that this opinion, confidently as it is maintained, is invalidated by authentic facts and records; but we must proceed to the consideration of our second problem, viz: II. _Does the matter of Contagion require the aid of a certain state of the air, (“Pestilential Constitution of the atmosphere.”) to give effect to its powers, and propagation; and to what causes are the decline and cessation of a contagious Pestilence to be attributed?_ It was laid down as a fundamental principle by Dr. _Mead_, that a “_corrupt_” state of the air is indispensable to the diffusion of a plague; and although we are at this day unable to ascertain in what this vitiated state of the air consists, yet there are too many stubborn facts on record to allow us to deny, or even to doubt the necessity of its existence for the propagation of a contagious fever. How are we otherwise to explain the fact of a malady like the plague, which, although it shall never be entirely absent from a city, rages only epidemically and fatally at particular times? Thus it is collected from the bills of mortality of London, that, although there were but four great plagues in this metropolis during the seventeenth century, _viz._ the years 1603, 1625, 1636, and 1665, (in the two first of which about 35,000, and in the last 68,000 died) yet that there were but three years, from the commencement of the bills of mortality in 1603 until 1670, which were entirely free from the plague.[199] _Diemerbroeck_ also remarks, that whenever the plague has been excited out of its proper season it has not spread; a fact corroborated by _Russel_ and _Hodges_.[200] It seems probable that a particular state of the atmosphere, in its relation to temperature and humidity, is one of the conditions, subordinate perhaps, of this “pestilential constitution.” Dr. _Russel_ has observed that, in _winter_, when infected persons have come to places about Aleppo, some of whom have died of the disease in the families where they lodged, the distemper was not by such means propagated. Dr. _Pugnet_ says that the susceptibility of a person for the contagion of plague is greatly increased by a moderately warm and moist atmosphere; and Dr. _Bancroft_[201] has adduced some observations made by himself in proof of the influence of atmospheric heat and cold, in both their extremes, in rendering the contagion dormant. The singular career which a pestilential epidemic runs, having a beginning, height, and decline, can only be explained on the idea of the pestilential constitution of the air undergoing corresponding changes; and it is probable that the return of a plague is a revival of infection that has been latent, or dormant, until a particular state of atmosphere rouses it to action. III. _Can Filth and Animal Putrefaction generate Contagion?_