Of the Causes of Fever.

The causes of fever are of two kinds; first, those which immediately produce the disease, and secondly, those which bring the system into a condition capable of being affected by the first: the former, are called the exciting, the latter, the predisposing causes: a third has been spoken of in relation to this as well as to other diseases, namely, the proximate. But what is really meant by the proximate cause of disease (if the term have any meaning) is the condition of the organ, or of the system, produced by the operation of the exciting cause: this term, therefore, designates an effect, not in any proper sense, a cause: it relates to the disease itself, not to that which produces it.

I. Of the Immediate, or Exciting Cause of Fever.

The immediate, or the exciting cause of fever is a poison formed by the corruption or the decomposition of organic matter. Vegetable and animal matter, during the process of putrefaction, give off a principle, or give origin to a new compound, which, when applied to the human body, produces the phenomena constituting fever. What this principle or compound is, whether it be one of the constituent substances which enter into the composition of organised matter, or whether the primary elements of organised matter, as they are disengaged in the process of putrefaction, enter into some new combination, and thus generate a new product, we are wholly ignorant. Of the composition of the poison, of the laws which regulate its formation, and of its properties when generated, we know nothing beyond its power to strike the human being with sickness or death. We know that, under certain circumstances, vegetable and animal substances will putrify: we know that a poison capable of producing fever will result from this putrefactive process, and we know nothing more.

Of the conditions which are ascertained to be essential to the putrefactive process of dead organic substance, whether vegetable or animal, those of heat and moisture are the most certain, and as far as we yet know, the most powerful. Accordingly, in every situation in which circumstances concur to produce great moisture, while the heat is maintained with some steadiness within a certain range, there the febrile poison is invariably generated in large quantity, and in great potency. Wherever generated, we have no means of ascertaining its existence but by the effects it produces on the human body. Now and then circumstances arise which illustrate these effects in an exceedingly striking manner. This is the case when large numbers of men, previously in a state of sound health, are simultaneously exposed to it. Examples of such occurrences, as numerous and as complete as can be desired, were long since recorded, among many others, by one very accurate observer, who is of the number (no small one) of those who have given valuable lessons to the world, which have been forgotten, and to which it is a useful labour to recall the attention of the present age.

“In the beginning of June, 1742,” says Sir John Pringle, in his Observations on the Diseases of the Army, “the British troops began to embark for Flanders. There were in all, of foot and cavalry, about 16,000: the winds were favourable, the several passages short, the men landed in good health, and went into their several garrisons. The head-quarters were at Ghent. During the Summer and Autumn the weather was good, the heats moderate, and the country in general healthy. The British officers continued well, but many of the common men sickened. Ghent is situated between the high and the low division of Flanders; one part of the town called St. Peter’s Hill, is much higher than the rest, and in this, the barracks, having drains and free air, were quite dry; so that the soldiers who lay there enjoyed perfect health. But those who were quartered in the lower part of the town (mostly on the ground-floors of waste houses, unprovided with drains, and of course damp) were sickly. The battalion of the first regiment of guards was a remarkable instance of this difference of quarters. Two of the companies lay on St. Peter’s Hill; the remaining eight in the lower part of the town, in rooms so very damp, that they could scarce keep their shoes and belts from moulding. In the month of July, the sick of this battalion amounted to about 140; of which number only two men belonged to the companies on the hill, and the rest to those in the lower town.”^[31]

It is further stated, that in the end of August, Ostend having surrendered, the garrison, consisting of five battalions British, was conducted to Mons, where they continued about three weeks: that these men had been so healthy that, when they marched out, upon the capitulation, they left only ten sick; but that the same corps having been put into damp barracks at Mons, while the town was surrounded with an inundation, fever immediately appeared, and prevailed to such an extent, that in this short space of time 250 were seized with the disease.^[32]

