Vapour, a term denoting those particles of bodies, that are rarefied by heat: being thus rendered specifically lighter than the atmosphere, they ascend to a considerable height, and are at length totally dissipated.

The aqueous vapours, exhaled from the earth by the solar rays, compose the clouds; from which those humidities are precipitated, in the form of Rain. But, there are others arising from metals, extremely pernicious to animal life: such, for instance, are those disengaged by the smelting or refining of lead, which communicate a deleterious property to the grass in their vicinity ; so that the cattle feeding on it frequently perish; and, if any stagnant water be impregnated with these fumes, it proves equally fatal to fish.

There are likewise mephitic vapours, discharged from the bowels of the earth ; and which are peculiarly injurious both to man and cattle: of this nature, are the fumes exhaled from the Grotto del Cani, in Italy; and those generated in some wells, and subterraneous places, in Britain. As there -are instances in which these were attended with melancholy effects, various expedients have been devised, with a view to render them innocuous ; but the cheapest and most simple, is that proposed by Mr. EbenezEr Robinson, in the Transactions of the American Philosophical Society. He directs a pair of smith's bellows to be fixed in a wooden frame, so as to work in the same manner as at the forge: this apparatus being placed at the edge of the well, one end of a leather tube must be fastened to the nozzle of the bellows ; while the opposite extremity is thrown into the well; and, on blowing them for about half an hour, all pernicious vapours will be effectually expelled. - If, however, any person should unfortunately have been suffocated by these exhalations, Dr. Van Marum, as well as M. Van Troostwyk, in such cases, advise the lungs to be inflated with dephlogisticated or vital air, through a bladder and tube contrived for this purpose. The treatment pointed out under the article Suffocation, may also be usefully employed on such melancholy occasions.

In March, 1792, a patent was obtained by Mr. Charles William Ward, for a method of converting the vapour or smoke arising from the combustion of different substances, into various materials (which are not stated) : our limits not permitting us to describe the machines by which such object is effected, the curious reader will consult the specification inserted in the 1st vol. of the Re~ pertory of Arts, etc.

A patent was 3lso granted, in May, 1794, to Mr. RobertStreet, for an invention designed to produce an inflammable vapour-force, by which engines, pumps, and machinery, may be set in motion. His method consists in placing an iron cylinder with a counter-sunk touch-hole in the lower surface, over a stove. When the bottom is sufficiently heated, a little spirit of turpentine, or of tar (in the proportion of 10 drops to a cubic foot), is poured into a funnel ; which falls on the hot part of the cylinder, and produces an inflammable vapour. The external, condensed air, is then admitted to the touch-hole, where such fumes become ignited; and, by their concentrated force, a piston is raised, that communicates with, and works the pump or machinery, at the opposite end. - A farther account of Mr. Street's contrivance, is also given in the volume above quoted, where it is illustrated by an engraving. - For an ingenious and economical method of heating rooms by the evaporation of water in the form of steam, we refer the reader to the article Warmth.