These bubbles contain gases the very same as are disengaged when animal and vegetable matter are putrefying among water. They are nearly the same as proceed from merely animal matter dead and putrefying, not incorporated with the soil, viz. carburetted hydrogen, or inflammable gas, carbonic acid gas, or fixed air, and sometimes a little phosphuretted and likewise sulphuretted hydrogen. These gases are sometimes appreciable to the organ of smell. Carburetted hydrogen is very strong, and is perceptible in many situations where there is much corruption going on; for instance, at the meadow-ground between the Dairy, on the Portobello road, and Comely Green, near Edinburgh, where the stench is so strong as to prove most offensive to passengers on the road. The source is the corrupting animal and vegetable materials, in the foul water conducted from Edinburgh, and made to overflow the ground, for the purposes of irrigation. In such situations, it is not uncommon to observe lights floating along during the night, and superstition has not failed to make them represent evil spirits. They are known by the name of “Will o’ Wisp” and “Jack o’ Lantern;” and have, on many occasions, proved objects of no slight dread to many ignorant persons. The lights are merely ignited carburetted hydrogen gas,—the same kind of gas as that used for lighting our shops and houses. The gas is ignited, perhaps, by the rising to the surface of the putrid water, of a bubble of phosphuretted hydrogen, which, as was before observed, burns the moment it comes in contact with the atmosphere. Other products of an aeriform kind may be evolved also, but we have no direct evidence of their existence,—but an atmosphere, loaded with vapours of the kind mentioned, is enough to account for the production of the observed disease, in all its varied forms, when there is conjoined with it other unwholesome agencies. In some countries, the pestilential air is present throughout the year, for instance, in the country around Rome, in the fens of Lincolnshire, where ague is seldom absent; in others it is periodical, chiefly confined to the hot and rainy seasons, as in India and in the West Indies, where fevers prevail to a great extent; and in others, again, it is observed only when the wind blows from a particular direction.