This section is from the book "A Treatise On Therapeutics, And Pharmacology Or Materia Medica Vol1", by George B. Wood. Also available from Amazon: Part 1 and Part 2.
This may be a state either of gas or vapour. Gases are aeriform fluids which retain their condition at common temperatures. Vapors are likewise aeriform fluids, but require an elevated temperature, under the ordinary degree of atmospheric pressure, to enable them to retain that state, and, upon the diminution of their temperature, become liquid or solid. Both gases and vapours arc employed as medicinal agents, the former seldom the latter very frequently. Gases are used chiefly by inhalation; vapours both in this manner, and by application to the surface of the body.