we can have no difficulty in comprehending their phenomena; for contagion, though more active under certain circumstances, and at some times than others, is capable occasionally of bidding defiance to all understood impediments, and of continuing pertinaciously its destined progress. When we understand better the minute chemical and meteorological changes which take place in the air about us, we shall be able, doubtless, to say why a contagion which spreads rapidly and becomes epidemic at one time, shall, at another, fail to extend itself, except by contact or near approach.

Prout tells us that, having been engaged in a series of nice experiments to determine the weight of a given portion of atmosphere in London, he suddenly found its specific gravity to undergo a notable, though not very great, increase. It was just at this period that cholera invaded the city, and he plausibly enough infers, from the coincidence in point of time, that between this superadded substance-the nature of which he failed to detect-and the terrible pestilence that ensued, there existed the relation of cause and effect. Suppose him to be correct in this, we are still at as great a loss to determine upon the origin of this heavier aerial matter. If any one contends that it was a telluric emanation, I object that the disease occurred above and upon all possible diversities of soil, rock of all varieties, alluvial deposits, clay, lime, black loam, and sand, and that it was even met with at sea, and in Russia, when the earth was icebound and covered with snow.

It would be difficult to maintain the possibility of an identical effluvium under all these varied conditions of soil, place, and temperature. No one has imagined a volcanic source for the cause of cholera. The spot where it commenced is as far as any other on the earth's surface from any known volcano, active or extinct. No earthquake announced its coming; no meteoric phenomena attended the approach of this scourge. In some places it was foreshown, as we are told, by myriads. of insects hovering over the devoted heads of its victims; but, if we give all the required weight to these statements, we cannot attribute it to their agency, because, in a large majority of other places, they were not seen-they did not, indeed, exist. In some localities, a sort of warning, it is affirmed, was given of its dreaded invasion, by the increased and still increasing malignity of its connate maladies-diarrhoeas, dysenteries, and all other bowel affections. This we do not deny; but it is equally true that, in many other places, its appearance was unexpected, and as sudden as the bursting of a thundercloud. Some of its seats were affected at the time of its access with a variety of malarious diseases, and many, perhaps, were attacked at the usual season of the prevalence of such diseases. This is true of New Orleans, Havana, and many of our western cities; but no one will assert it of Moscow, Hamburg, Paris, or Edinburgh.

If it be proved that a ponderable agent, such as was detected by Prout, is the cause of cholera, why may it not consist of a solution or diffusion of the matter of contagion. It is not ordinary malaria, nor a volcanic nor telluric emanation; some regard it as a cloud of fungous sporules, others as a congeries of animalcule; no chemical properties are attributed to it, unless we are to believe the stories of the more

ready putrefaction of animal matter during its presence, half told indeed, and not proved at all; no sensible quality is ascribed to it but weight, and this must belong to the matter of contagion. Matter may be dissolved and diffused in the air in a state of the most incomprehensible tenuity, and yet preserve the specific qualities belonging to it; a familiar example of which may be referred to in the fragrance of flowers and other odoriferous bodies, and magnificent instances recorded as occurring in 1782 and 1783, when the atmosphere, almost over the whole globe, was filled with a coloring or colored matter, which tinged the sunlight and the moonbeams of a reddish hue, and in August, 1832, when the whole air assumed a greenish tint. These phenomena were, at the time, themes of high discussion, and were by the superstitious supposed to prognosticate wonderful things.

"In the year 1782, and still more in the year following, a remarka ble haze spread over the whole of Europe. Seen in mass, this haze was of a pale blue color. It was thickest at noon, when the sun appeared through it of a red color. Rain did not in the least degree af fect it. This haze is said to have possessed drying properties, and to have occasionally yielded a strong and peculiar odor. It is also said to have deposited in some places a viscid liquid of an acid taste and an unpleasant smell. About the same time, there were in Calabria and Iceland, terrible earthquakes, accompanied by volcanic eruptions. Its dispersion, in the summer of 1783, was attended by severe thunderstorms. During the above-mentioned year, an epidemic catarrh or influenza prevailed through Europe, affecting not only mankind, but likewise other animals."

The contagion of smallpox, we know, spreads itself at all times a few inches, or a few feet from the diseased body; sometimes fills the chamber of the sick, sometimes infects the air of a whole city. This is the diffusion, locally epidemic of the matter of contagion. Upon what principle does any one venture to deny the possibility of its further diffusion to an indefinite extent? It seems to me fair and logical to follow out the analogies offered us by the ascertained and undis puted facts which have been referred to, and to conclude that a diffused contagion is the true source of this wide spreading epidemic.