Of the campaign in 1748, it is stated that the troops had scarcely been a month in the cantonments, when the returns of the sick amounted to 2000: that afterwards the number became much greater: that those who were near the marshes suffered by far the most, both in the number and the violence of the symptoms; that the Greys, cantoned at Vucht (a village within a league of Bois-le-duc, surrounded with meadows, either then under water, or but lately drained) were the most sickly; that for the first fortnight they had no sick, but, after continuing five weeks in that situation, they returned about 150; after two months, 260, which was about half the regiment; and at the end of the campaign, they had in all but 30 men who had never been ill: that a regiment at Nieuland, where the meadows had been floated all Winter, and were but just drained, returned sometimes above half their number: that the Scotch Fuzileers at Dinther, though lying at a greater distance from the inundations, yet being quartered in a low and moist village, had above 300 ill at one time, while a regiment of dragoons, cantoned only half a league south-west of Vucht, were in a good measure exempted from the distress of their neighbours, such was the advantage even of that distance from the marshes, of the wind blowing mostly from the dry grounds, and of a situation upon an open heath, somewhat higher than the rest.

When the troops were in Zealand, where the poison was in a high degree of concentration, they had not been a fortnight in the cantonments, before several of the men belonging to those regiments which were stationed nearest the inundations, were seized simultaneously with lassitude and inquietude, a sensation of burning heat, intense thirst, frequent nausea, sickness and vomiting, aching of the bones, pain in the back, and violent headache. There were some instances of the head being so suddenly and violently affected that, without any previous complaint, the men ran about in a wild manner, and were believed to be mad, till the solution of the fit by a sweat, and its periodic return, discovered the true nature of their delirium. Most of the men were first taken ill upon their return from forage. The regiment being cantoned close upon the inundations, and many of the quarters being above two leagues from the place where the magazines were kept, the men were obliged to set out about four in the morning, in order to get back before the greatest heat of the day. At this early hour, the meadows and marshes on each side of the road were covered with a thick fog, of an offensive smell. The party generally returned before noon; but several of the men, even before they could get back to their quarters, were already in a violent fever; some, in this short space of time, were actually delirious; and a few, on their way home, were so suddenly taken with a phrenzy, as to throw themselves from their trusses into the water, imagining they were to swim to their quarters. One man, on reaching home, was suddenly seized with intense headache, got out of his quarters, and ran about the fields like one distracted. Three years after this sickness, it was found that two of the men who were thus suddenly affected with phrenzy, though they recovered of their fever, had ever since been epileptic, and that all the rest who had been ill, remained exceedingly liable to returns of an intermitting fever.

The suddenness with which fever sometimes attacks individuals on board a ship, or even an entire ship’s crew, on the approach of the vessel to a shore where this poison is generated in large quantity, and in a high state of concentration, illustrates its operation, perhaps, in a still more striking manner. Dr. Maculloch, who has laboured with great ability and zeal to recal attention to the most important and long-forgotten subject of malaria, relates an instance of some men on board a ship, who were seized, while the vessel was five miles from shore with fatal cholera, the very instant the land-smell first became perceptible. Several of these men, who were unavoidably employed on deck, died of the disease in a few hours. The armourer of the ship, who, before he could protect himself from the noxious blast, was accidentally delayed on deck a few minutes, to clear an obstruction in the chain cable, was seized with the malady while in that act, and was dead in a few hours.

