There is little known of the composition of these effluvia. We are most conversant with their situations, and their effects upon health. In different situations, they produce different diseases. But no known facts entitle us positively to say that their composition is different. It is a remarkable fact, but one well ascertained, that the atmosphere, in all parts of the world, in all climates, and in all situations, is much the same in its chemical composition. It manifests the same general physical characters in all situations, whether healthy or pestilential, and the nicest investigations have detected nothing in an atmosphere known to be pestilential, that is not found in the most wholesome. However, there is much reason to think, that this circumstance is owing, not to the absence of hurtful gases, but to the comparative insignificance of their volume beside that of the atmosphere itself, so vast in its dimensions. Medical men have been disposed to think that effluvia which cause one disease, say the yellow fever, are not the same as that which cause another, say the fever of the coast of Africa. The only reason offered is the difference of the diseases; but that is not enough to prove that the effluvia are different in their nature. Different effects, or effects so modified as to appear very different, are the results of the same cause on many occasions. The smoke of tobacco will make one person feel comfortable, another merry, another sick, another faint, and so on; but it would be unfair, from these differences in the effects, to pronounce that tobacco-smoke was in all these cases different in its own nature. We are satisfied that the effluvia or gases arising from marshy or unwholesome soil, are the same, generally speaking, in all situations, whatever disease is produced; and that the difference in the results is to be attributed to the varying circumstances under which they act,—for instance, the constancy or inconstancy of their operation, their greater or less intensity, the greater or less degree of concomitant moisture and heat, the greater or less amount of motion of the air,—the sheltered situation of human habitations,—the condition of the body, its predispositions from native country and the like, and the individual being accustomed or unaccustomed to the action of effluvia. Gases are known to arise from the marshy grounds mentioned, where animal and vegetable matter is putrefying, from the fact, that the neighbourhood of swamps is most unwholesome, the inhabitants and visitors almost uniformly suffering, because unwholesome effluvia are invariably known to emanate where animal and vegetable matter is thus corrupting, and because the gases themselves may be seen rising in bubbles out of putrid water, containing dead animal and vegetable matter in a state of corruption.