XIX. Three papers, containing experiments on factitious air

By factitious air, I mean in general any kind of air which is contained in other bodies in an unelastic state, and is produced from thence by art. By fixed air, I mean that particular species of factitious air, which is separated from alcaline substances by solution in acids or by calcination; and to which Dr. Black has given that name in his treatise on quicklime. As fixed air make a considerable part of the subject of the following papers; and as the name might incline on to think, that it signified any sort of air which is contained in other bodies in an unelastic form; I thought it best to give this explanation before I went any farther.