Dr. Potter states^[33] that he witnessed the rise of a most malignant yellow fever, in a valley in Pennsylvania, which contained numerous ponds of fresh water, and which, from the heat and dryness of the season, emitted a most offensive smell: that the fever prevailed most, and with the greatest degree of malignity among the people who lived nearest these ponds; and adds an exceedingly instructive case, illustrative of the generation and operation of this cause of fever, recorded by Major Prior, in his account of a fever which attacked the army of the United States at Galliopolis. The source of the malady was clearly traced to a large pond near the cantonment. When the disease was most severe, it assumed the continued form, and was accompanied with yellowness of the skin: when proper means were taken to destroy the pond, the fever immediately lost its continued form, and became first remittent, then intermittent, and ultimately disappeared. “The fever,” says this intelligent officer, “was, I think, justly charged to a large pond near the cantonment. An attempt had been made two or three years before to fill it up, by felling a number of large trees that grew on and near its margin, and by covering the wood thus fallen with earth. This intention had not been fulfilled. In August, the weather was extremely hot, and uncommonly dry: the water had evaporated considerably, leaving a great quantity of muddy water, with a thick slimy mixture of putrefying vegetables, which emitted a stench almost intolerable. The inhabitants of the village, principally French, and very poor, as well as filthy in their mode of living, began to suffer first, and died so rapidly, that a general consternation seized the whole settlement. The garrison continued healthy for some days, and we began to console ourselves with the hope that we should escape altogether: we were, however, soon undeceived, and the reason of our exemption heretofore was soon discovered. The wind had blown the air arising from the pond from the camp; but, as soon as it shifted to the reverse point, the soldiers began to sicken: in five days, half the garrison was on the sick list, and in ten, half of them were dead. They were generally seized with a chill, followed by headache, pains in the back and limbs, red eyes, constant sickness at stomach, or vomiting, and generally, just before death, with a vomiting of matter like coffee-grounds. They were often yellow before, but almost always after death. The sick died generally on the seventh, ninth, and eleventh days, though sometimes on the fifth, and on the third. As some decisive measures became necessary to save the remainder of the troops, I first thought of changing my quarters, but as the station was in every respect more eligible than any other, and had been made so by much labour and expense, I determined to try the experiment of changing the condition of the pond, from which the disease was believed to have arisen. A ditch was accordingly cut; what little water remained was conveyed off, and the whole surface covered with fresh earth. The effects of this scheme were soon obvious. Not a man was seized with the worst form of the fever after the work was finished, and the sick were not a little benefitted, for they generally recovered, though slowly, because the fever became a common remittent, or gradually assumed the intermitting form. A few cases of remitting and intermitting fever occurred occasionally, till frost put an end to it in every form. As soon as the contents of the pond were changed, by cutting the ditch, the cause, whatever it was, seems to have been rendered incapable of communicating the disease in its worst form.”

Dr. Potter further states that, on one occasion, he saw a lady, who had been confined three days only, and whom he found in the agonies of death, with the skin of a deep orange colour, the eyes red and prominent, the pulse intermittent, and ejecting copiously from the stomach every eight or ten minutes, the secretion now known by the name of the black vomit; that she expired in a convulsion, while he sat at her side; that petechiÆ appeared immediately after death, and that putrefaction succeeded so rapidly, that it was necessary to order immediate interment: that, shortly afterwards, he was called to a gentleman who had been ill five days, and who, having expired in an hour or two after his visit, was removed into the coffin with the utmost difficulty, the flesh literally dropping from the bones: that, in one family residing in a house which stood on a level piece of ground, apparently beyond the reach of noxious exhalation, there being no stagnant water, as was supposed, within a mile of it, he found the mother labouring under a bilious remitting fever, which had continued eleven days; the daughter, seventeen years of age, suffering from a similar fever; two sons, the one between eight and nine, and the other six, ill with dysentery; and the father, on the brink of the grave, from a most malignant fever. There being no apparent cause for the condition of this afflicted family, the immediate neighbourhood of the house being free from the ordinary sources of malaria, and the adjacent country being not unhealthy, the condition of the house itself was minutely investigated. The cause of the evil was manifest. It appeared that the present family had resided in the house only about five weeks; that immediately preceding their occupation of it, a man had died suddenly in it; that he himself (Dr. Potter) was seized with nausea and general lassitude, immediately on leaving the house after his first visit; and that a fever, as he supposes, was arrested by a strong dose of tartarized antimony, which operated violently by vomiting and purging. On examining the premises, it was found that the cellar contained water about two feet deep, which had remained there from the first week in June, the country having been then inundated by torrents of rain. The cellar being useless, the door had been closed, and the only vent for the pestiferous gases was through the floor, which was open in several places. The family being immediately removed, all the sick became convalescent from the time they ceased to breathe the air of the place. The owner of the house hired two men to empty the cellar. These men having ripped up the floor, and placed a pump in the deepest part of the water, evacuated the cellar to the dregs in one day. On the second day after the execution of this task, one of these men was seized with a chilliness, succeeded by an ardent fever, which terminated with the usual symptoms of yellow fever; namely, hÆmorrhages, yellow skin and petechiÆ, and proved fatal on the third day from the attack: the day following the seizure of the first, the second man was attacked with similar symptoms, and died on the seventh day of the disease, with the black vomit, in addition to the ordinary symptoms of the yellow fever.

