There is no reason to believe that it was the design of the Almighty, that the process of putrefaction, which is so essential to the great plan of successive races of animals and vegetables, should be the active engine of pestilence, which it is in many situations. That is not the necessary consequence of putrefaction; and when it does occur, it is rather the effect of accidental circumstances. Under ordinary circumstances, where putrefaction goes on, as among vegetables moderately moist, exposed to currents of air, and mixed up with the soil, as in the various processes of agriculture, no bad results are experienced. But when vegetation is allowed to go on year after year, without being cropped, where, as it ripens, it withers and dies; and when it dies, is allowed to accumulate and putrefy, where there is much moisture, much solar heat, and little motion of the air, where, perhaps, other circumstances are operating, favourable to rapid decomposition, effluvia are wont to ascend and vitiate the atmosphere. Such a vitiated atmosphere has acquired various appellations, according to the place in which it has been observed, and according to the effects or diseases it produces. But, under whatever name it passes, its origin is the same, namely, decomposing vegetable matter on the surface of the earth, perhaps, in some situations, mixed more or less with dead animal matter. It is decomposing vegetable matter which produces the yellow fever of the West Indies, the jungle fever of India, the deadly pestilential fever of the coast of Africa, the ague in this country and in many others, the cretinism of Switzerland, the pellagra of Milan, the unwholesome condition of humanity in many parts of Italy, and especially in the country surrounding Rome, or the Campagna of Rome, as it is called. The decomposition, however, takes place under circumstances somewhat different, and hence the difference in the results of its action. These effects are attributed to the decomposition of vegetable matter; but there is room to think that, along with that, there is combined no very insignificant proportion of matter of an animal nature. It may safely be inferred, that wherever there is vegetation, there animals are found also; and it is well known that vast numbers of many kinds of animals live wherever decomposition is taking place, especially if the situation is warm and sheltered. The carcasses of these animals will be added to the vegetable matter, and add to the common mass of corruption. That matter in swamps, and in unwholesome situations, said to be purely vegetable, is then a compound of animal and vegetable origin; and these effluvia arise, not from vegetable decomposition only, but from both dead animal and vegetable substances in a state of putrefaction.