that the vapour every where arising from the surface of the globe necessarily disentangles noxious exhalations, but, that where the surface does contain the materials for their production, there aqueous vapour promotes their disengagement, is the agent through whose means they escape, and associated with which they rise into the higher regions; and, therefore, that it is important the various forms and changes to which this the more tangible material (if we use the expression) is subject, should be attended to.
Water can exist in an aeriform and invisible shape only under the influence of caloric, and the quantity of vapour present in the atmosphere has always a reference to the temperature of that medium; there being a variable point at which it must assume a different, a visible shape. This variable point, in Daniell's hygrometer, is exhibited in degrees of the thermometer, and is called the dew-point; the instrument shewing the exact thermometric degree to which, if the temperature of the air falls, the vapour mingled with it will begin to assume the form either of dew, fog, or cloud.
When the lower strata of aqueous vapour of the atmosphere are condensed in the form of dew, those above descend and experience a similar change. Now, if vapour undergoing this process is associated with extraneous invisible exhalations, it is probable that the latter may continue floating about in the air after the vapour with which they were conjoined has been deposited upon the ground, and the process continuing, of course they become concentrated, and, if of a noxious quality, dangerous.