These examples may suffice to illustrate the operation of that febrile poison which arises chiefly from the decomposition of vegetable matter. The poison derived from the putrefaction of animal matter is still more pernicious: its effects are more powerful in degree, and worse in character; it operates more intensely on the nervous system, and less on the vascular; and the fevers it produces are invariably of the typhoid type, and of the continued form.

Without doubt, a febrile poison, purely of animal origin, in a high degree of concentration, would kill instantaneously; and when not intense enough to strike with instantaneous death, it would produce a continued fever with the typhoid characters, in the greatest possible degree of completeness and perfection. And this appears to afford the true solution of the origin of the plague. The more closely the localities are examined of every situation in which the plague prevails, the more abundant the sources of putrefying animal matter will appear, and the more manifest it will become, not only that such matter must be present, but that it must abound. And this also is one of the truths which was known to the observers of former times, but which has been forgotten. Were it not that the professional reading of an age, is bounded by as strict a line as that which divides century from century; were it not that no one reads back beyond the authority which happens to give to the day its prevailing doctrines; were it not that the great repository of facts treasured up in the volumes of the close observers, though sometimes the bad reasoners of former days, thus becomes neglected for the dogmas of some modern writer, who reasons as ill, and who observes less, the notion that vegetable malaria produces only intermittent fever, never could have become so prevalent as it is at present, nor could the influence of animal malaria ever have been so entirely overlooked. But it chanced that Cullen, in his definition of intermittent fever, assigned the miasma of marshes as the origin of the disease, while he makes no mention of animal malaria in his definition of any of the forms of fever; and as this author superseded all former authorities, by becoming the great authority of the age, few of his successors are acquainted in the slightest degree with the writings anterior to his period: whence it has happened that the numerous and invaluable facts observed and recorded by his predecessors, relative to the cause of fever, have been disregarded until they have become wholly unknown. To cite the antient and the more modern authorities who have observed and recorded the influence of animal malaria in the product of plague, would be to enumerate every distinguished writer, from Pliny and Diodorus Sicculus, down to Galen, from Galen to Mead, and from Mead to Pringle.

In assigning the reason why Grand Cairo, in Egypt, is the birth-place and the cradle of the plague, Mead states that this city is crowded with vast numbers of inhabitants, who live not only poorly, but nastily; that the streets are narrow and close; that the city itself is situated in a sandy plain, at the foot of a mountain, which keeps off the winds that might refresh the air; that consequently the heat is rendered extremely stifling; that a great canal passes through the midst of the city, which at the overflowing of the Nile is filled with water; that on the decrease of the river, this canal is gradually dried up, and the people throw into it all manner of filth, carrion, offal, and so on; that the stench which arises from this, and the mud together, is intolerably offensive; and that, from this source, the plague constantly springing up every year, preys upon the inhabitants, and is stopped only by the return of the Nile, the overflowing of which washes away this load of filth: that in Ethiopia the swarms of locusts are so prodigious, that they sometimes cause a famine, by devouring the fruits of the earth, and when they die, create a pestilence, by the putrefaction of their bodies; that this putrefaction is greatly increased by the dampness of the climate which, during the sultry heats of July and August, is often excessive; that the effluvia which arise from this immense quantity of putrefying animal substance, combined with so much heat and moisture, continually generate the plague in its intensest form; and that the Egyptians of old were so sensible how much the putrefaction of dead animals contributed towards breeding the plague, that they worshipped the bird Ibis, from the services it did in devouring great numbers of serpents, which they observed injured by their stench when dead, as much as by their bite when alive.

