It has long been known, that in the vacuum of the barometer, mercury rises in a vaporous state at the usual temperature of this climate, and that persons employed in the mines from whence this metal is procured, as well as those who are employed in gilding and plating, have suffered paralytic and other constitutional affections, from inhaling the air saturated with mercurial vapours: had any doubt remained of mercury existing in the state alluded to, it would be effectually removed by the experiments made by Mr. Faraday, detailed in the twentieth number of the Journal of Science, &c. An unprecedented event, which occurred in one of His Majesty's ships of the line, at Cadiz, in the year 1810, a short time before I took upon me the charge of the Medical Department of the Mediterranean Fleet, has afforded me an opportunity of illustrating this subject on a very extensive scale, the details of which may not, perhaps, be uninteresting to the Royal Society.