From this it is not very easy to determine what the Doctor means when he speaks of _specific contagion_ as the cause of the plague. Dr. Russel plainly ascribes it to human effluvia. “The plague (says he) is a contagious disease; that is, an _emanation from a body diseased_, passing into one which is sound, produces, in time, the same disease,” &c. There must, however, undoubtedly have been something originally distinct from the human body which gave rise at least to the _first_ plague that was in the world; and some plagues recorded in history are said to have arisen in this way. Thus, Ammianus Marcellinus says that the plague which broke out in the Roman army in the time of Marcus Aurelius arose from a pestilential vapour confined in a golden coffer dedicated to Apollo. Upon opening this, the _contagion_ diffused itself all around, and the _infection_ spreading from one to another, produced an almost universal pestilence. Ammianus indeed is the only historian who relates this; another account of its origin is given, p. 14, but whether we believe the account of Ammianus or not (which indeed does not appear probable) it is sufficient to show what were the received opinions at the time. In like manner every one has heard of pestilential effluvia breaking out from the earth, from graves, &c. so that we certainly look upon this doctrine of _contagion_ as the cause of diseases to have been pretty generally received. We are also informed by Dr. Mead, from M. Villani, who wrote the history of those times, that the great plague of 1346 began in China, where, according to the report of some Genoese sailors, it was occasioned by a great ball of fire that either burst out of the earth, or fell down from heaven. This is thought incredible by Dr. Mead, and no doubt is so, but it shows the general opinion, that the original cause or _contagion_ which produces a plague is distinct from the _infection_ which is afterwards communicated from one to another. In the French Encyclopedie, we have this account of the ball of fire, or fiery vapour, without any comment.