1. It appears only in those climates and seasons of the year in which heat, acting upon moist animal and vegetable matters, fills the air with their putrid exhalations. A vertical sun, pouring its beams for ages upon a dry soil; and swamps, defended from the influence of the sun by extensive forests, have not, in a single instance, produced this disease.
2. It is unknown in places where a connection is not perceptible between it, and marshes, mill-ponds, docks, gutters, sinks, unventilated ships, and other sources of noxious air. The truth of this remark is established by the testimonies of Dr. Lind and Dr. Chisholm, and by many facts in Lempriere's excellent History of the Diseases of Jamaica. Dr. Davidson furnished me with a striking confirmation of their remarks, in the following extract from a letter, dated November 12th, 1794. “I have mentioned (says the doctor) an instance of the remarkable good health which the 66th regiment enjoyed at St. Vincents for several years, upon a high hill above the town, removed from all exhalations, and in a situation kept at all times cool by the blowing of a constant trade wind. They did not lose, during eighteen months, above two or three men (the regiment was completed to the peace establishment), and during eight years they lost but two officers, one of whom, the quarter-master, resided constantly in town, and died from over fatigue; the other arrived very ill from Antigua, and died within a few days afterwards.”
In the United States, no advocate for the specific nature or importation of the yellow fever, has ever been able to discover a single case of it beyond the influence of an atmosphere rendered impure by putrid exhalations.
It is no objection to the truth of this remark, that malignant bilious fevers sometimes appear upon the summits of hills, while their declivities, and the vallies below, are exempted from them. The miasmata, in all these cases, are arrested by those heights, and are always to be traced to putrefaction and exhalation in their neighbourhood. Nor is it any objection to the indissoluble connection between putrid exhalations and the yellow fever, which has been mentioned, that the disease sometimes appears in places remote from the source of miasmata in time and place. The bilious pleurisies, which occur in the winter and spring, after a sickly autumn, prove that they are retained in the body for many months, and although they are sometimes limited in their extent to a single house, and often to a village, a city, and the banks of a creek or river, yet they are now and then carried to a much greater distance. Mr. Lempriere, in his valuable Observations upon the Diseases of the British Army in Jamaica, informs us, that Kingston is sometimes rendered sickly by exhalations from a lagoon, which lies nine miles to the eastward of that town[34]. The greater or less distance, to which miasmata are carried from the place where they are generated, appears to depend upon their quantity, upon the force and duration of currents of wind which act upon them, and upon their being more or less opposed by rivers, woods, water, houses, wells, or mountains.
3. It is destroyed, like its fraternal diseases, the common bilious and intermitting fevers, by means of long-continued and heavy rains[35]. When rains are heavy, but of short duration, they suspend it only in warm weather; but when they are succeeded by cold weather, they destroy all the forms of bilious fever. The malignant tertians, described by Dr. Cleghorn, always ceased about the autumnal equinox; for at that time, says the doctor, “Rain falls in such torrents as to tear up trees by the roots, carry away cattle, break down fences, and do considerable mischief to the gardens and vineyards; but, after a long and scorching summer, they are very acceptable and beneficial, for they mitigate the excessive heat of the air, and give a check to epidemical diseases[36].” There are facts, however, which would seem to contradict the assertion that miasmata are suspended or destroyed by heavy rains. Dr. Lind, in his Treatise upon the Diseases of Hot Climates, mentions instances in which they suddenly created fevers. It is probable, in these cases the rains may have had that effect, by disturbing the pellicle which time often throws over the surface of stagnating pools of water, and putrid matters on dry land. I was led to entertain this opinion by a fact mentioned in a letter I received from Dr. Davidson, dated November 4th, 1794. “Being ordered (says the doctor) up to Barbadoes, last November, upon service, I found that the troops had suffered considerably by that formidable scourge, the yellow fever. The season had been remarkably dry. It was observed, a rainy season contributed to make the season healthier, excepting at Constitution-Hill, where the sixth regiment was stationed, and where a heavy shower of rain seldom failed to bring back the fever, after it had ceased for some time. I found the barrack, where this regiment was, surrounded by a pond of brackish water, which, being but imperfectly drained by the continuance of the drought, the surface was covered with a green scum, which prevented the exhalation of marshy putrefaction. After a heavy shower of rain, this scum was broken, and the miasmata evolved, and acted with double force, according to the time of their secretion.”
4. It is completely destroyed by frost. As neither rains nor frosts act in sick rooms, nor affect the bodies of sick people, they must annihilate the disease by acting exclusively upon the atmosphere. Very different in their nature are the small-pox and measles, which are propagated by specific contagion. They do not wait for the suns of July or August, nor do they require an impure atmosphere, or an exciting cause, to give them activity. They spread in the winter and spring, as well as in the summer and autumnal months: wet and dry weather do not arrest their progress, and frost (so fatal to the yellow fever), by rendering it necessary to exclude cold air from sick rooms, increases the force of their contagion, and thereby propagates them more certainly through a country.
5. It is likewise destroyed, by intense heat, and high winds. The latter, we are sure, like heavy rains and frost, do not produce that salutary effect by acting upon the bodies, or in the rooms of sick people.
It is worthy of notice, that while the activity of miasmata is destroyed by cold, when it descends to frost; by heat, when it is so intense as to dry up all the sources of putrid exhalation; by heavy rains, when they are succeeded by cool weather; and by high winds, when they are not succeeded by warm weather; they are rendered more active by cool, warm, and damp weather, and by light winds. The influence of damp weather, in retaining and propagating miasmata, will be readily admitted, by recollecting how much more easily hounds track their prey, and how much more extensively odours of all kinds pervade the atmosphere, when it is charged with moisture, than in dry weather.
It has been asked, if putrid matters produce malignant bilious fevers in our cities, why do they not produce them in Lisbon, and in several other of the filthiest cities in the south of Europe? To this I answer, that filth and dirt are two distinct things. The streets of a city may be very dirty, that is, covered with mud composed of inoffensive clay, sand, or lime, and, at the same time, be perfectly free from those filthy vegetable and animal matters which, by putrefaction, contaminate the air. But, admitting the streets of those cities to abound with the filthy matters that produce pestilential diseases in other countries, it is possible the exhalations from them may be so constant, and so powerful, in their impressions upon the bodies of the inhabitants, as to produce, from habit, no morbid effects, or but feeble diseases, as was remarked formerly, is the case in the natives and old settlers in the East and West-Indies. But if this explanation be not satisfactory, it may be resolved into a partial absence of an inflammatory constitution of the air, which, I shall say presently, must concur in producing pestilential diseases. Such deviations from uniformity in the works of Nature are universal. In the present instances, they no more invalidate the general proposition of malignant fevers being every where of domestic origin, than the exemption of Ireland from venomous reptiles, proves they are not generated in other countries, or that the pleurisy and rheumatism are not the effects of the alternate action of cold and heat upon the body, because hundreds, who have been exposed to them under equal circumstances, have not been affected by those diseases. There may be other parts of the world in which putrid matters do not produce bilious malignant diseases from the causes that have been mentioned, or from some unknown cause, but I am safe in repeating, there never was a bilious epidemic yellow fever that could not be traced to putrid exhalation.