to lead to the conclusion that the elevation from its source