Such is the usually mild character of this affection; but if its progress has been interrupted by any means, more particularly by mercury, it assumes a more complicated form, and a less tractable nature. If that mineral is administered in the usual style, and at the commencement, when the fever and other symptoms are high, the patient’s sufferings are all much aggravated. After the fever has subsided, the eruption will often be found to disappear under the use of mercury; but it is extremely apt to recur, as soon as the system has shaken itself free from the effects of that medicine. The mercury produces an irritation, which supersedes the eruption, but by frequent repetition its effects on the system diminish: it at last fails to create an irritation more powerful than the disease to which it is opposed, and, consequently, the eruption does not yield, but during its use is frequently reinforced by fresh crops of papulæ. If the eruptive fever, and advanced stage of the disease, are imprudently and suddenly arrested by the use of mercury, by exposure to cold, or by other means, inflammation of the iris or joints often follows, of a very violent form, and not to be easily moderated. No one thinks of repelling measles or other eruptive diseases, and with good reason, for such practice would almost certainly induce serious affections of internal organs. For the same reason, every precaution must be used to allow this form of eruption to take its own course, while we merely regulate the constitutional symptoms as they obtrude. Another form of eruption, which occasionally, though much less frequently, results from either of the above mentioned sores, is the pustular. It is preceded by fever, and consists of rather large pustules, separated from one another, and not very numerous. After their apices give way, and the contained pus is discharged, a thin scab is formed, and on its separation a small ulcer is left, which in general soon heals from its margins, leaving a dark-coloured spot to mark its situation. The papular and pustular eruptions are sometimes blended; a few pustules appearing amongst numerous papulæ, or _vice versâ_. The pustular disease is not of frequent occurrence; and in proportion as it approaches the papular, with desquamation, it becomes milder and more easily removed. In it, as in the papular, mercury proves injurious. The phagedenic form of ulcer is the most dreadful and unmanageable of all; most uncertain in progress, and direful in event, and often rendered still more destructive by the mode of treatment adopted. Fortunately, it is now seldom seen, though not long ago it was well known, as a perpetrator of dreadful havoc, under the name of black pox.