Nothing can be more striking than the cases recorded by Pringle, and which daily occurred to him of the production of fever, exquisitely typhoid, (according to the language of that day, jail and hospital fever) and of the sudden transition of intermittent and remittent into the continued and typhoid type, from the presence of a poison clearly and certainly of animal origin. Whenever wounded soldiers, with malignant sores, or mortified limbs, were crowded together, or whenever only a few of such diseased persons were placed in a room with the sick from other diseases, with those labouring under intermittent and remittent, for example, a severe and mortal typhus immediately arose; nay, whenever men, previously in a state of sound health, were too much crowded together for any considerable time, typhus (jail or hospital fever) was sure to be produced. The instances of such occurrences that are detailed, are too numerous to be cited, but they are so clearly stated, and so striking, that they well deserve to be consulted by whoever is desirous of clearly tracing the operation of this great cause of fever.

But by far the most potent febrile poison, derived from an animal origin, is that which is formed by exhalations given off from the living bodies of those who are affected with fever, especially when such exhalations are pent up in a close and confined apartment. The room of a fever-patient, in a small and heated apartment in London, with no perflation of fresh air, is perfectly analogous to a stagnant pool in Ethiopia, full of the bodies of dead locusts. The poison generated in both cases is the same; the difference is merely in the degree of its potency. Nature, with her burning sun, her stilled and pent-up wind, her stagnant and teeming marsh, manufactures plague on a large and fearful scale: poverty in her hut, covered with her rags, surrounded with her filth, striving with all her might, to keep out the pure air, and to increase the heat, imitates nature but too successfully; the process and the product are the same, the only difference is in the magnitude of the result. Penury and ignorance can thus at any time, and in any place, create a mortal plague. And of this no one has ever doubted. Of the power of the living body, even when in sound health, much more when in disease, and above all, when that disease is fever, to produce a poison capable of generating fever, no one disputes, and the fact has never been called in question. Thus far the agreement among all medical men, of all sects, and of all ages, is perfect.

But it happens that there is another form of animal matter capable of producing fever: namely, a matter secreted by the living body, constituting not only a poison, but a peculiar and specific poison. This specific poison produces not merely fever, but fever with a specific train of symptoms. In the acknowledgment of this fact, also, the agreement among all medical men is equally perfect.

But some contend that the poison generated in the first case, and that generated in the second, may both be properly called contagions: others maintain that the application of the same term to two cases so specifically different, destroys a distinction which it is useful to preserve, and that it would be more correct, as well as more conducive to clearness of conception, to call the poison generated in the first case an infection, and to restrict the term contagion, to designate the poison generated in the latter. Vast and immeasurable as the difference appears to be between the contagionists and the anti-contagionists, if regard be had merely to their language, yet if attention be paid only to their ideas, to this, and to this only, narrow as the compass is, the whole controversy is reduced. It resolves itself wholly into the question, whether one word shall be used to express two cases which differ from each other in some important circumstances, or whether it may not be more convenient to employ two terms, and strictly to appropriate each to designate its own specific class. It must be manifest that, since both sects are perfectly agreed about the facts, the dispute can be only verbal. If the one would consent to restrict their use of the term contagious, for which there is the best authority and ancient custom, to those diseases which arise from a specific contagion, and would call those which arise from every other poison infectious, there would be an end to this apparently interminable, and in many respects mischievous, controversy.

