To contagion acting in the first-mentioned manner, has almost universally been applied the title, by distinction, Contagion, or immediate contagion; but in order to promote perspicuity, we shall call it Contactual or Palpable Contagion. To contagion acting in the second-mentioned manner, has been applied the term Fomites (impregnated clothes), but we shall call it Fomitic Contagion. To contagion acting in the third-mentioned manner, many terms have been applied indiscriminately, Contagion, Infection, Contagious Miasm, Infectious Air, &c. &c.; but to preserve distinctness, and to shew its relation to the other modes, we shall apply to it the title Atmospheric Contagion. With Atmospheric Contagion, the third mode in which contagion acts, has been confounded by many, air holding in solution, or having commingled with it, gases or impurities, not producing exclusively one disease, as contagious matter does; but productive of deranged health—or at least hurtful to life. Air thus tainted, has also been called Contagious, Infectious, &c. &c.; but as it is widely different, for the reason mentioned, they should not be confounded; and in order to prevent any accidental confusion, we shall term it vitiated, or, simply, impure air. There is yet another pestiferous principle called Marsh Miasm, which has sometimes, but less frequently, been confounded with the third mode in which contagion acts, viz. atmospheric contagion. They are very different: the former is confined to marshy lands, and produces exclusively disease of an intermittent character. Of the first mode in which contagion is said to act, contactual or palpable contagion, there is the most positive proof. That is a settled point capable of demonstration. Of the second, viz. fomitic contagion, there seems to be no good room to doubt. It is consistent with our knowledge, on points of a like nature, to admit the possibility of its existence; and there is evidence of pretty good character, that contagion does act in that shape, though we are disposed to think that it is not the cause of pestilence so often as is generally understood. It is to the third mode, viz. atmospheric contagion, that we object. We question its existence for these reasons, _first_, That in the whole course of its history, it fails to supply us with sufficient evidence thereof; _secondly_, That its supposed career is not marked with the same uniformity of effect, and constancy of character, cognisable among other powerful agents, but appears rather to be regulated by no fixed laws; _thirdly_, That the phenomena of disease do not go to shew that it is dependent on atmospheric contagion, the occurrence and dissemination of which, moreover, it could not explain. We are further disposed to deny its existence at all, for this reason, that its admission is opposed to the testimony of direct observation and of experiments instituted for the purpose.