Sydenham, who writes of the changes produced in the amount of disease according to locality, the constitutions of the seasons, &c., notices that London in his day was ill-built, ill-drained, ill-supplied with water, and the neighbouring country to the very suburbs so badly cleared, as to subject the inhabitants to a return of agues regularly in spring and autumn; and in the journal of De Foe we have assigned as one of the many causes of the pestilence that occurred in this country in 1665, the crowded state of the city and suburbs, which were prodigiously full in consequence of the disbanding of the armies after the restoration of the royal family and of the monarchy. Our city, says De Foe, was computed to have had in it 100,000 inhabitants _more_ than it ever entertained before; some computed the number as being twice as many, so that pestilence became in one year (1665) more terribly destructive from the immense numbers crowded together, aided by peculiar elemental disturbances, which had been rife for several years previously; pestilence indeed may be said to have existed in London continuously from the year 1661 unto 1666. In the year 1665 the terrible plague became aggravated in the month of July, from the sultriness of the atmosphere, cold nocturnal dews or damp fogs, frequent cold showers and cold winds alternating with the warmth and dryness of the air; in short, every extreme vicissitude seemed to exist in that dismal period of disease and death, producing agues, bilious disorders, bowel complaints, &c. In the next month the great general pestilence appeared to swallow up as it were all previous diseases in one common destruction. Indeed, from the month of August all other diseases were absorbed in the pestilence: all distempers, commencing in their usual form, were uniformly resolved into the deadly plague; and in the month of September, says the great historian, “Death rode triumphant among the devoted inhabitants of London; having borrowed the fatal scythe of Time, he mowed down the people like grass, and immerged the poor remains in horror and despair.” Having advanced thus much in support of the natural causes of universal distempers or epidemic pestilences, culled from the histories of pestilences from the earliest ages, we arrive at the consideration of that ‘vexata quæstio,’--contagion. CHAPTER XI. OF CONTAGION. Contention on such a subject as contagion is not to be wondered at, when we consider the obscurity attendant on all that relates to subtle agencies (the supposed qualities of contagion); and inasmuch as we observe that different effects arise from the same causes, as also similar effects from different causes in the human body, a vast field is necessarily opened up for contention, as every impartial observer will admit.