The doubt that had arisen in the minds of several experienced practitioners in regard to the non-contagious nature of the plague, is a matter of vital interest, since it not only concerns the health of nations, but in a commercial point of view it becomes a question of political economy of the utmost importance, as the severity of the quarantine laws, which must materially effect the prosperity of trade, would become useless if it could be proved that no contagion is to be apprehended from a free intercourse. It is somewhat curious that Dr. Mead long ago expressed his decided opinion that whenever the doctrine of non-contagion should be revived in England, (and it will be so, he adds, even a hundred years hence,) it will always excite alarm amongst those nations who are more prudent than ourselves, and less eager to entertain every kind of wild and visionary speculation. The contagionists affirm that the destructive ravages of the plague of Marseilles in 1720, when 60,000 inhabitants were carried off, arose from neglect in enforcing a rigid separation of the diseased from the healthy part of the community. The mortality in the plague of Messina, in 1743, during which 43,000 people fell victims to the disorder, is also referred to similar causes. They also maintain that the London plague of 1593, which destroyed 11,503 persons, was ascertained to have been introduced from Alkmaer; that the pestilence of London in 1603, which carried off 36,269 inhabitants, was brought from Ostend, and further that in 1636, the scourge which destroyed 13,480 victims in our metropolis, had been imported from Leyden. In 1665, when its still more fatal ravages swept away 68,596 citizens, it had also been traced to our foreign intercourse. Dr. Merlens who has accurately described the plague that raged at Moscow in 1771, asserts that it was introduced by a communication with the Turkish army. Notwithstanding which, by keeping the patients strictly guarded, the city was maintained free from infection, while the disease raged around in every quarter.