_FIRST_, What Cause can be assigned for such an uncertain Return of the Paroxysms? And, _SECONDLY_, What Reason can be given for the unequal Exacerbations when the Fits do recur? CAN any one doubt what Tumults and Disorders may be excited in the Blood, and other animal Juices, by that saline _Seminium_ of a Pestilence, which we have already described? The Uncertainty then of such Disorders must regard either the morbifick Venom, or the Nature and Motion of the Fluids upon which it operates: The morbifick Venom, in Proportion to its Energy, and Disagreement in Figure, irritates Nature, always ready in her own Preservation to expel the Enemy; but when her Exertions are slow, or imperfect, or quite languid, such a Depuration is not obtained; but upon a Remission of the Conflict, there is Space given for interval; and this indeed happens, when the Nature and peculiar Figures of the noxious Particles are such, as may in the first Struggle be broke and subdued, but after some Truce insinuates its Virulence further into the Habit, and imprints upon every Part the true Characteristicks of a fatal Pestilence. BUT to subdue and throw out the Enemy, the Spirits are at continual Strife, although their Efforts are not always successful; to dispatch this Matter therefore in one Word, as the Assimulation and perfect Mixture of the heterogeneous Particles procures a Motion regular and conformable to the Blood, so from an Inequality and disproportionate Mixture, arises an irregular Circulation and Fermentation, so that the Reason for that Uncertainty in these Fevers, and their irregular Returns and Exacerbations, is to be fought for in the Fluids and their circulating Vessels, and not from any Corruption, or Degree of Putrefaction, according to the Opinion of the Ancients. AND as for my own Part, I can affirm, that I never could in any one single Instance amongst the infected, find the least Impressions of Corruption in the Blood; and this even those Empiricks, though unwillingly, confess, who, to the vast Detriment of the Sick, let them Blood upon such a Notion; none of them having been ever able to discover any Signs of Corruption in their Blood, which as conscious of it self blushed for their fatal Mistake, and in this Distemper commonly appeared more florid than at other Times. THAT the Times of the Paroxysms should be uncertain, I take owing to the Inability of the Constitution to struggle with the pestilential Venom; for as every Fever is accounted regular, where all its Changes are uniform and distinct, by Reason of the managable and ductile Disposition of the morbifick Matter; so, on the contrary, where the pestilential _Miasmata_ uncertainly exert themselves, and excite new Commotions, either by the Obstinacy or Asperity of their Parts not yielding to Comminution, there a Fever returns with inconstant and unexpected Exacerbations: But to hasten to the subsequent Symptoms.