This sudden departure or cessation of plague, upon the overflowing of the Nile, proves that contagion, though it may be the cause of some cases of that disease, is not the occasion of the vast majority,—the great mass of cases, in short, which constitute the Epidemic; and goes far to prove that distemper to be dependent upon an unwholesome condition of the soil, or vitiated atmosphere, and other widely extended and unwholesome agencies, of a nature totally different from specific contagious poison. That fact goes to prove, in reference to one disease, viz. Plague, what I believe holds with all other contagious distempers, that contagion, at most, is only an occasional, while such influences as those to which reference has been made, are the constant and general causes of sickness. CHAPTER XII. THE PREVENTION AND CORRECTION OF VITIATED AIR. The important part which vitiated air enacts in the production of many forms of disease, has been already fully shewn; and it must be admitted, that whatever has for its objects the prevention and correction of that principle, is deserving of attention. By preventing the production, and by removing vitiated air when already formed, a vast amount of disease may be arrested, and much of that benefit will actually be accomplished, which it was boldly but fallaciously pronounced would revert from many absurd measures which were adopted, and which are still recommended for the avoidance of contagion, and would realize almost all the advantages which Quarantine Regulations, and the most efficient systems of Contagion Police can or propose to afford; and that, too, at no inconvenience to individuals, no restraint upon communication, after certain processes of purification have been undergone, and no ruinous hindrance of commercial transactions. The various sources of vitiated air have been already noted. Some of them are beyond any present remedy, as the unwholesome condition of the surface of the earth in many regions, within the tropics, for whose correction or improvement, time, capital, enterprise, labour, and perhaps new climates, are essential. To that source of vitiated air, draining, cutting down superabundant wood, embanking rivers, reclaiming partially inundated land, and cultivation, must be applied before the emanations which infest these situations can be prevented from arising. Another source of vitiated air is, men being crowded together in close and confined apartments, where no attention is paid to the preservation of cleanliness and the removal of impurities, as in some jails and other places for the confinement of criminals. That source of vitiated air is particularly worthy of notice here, because a very common form of disease which it induces is what is well known as Jail or Contagious Fever.