The same reasonings will apply to the case of plague, measles, sore throat, scarlet fever, etc., when they spread beyond their usual limits, and assume the character of general epidemics.

Much has been said of the laws of epidemics, but no confidence is to be placed in the doctrines, so positively laid down on this subject. Each individual is sui generis; arises, progresses, and is limited in its own peculiar manner. Some, as plague, pneumonia typhoides, and yellow fever, are under the control of temperature; others are entirely independent of such control. Some are confessedly contagious; others are by many writers denied this property, as yellow fever and influenza. Some are endemic or locally epidemic, as typhus-others present in their history no reference to place, as measles and scarlatina.

Among the laws so dogmatically laid down as governing epidemic diseases, much stress has been laid upon their supposed exclusiveness. They are affirmed to "bear, like the Turk, no brother near the throne."

"They drive out," says Rush, "all other diseases, or compel them to wear their livery." If this be a general rule, the exceptions are extremely numerous and striking. Yellow fever and bilious remittent prevailed together in Charleston in 1827, and each of them to an almost unprecedented extent. In New Orleans and Havana, cholera and yellow fever pursued their ravages simultaneously, and, so far as is known, without any modifying influences upon each other. In Smyrna, in 1848, cholera and the plague were epidemic at the same time.

It is affirmed, also, that without any observable change in the appreciable qualities or conditions of the air, they exhibit a regular tendency to their own decay and extinction. "Entering," says an American writer, "like the lion, they retire like the lamb." And this peculiar trait, as it is considered to be, is attributed by some to the influence of habit on the constitution, which becomes accustomed to the presence and excitement of the malignant cause, whatever we may suppose it to be. At any rate, the doctrine, if true, must be taken as the expression of a fact which relates to the condition of the subjects of a pestilence, rather than to any quality or property, or characteristic of the disease itself. There seems to me, however, nothing peculiar in this matter, nor can any general law be laid down which shall include a majority of epidemics. Thus, yellow fever does not usually subside anywhere, until the heat of the season has passed by; typhus continues to rage as long as the favorable or fostering conditions which gave it prevalence are present; measles, hooping-cough, smallpox, etc., as long as they find subjects. If a greater number are attacked when the disease first invades, it is because no means of evasion are attempted for awhile, and because there are a greater number then ready to receive it. But in all the histories of the cholera and the plague, there is a gradual, but not a regular progress, from bad to worse. The bills of mortality of 1836 show the number of deaths from cholera, in Charleston, to have been continually vacillating; nor did it reach the greatest amount until it had been six weeks prevailing. Even then, its subsidence was not regularly progressive.

I have said that I believe "the epidemic constitution of the air" in any given case, to consist in the contamination of the affected atmosphere by the diffusion in it of the material cause of the pestilential disease, whatever that may be.

Thus an endemic or malarious disease may become epidemic, and thus, and in no other way, can a contagious disease become epidemic, either locally or generally. There are many who employ the phrase, infected atmosphere, who carefully repel the suspicion of acknowledg ing, as to the majority of epidemics, the possibility of a contagious infection or contamination. Let us take an example. The atmosphere of a rice field is at this moment (August, 1854) capable of exciting bilious remittent or intermittent; that of New Orleans or Vera Cruz, capable of generating yellow fever, in a fit subject; that of the fever hospitals of London or Dublin, of generating typhus; that of Palermo or Breslau, of generating cholera. These, then, are "infected atmospheres." Let us, with this contaminated air, fill a ship's hold and cabin, and bring her into the port of Baltimore, Philadelphia, New

York, or Charleston. What is likely to happen in each of these cases? Let experience answer. Any number of persons exposed to the rice field atmosphere, especially if in the state of sleep, will be attacked with intermittent or remittent; the typhus atmosphere will scarcely produce any effect, unless applied for a considerable length of time, and in a deteriorated constitution, depressed either by low living or by residence in an uncleanly, vitiated chamber; the yellow fever atmosphere will not affect a southerner, no matter how his bodily strength may be disordered or impaired, but will attack a northern man in the most robust and vigorous health. The cholera atmosphere will assail almost any one who enters it, but is favored by the same conditions which predispose to typhus.

