In order to show that the “air” which exists in combination with lime or alkalies is not the air which is contained in solution in water, lime-water was placed under an air-pump, along with an equal quantity of pure water; on making a vacuum, an approximately equal amount of air was evolved from each. “Quicklime, therefore, does not attract air when in its most ordinary form, but is capable of being joined to one particular species only, which is dispersed through the atmosphere, either in the shape of an exceedingly subtile powder, or more probably in that of an elastic fluid. To this I have given the name of _fixed air_, and perhaps very improperly; but I thought it better to use a word already familiar in philosophy than to invent a new name, before we be more fully acquainted with the nature and properties of this substance, which will probably be the subject of my further inquiry.” The next proceeding was to render “mild alkali” caustic by means of lime, and to determine that nearly the same amount of acid is required to saturate the caustic alkali as to saturate the mild alkali from which the caustic alkali had been prepared. On exposure to air for a fortnight, the caustic alkali again became mild, owing to its absorption of fixed air. Careful experiments were made to prove that such caustic alkali contains no lime, and does not therefore owe its causticity and corrosive properties to the presence of that ingredient. The volatile alkali (ammonium carbonate) was also rendered caustic, and Black “obtained an exceedingly volatile and acrid spirit, which neither effervesced with acids nor altered in the least the transparency of lime-water; and although very strong was lighter than water, and floated upon it like spirit of wine.”