Is the febrile poison, whether of vegetable or animal origin, or whether composed of both, capable of adhering to clothes, apparel, and other substances, in such a manner as truly to infect them, so that when applied to the bodies of the healthy, at any distance of place, and at some distance of time, the specific effects of the poison are produced? That such substances may be so imbued with the poison of the small-pox, all admit: that the evidence should not be as complete relative to the power, or the inability of such substances to convey and communicate the poison of ordinary continued fever, is alike disgraceful to the state of our science, and injurious to the cause of humanity. There is no reason why the question should not be settled with absolute certainty; there is no manner of difficulty in determining it. Experiments the most direct, complete, and decisive, might be performed, which, if observed, during their progress, by competent witnesses, and duly authenticated, might ascertain the point with sufficient clearness and certainty, to satisfy not only the present age, but future generations. Once, for all, the full trial might be made, and if the trial were really full, it need never be repeated. A series of experiments completely decisive of the question, as far as regards the fever of our own country, which might be easily extended to the plague, were some time ago drawn out, and exertions were made to carry them into effect; but in the prevailing state of public opinion and feeling, it was found absolutely impossible to institute them on a scale at all adequate to render them decisive, without the aid of Government. There seems to be no possible mode of performing them effectually, unless Government will co-operate, by granting a free pardon to such convicts, as will voluntarily allow themselves to be made the subjects of them. The risk to them would be slight, the evil to the community none; while the danger, the suffering, the disease, the mortality that would be prevented, to say nothing of the expense that would be spared by the decision of the question, would be incalculable. It is earnestly to be hoped that those who have it in their power to afford the means of putting this question at rest, will not allow it to remain in its present unsettled state. Science, commerce, humanity, alike demand that the truth should be ascertained.

This subject, it is my intention to take up, and to discuss fully in a future publication, in which will also be investigated some inquiries, which it has been found impossible to include in the present volume; such as whether the vegetable and animal poison we have been considering, be the only true exciting cause of fever; by what means its general diffusion is effected; on what conditions its propagation depends; by what measures its extension may be checked, and its power diminished or destroyed; what circumstances in the modes of life, in the habits of society, in the structure of houses, in the condition of the public streets and the common sewers, in the state of the soil over large districts of the country, as influenced by the mode of agriculture, drainage, and so on, favour or check the origin and propagation of this great curse of civilized, no less than of uncivilized man. It is obvious that these inquiries will include the investigation of several exceedingly curious and important statistical questions; and the object of these researches will be accomplished should they lead to the establishment of any useful principles of extensive application.^[34]

II. Of the Remote or Predisposing Causes of Fever.

The remote or the predisposing causes of fever have been stated to be those circumstances which bring the body into a condition capable of being affected by the immediate or the exciting cause. Whatever diminishes the vigorous action of the organs, impairs their functions, and so weakens the general strength of the system, is capable of becoming a predisposing cause of fever; and every predisposing cause acts in one or other of these modes, and becomes a predisposing cause only and in proportion as it lessens the energy of the system, or disturbs the balance of its actions, which in fact is to render some portion of it weak. During a state of vigorous health the body is endowed with the power of resisting the influence of noxious agents, which in a less perfect state of health are capable of producing intense and fatal disease; and the action of all predisposing causes is to lessen this resisting power, or to weaken the energies of life.

Of all predisposing causes, the most powerful is the continued presence and the slow operation of the immediate or the exciting cause. It is a matter of constant observation, that the febrile poison may be present in sufficient intensity to affect the health, without being sufficiently potent to produce fever. In this case the energy of the action of the organs is diminished, their functions are languidly performed, the entire system is weakened, and this increases, until at length the power of resistance is less than the power of the poison. Whenever this happens, fever is induced; not that the power of the poison may be at all increased; but the condition of the system is changed, in consequence of which, it is capable of offering to the noxious agent that assails it less resistance.

