Some other agents may be said, under some circumstances, to propagate themselves, but it is in a very remote way, and by no means by that direct and uniform operation which marks the propagation of contagious poisons and yeast, which is obviously as well defined as germination among animal and vegetable bodies. _Heat_, under some circumstances, does cause the production or evolution of heat, but that is rather an accidental circumstance, brought about remotely by the chemical operation produced, and would have taken place whatever had been the cause of that process, and is not the result of an immediate and particular property. Vitiated air also is calculated much in the same way to reproduce itself; but, instead of being in virtue of a quality possessed by the palpable contagious poisons, vitiated air of itself produces disease, and a common result of disease is vitiated or impure air. The close analogy subsisting between yeast and the palpable contagious poisons, it is hoped, has been fully made out; and though it is not permitted, by the rules of logic, positively to determine, that the laws which regulate the action of the one, necessarily hold with the other agents; yet, where there is no evidence of a contrary nature, the closeness of the connection lends countenance to the idea. That analogy seems remarkably strong when it is considered, that both yeast and the palpable contagious poisons produce their peculiar effects only once upon the same object. Many instances are known where the palpable contagious poisons have produced their peculiar effects more than once, but these deserve rather to be held as exceptions to the general law than as a proof against its existence. PART II. CHAPTER I. THE NEGATION OF ATMOSPHERIC CONTAGION FROM THE HISTORY AND ACTUAL OBSERVATION OF DISEASE. Ce qu’il y’a d’extraordinaire c’est que ceux qui fatiguent leur raison pour lui faire rapporter de certains événements à des vertus occultes n’ont pas un moindre effort à faire pour s’empêcher d’en voir la véritable cause. MONTESQUIEU. It has been attempted, in the preceding part of this work, to prove, on general principles, and by a reference to analogous objects, that atmospheric contagion cannot exist; but, lest that object should appear unaccomplished, and that the data are insufficient for the conclusion proposed, it is purposed to test the merits of the question by the consideration of the history and phenomena of disease.