FIXED, or Fixable Air, an invisible and permanently elastic fluid, superior in gravity to common atmospheric air and most other aerial fluids, exceedingly destructive to animal life; produced in great quantities, naturally from combustible bodies, and artificially by many chemical processes. From its acid properties it has obtained the name of aerial acid, cretaceous acid, and carbonic acid; from its noxious qualities, it has been called mephitic air; or mephitic gas; and, from the circumstance of being produced in vast quantities during the combustion of charcoal, it first obtained from Van Helmont the name of gas sylvestre. The term fixed air has been given from its property of readily losing its elasticity, and fixing itself in many bodies, particularly those of the calcareous kind; and though some objected to the propriety of the term, the fluid in question is so well known by the name of fixed air, that we choose still to retain it. See CHEMISTRY Index. For an account of the apparatus for impregnating water with fixed air or carbonic acid, see MATERIA MEDICA Index.