We have seen that the vegetable or animal poison may exist in sufficient intensity to produce fever on the slightest exposure to it, without the operation of any predisposing cause, in a body in the state of the soundest health, and endowed with the greatest degree of strength. Examples of this kind are but too frequent in tropical climates. In countries where the temperature never rises so high, and seldom continues so long, it is rare that fever is produced immediately, on exposure to the exciting cause. Concentrated and potent as that poison is in many parts of Flanders, yet Sir John Pringle states that, in removing to an unhealthy situation, the men rarely became ill at once; that they generally continued in tolerable health for some days; and that recruits recently arrived in the country, resisted the noxious agent longer than the men who had been long there. Dr. Potter gives a remarkable example of the same fact, with regard to the yellow fever, which fell under his own observation, and states other facts, strikingly illustrative of the influence and operation of the predisposing causes. Strangers, from certain countries, he informs us, are insusceptible of yellow fever in America. In the most malignant and protracted epidemics which afflict that country, these strangers uniformly escape: emigrants from the West Indies, and other warm latitudes, for example, invariably resist the cause which produces these maladies in the native inhabitants. But the curious fact is, that such persons are unable permanently to resist the operation of the exciting cause; for, after a residence in America of some years, their constitution is so completely assimilated by the influence of the climate to that of the American, that they become equally sensible to its febrile miasma, and are as exquisitely impressed by them, as the American citizens themselves. The illustration is equally striking and instructive, if the position be reversed. The natives of northern climates are extremely susceptible to the influence of these miasma; that susceptibility is in exact proportion to the latitude of their country: those from the north of Europe scarcely ever escape an attack; the natives of Great Britain are nearly as susceptible to the influence of the poison, while persons even from the more northern countries of the United States are more liable to the disease than the citizens of the southern and middle states.

Dr. Potter performed some experiments, to show that the continual presence of the exciting cause not only operates upon the general system, but actually produces a morbid change in the blood, before it induces fever. During the prevalence of an epidemic, it was observed that, in all the cases in which the patients were bled, the general appearance of the blood was precisely the same; that the coagulum was either of a yellow, or of a deep orange colour, and that a portion of the red particles was invariably precipitated. It occurred to Dr. Potter that, if the cause of the disease were contained in the common atmosphere, the blood of those who had inhaled it a certain time would exhibit similar phenomena; and that, should this be the case, it would prove that the cause, before actually producing the disease, brought about a state of the system, which predisposed it to be affected by the poison. To ascertain the appearances of the blood in persons who were exposed to the febrile poison, but who still remained apparently in perfect health, he drew a quantity of blood from five persons, who had lived during the whole epidemic season in the most infected parts of the city. To external appearance and inward feeling, each of these persons was in sound health. Their blood could in no respect be distinguished from the blood of those who laboured under the most intense forms of the prevailing fever. As it was necessary to the conclusiveness of the experiment that their blood should be compared with the blood of those who lived in an atmosphere unquestionably pure, Dr. Potter selected an equal number of persons who dwelt on the hills in Baltimore country, and drew from each of them ten ounces of blood. The contrast was most manifest. The serum was neither of a yellow, nor of an orange colour; there was no red precipitate; the appearances were such as are found in the blood of persons in perfect health.

A young gentleman having returned to the city from the western part of Pennsylvania, on the 10th of September, in a state of sound health, Dr. Potter drew a few ounces of blood from a vein, on the day of his arrival; it exhibited no deviation from that of a healthy person. He remained in the family until the 26th of the month, that is sixteen days. On the 16th day the bleeding was repeated. The serum had assumed a deep yellow hue, and a copious precipitation of red globules had likewise fallen to the bottom of the vessel.

In these experiments, the blood in six persons indicated the operation of the morbid cause, while each remained in a state of apparent health. Of these six persons, four were actually seized with yellow fever during the prevalence of the epidemic; and the other two, though they escaped any formal attack, did not escape indisposition. They were affected with headache, nausea, and other indications of disease, like hundreds besides, who were never absolutely confined to the house, and who never took any medicine, but who still experienced in nausea, giddiness, headache, pain in the extremities, and so on, abundant intimations of the presence of the poison.

These examples may suffice to show how the exciting, may itself become a most powerful predisposing cause. The predisposition to subsequent attacks, after the system has once suffered from the disease, is very remarkable; that predisposition remains for a considerable period after convalescence and apparent recovery. Of this, striking examples continually occur both with regard to intermittent, and to continued fever. In fact, the disposition to relapse, remains until the constitution has recovered its previous strength and vigour, however distant that period may be. The influence of cold, moisture, fatigue, intemperance, constipation, anxiety, fear, and all the depressing passions, are likewise extremely powerful predisposing causes. They enable a less dose of the poison to produce fever, and they increase the intensity of the fever when it is established. They all act by weakening the resisting power inherent in the constitution, that is, by enfeebling the powers of life.