It would appear, however, that to reconcile conflicting opinions, various modern writers of experience and talent have endeavoured to show that epidemic diseases, _not_ contagious at their commencement, may acquire that character from confined air, filth, and other accumulations. That impure air, want of light, crowding together _unwashed_ numbers, &c., will by predisposition contribute to the production and aggravation of disease, as we observe to be the case in jails, hospitals, and in camp, I can fully understand; but that epidemic diseases, such, for instance, as plague, yellow fever, or cholera, are ever propagated by contact--contagion, I, after twenty-five years’ residence in pestilential countries, have no reason to believe; for I have seen epidemics seize on vast numbers at once,--I have sometimes seen them attack a whole people, or a part thereof, whilst at other times they have prevailed amongst the inhabitants of particular provinces and cities only. On what principle of contagion or infection, I would ask, can such universality or partiality of disease be accounted for or explained? Surely not from any cognizable property of contagion. All that I have ever read and seen of the nature of epidemics militates against the doctrine of contagion, whilst all that has been adduced, as far as my researches show, in support of contagion, has been of a negative, or at all events not of a positive, nature. Epidemic diseases, which have appeared and spread at different seasons,--in fact, at all times of the year,--in the middle of summer, for instance, as well as in the depth of winter,--which have also been found traversing whole continents, continuing their course for many successive months, and often assuming a definite direction or progress, often affecting large masses of people living on the same spot, while others in adjoining localities are exempt,--cannot, I contend, be attributed to contagion, but to the qualities and influences of the surrounding atmosphere, coupled with enervating habits, &c. AGAIN, CONTAGIOUS DISEASES ARE RECOGNIZABLE BY THE DETERMINATE PERIODS OF THEIR PHENOMENA, ESPECIALLY AS REGARDS SPECIFICATION, AS ALSO BY THEIR MODE OF PROPAGATION, WHILST, IF EPIDEMIC PESTILENCES WERE DEPENDENT ON, OR CAUSED BY, CONTAGION, THEY WOULD NEVER CEASE UNTIL WHOLE COMMUNITIES BECAME EXTINCT!