is also known that aqueous vapour increases the vaporizing force of all substances, becoming, as it were, an active agent, carrying off every diffusive emanation into the air. We can readily distinguish several earths and stones by their smell, nay, the metals themselves are not all devoid of odour, some particles, therefore, however minute they may be, pass from them into the air; added to these, are the attenuated matters given off in the processes of vegetation, and in the chemical changes which take place during decomposition.
All these emanations, as well as the vapour of water itself, are, in their ascent, so extremely diluted as not to affect, in any way, the senses or functions of the human frame. But, when they descend again upon the earth, they accumulate in the lowest strata of the atmosphere, are often appreciable by the senses, and, in many localities, unquestionably prejudicial. Whether the extraneous matters always descend when the aqueous vapour which bears them upward is condensed, or whether they sometimes remain in the upper regions an indefinite period, and then gradually, by some natural operation, accumulate and fall down, is a question not, perhaps, easily determined. If we consider the production of epidemical diseases an evidence of their effects, we may then reasonably conclude that they generally descend when the vapour with which they are associated is condensed, though, perhaps, they sometimes remain and accumulate until some hidden operation in the higher regions favours their precipitation. I by no means intend to be understood as implying
