These are the phenomena of fevers in their acute, regular, and distinct form. In intermittents they appear regularly, nearly as described; and the most perfect undisguised form of fever is the tertian. In remittents these various changes are less distinct; but the chief difference consists in the termination: the sweat is inconsiderable, and the relief imperfect, though the exacerbation is generally distinctly marked. In hectics the whole stage is often regular, and the intermission tolerably perfect; but the remaining state of debility very considerable. In continued fevers the exacerbations and remissions are indistinct; but an attentive practitioner will generally, we believe always, discover them. Like critical days, they require a minute examination for their detection; a minuteness of discrimination which some do not possess; an acuteness which others will not exert.

These three stages are evidently connected; and Dr. Cullen thinks, with great reason, that they are the causes of each other. In every fever each is probably present, though not always observed. The immediate cause of these very singular appearances, physicians in every period have been anxious to discover; but to collect all the theories, all the absurdities, which the imagination has suggested, would extend our article very far. In justice, however, to some of the ablest of our predecessors, we shall notice a few of the principal opinions.

When a violent commotion was observed in the system, followed by an evacuation from the skin and the kidneys, it was an obvious conclusion that something noxious had been introduced, which, at the conclusion of the paroxysm, was discharged. The opinion was at least innocent, if not ingenious; but when to this first idea was added, that the morbid matter was prepared for expulsion by the febrile commotion, and particularly the hot fit, the opinion produced the most fatal consequences, as it led to every means of increasing the heat, in order to assist this unknown process which the morbid matter was to undergo. The simplest observations were couched in the language of this theory; and when, for instance, the authors had observed that, during the violence of the heat, it was injurious to attempt forcing any evacuation, they told us, that cocta non cruda sunt evacuanda. Many of the aphorisms of Hippocrates, which have thus a profound scientific appearance, are only the commonest remarks in the jargon of a system.

As it could not escape the most superficial observers that, after an apparently complete solution, the disease .returned, this morbid matter was supposed not to be wholly discharged; but the little remains possessing an assimilating power, was augmented in quantity, till it was again sufficiently powerful to reproduce the paroxysm. At last, either by superior exertion, or some unknown cause, it was wholly evacuated, and health restored. In support of this, they usually found, what is indeed true, that the last paroxysm was the most violent.

The theory was certainly highly probable; but, as if impregnable, succeeding authors attempted only to proceed another step, and the morbid matter was acid, alkaline, earthy, inflammable; for a time any thing which the predisposing causes could in the slightest degree support. At last Boerhaavc supposed it no foreign substance introduced, but a lentor or viscidity in the blood itself; and it was no little triumph that the inflammatory-crust on the blood was found peculiarly dense. In the latter period of his life, he wished to introduce a similar density, or rather inactivity, into, the nervous fluid.

The various theories respecting the nature of this matter, each of which is supported with equal plausibility, would immediately lead us to conclude that no such exists; for, after the lapse of near three thousand years, it would be singular if some lucky conjecture had not supplied what investigation was inadequate to, and revealed the secret. In reality, however, there is no real evidence of any such morbid cause. The discharges are the usual ones; the pus of abscesses, when such is the termination of fever, displays no peculiar qualities; the blood, which in other cases apparently relieves the fever by flowing from the nose, is equally pure with that taken from the arm. That it is concocted or meliorated by the hot fit is less probable; for if from the hot fit of fevers any change is produced on the fluids, it is of a kind perfectly different; nor is there any similar process in any part of the animal economy. We find that fevers may be often checked in their attack, without waiting for this process; and by mitigating the heat, the supposed great agent in producing this change, we cure the fever more quickly and more safely.

Any change in the physical qualities of the blood is wholly gratuitous. It has been examined by every test, and all its properties are found with scarcely any alteration. The buffy coat is now known to be owing chiefly to a more perfect state of mixture of the incongruous portions of the blood rather than viscidity.

Allied to these systems is an equally dangerous error, that fever is a cure of other disorders. Intermittents have been considered as such; and a practitioner of the Boerhaavian school is said to have asserted, that if he could produce a fever as easily as he could cure it, he should be well satisfied with his own skill. Luckily for his patients, this power was denied him; for we know-no disease, which, if at all within the reach of art, may not be at least as soon and as perfectly relieved without its assistance.

A later opinion has been, that fever is heat only; and to be cured by its opposite, cold. Cold undoubtedly -mitigates the increased action of the sanguiferous system, and if it does not cure, greatly relieves fever. Like the best of our remedies, it contributes to conduct fever safely to its termination; but we receive no information from this system respecting the cause of the heat, or its connection with the other symptoms.