Again, setting aside all reference to contagion, the poison which infects each of these atmospheres, possesses a separate tendency to adhere to fomites, or to spread itself through surrounding air. This latter property of self-repulsion, so common among aerial or gaseous matters, seems scarcely to belong to malaria; nor is it at all tenacious. If any one enters the rice field air then, supposing him to escape the effects of the poison, he brings away none, and may be approached safely. Thus is its influence limited within narrow bounds. But if a vigorous and robust individual undergo, without injury, exposure to a typhus air, he may, on issuing forth, carry with him, such is its tenacity, in adhesion with his clothes, and forming a sort of personal atmosphere, quite a sufficient quantity to affect any one who may approach him in a condition more favorable for the development of the disease. Instances of this sort have often occurred; thus at the famous Black Assizes at Oxford in 1577, and in Exeter, and in Taunton, and at the Old Bailey, in London, the prisoners brought into court for trial, communicated to the court and spectators the infection from their jail, of a virulent typhus.

As to the tenacity of the aerial cause of yellow fever and of cholera, I do not think it remarkable, yet there are some facts which seem to show that it may adhere to fomites. The quality of self-repulsion which I have said does not probably belong to malaria at all, is certainly very feeble in the poison of typhus. Haygarth, and others, would limit it to a very few feet, yet, as in the instance of smallpox, it seems sometimes to assume wings, and spread with fearful promptness. With respect to yellow fever, the question is hotly disputed, but we must believe its capacity for extension, in other words, its self-repulsive power to be considerable, unless we are willing to pronounce it a disease of very wide and varied origin. It has either arisen or been imported into Philadelphia, New York, New Haven, Boston, Cadiz, Seville, Gibraltar and Xeres, and soon after being noticed at, or in the neigh borhood of some infected vessel, or other centre, has pervaded those cities very rapidly. Still more forcible must be the self-repulsion of the cause of cholera, which, from its several centres, rolls on every side its waves of pestilence so diffusively, and, as at Gateshead, near Sunderland, with such incredible intensity and suddenness.

Suppose lastly, then, that the several holds and cabins of the infected ships, above described, were broken open in the midst of the harbor of a populous city, and not entered by a single individual. The

malaria of the rice field would continue harmless in its place of deposit; so, probably, would the malaria of the typhus hospital; it is not so clear that the malaria of yellow fever would remain innocuous, nay, if season and temperature favored, it is highly probable that cases of yellow fever would be developed; but I do not entertain a doubt that the atmosphere of cholera would immediately diffuse itself, and excite this terrible and devouring pestilence.

PARASITES-ANIMAL AND VEGETABLE.

All living beings, as far downwards as we are able to trace the history of created things, are subject to the presence of parasites, or dependents, who subsist upon them, and in turn supply a still lower race of adherents with a portion of the superfluous fluids which distend their vessels. This mutual relation is one among the regular ordinances of nature, and, with regard to a very extensive class of parasites, does not seem to do any injury to the organism. Nothing can be more beautifully impressive than one, or an avenue, of our old, rugged, and majestic live oaks, adorned with the graceful garniture of the long "gray-beard," or Spanish moss, the Tillandsia usneoides, which hangs from it without impeding its wide growth, impairing its vast strength, or subtracting from its venerable longevity. And so it is with many other forms of vegetation. Nor do the infinitely dense colonies of small animals-aphis and others-which infest all our succulent plants, in their usual numbers, affect the life or vigor of the vegetation which gives them support. Doubtless, when they multiply beyond their ordinary abundance, they inflict injury; and some are uniformly hurtful. They, therefore, deserve to be considered in this place among causes of disease.

The vegetable parasites which infest animal bodies are of the lowest order-algae and fungi. Fourteen genera of algae, says Robin, and thirty-eight species, sixteen genera of fungi and forty-eight species, are found in the animal body. They are often found on our common horsefly, on the silkworm, on frogs, in the yelk of a hen's egg. Of epiphytes, growing on the human skin and in the bulb of the hair, there are several varieties; they are found in herpes, porrigo, mentagra, favus, pityriasis, and plica polonica. Endophytes are found upon the mucous surfaces, as the sarcina of the stomach, and the torula cerevisii; in the mouth, as the algae of diphtherite and of aphtha, and in the sordes of typhus; in the intestines, as the navicula and the cholera fungi; in the lungs, by Burnett; in the eye, by Hembrecht; in the urine, in milk, mucus, and pus, by many observers; in the perspiration of cholera, by Cowdell and Curme.

Cryptogamic plants have been found on the human skin, in favus, by Remak, Gruby, and Bennett; in mentagra, and porrigo decalvans, or herpes tonsurans, by Gruby; in trichoma, by Gemsburg; on ulcerated spots in the intestines, by Langenbeck; in pityriasis versicolor, by Sluyter and Simon; on the mucous membrane of the mouth, in aphtha (muguet), by Vogel and Gruby; on the coating of the tongue, by Hannover and Bennett; on the fauces and oesophagus, by Hanno