The disappearance of the disease, while all the causes that we suppose capable of producing it remain in full force, is a demonstration that it depends on something entirely distinct from the human body, and from all those powers which perceptibly act upon it. It proves that this unknown power has only a temporary existence, coming to perfection at one season, and dying away in another; sometimes capable of being revived, and sometimes not. This corresponds entirely with what has been laid down concerning contagion itself, viz. that like other aerial vapours it is capable of decomposition, and remaining for an uncertain length of time in a dormant state; but that occasionally it may revive, and appear unexpectedly, as _mofetes_ arise from lavas, or damps in mines. After a city has been thoroughly infected with a pestilential disorder, therefore, there can be no security against its re-appearance; it being impossible to know whether the contagion may not be still existing and capable of being revived by some unknown cause, though it has been dormant ever so long. In such cases it may with propriety be said to have arisen _spontaneously_, though, had it not been there at a former period, there could be no reason to think that it would have appeared at that time. Dr. Russel next takes into consideration the plague at Marseilles in 1720, of which he says the accounts “are more full, and circumstances better authenticated, than most of the accounts of anterior plagues to be met with in books.” From the opposition to the doctrine of contagion at the time, he also supposes that the facts relative to its introduction would be severely scrutinized, and falsehoods detected: “but (says he) if, instead of such detection, the most material have been passed over in silence, and little more than hypothetical reasoning opposed to others, the main facts may be considered as established, if possible, more firmly than they were before.” These facts are stated as follows: “1. That the plague did not exist in France before the 25th of May, 1720. 2. That it was imported in goods from the Levant, by a ship which left the coast of Syria the beginning of February, and arrived at Marseilles the 25th of May. Two days after her arrival one of the sailors died; an officer of quarantine who had been put on board died on the 12th of June, and a cabin boy on the 23d. Some porters employed in opening the merchandise at the lazaretto also died about this time. Three others were taken ill in the beginning of July, with buboes in the groin and axilla. This alarmed the surgeon of the lazaretto; a consultation was held with two other surgeons on the 28th; the disease was unanimously declared to be the plague, and the three patients died next day: the surgeon of the lazaretto, with part of his family, and the priest who attended the sick, were also taken ill and died.”