Alkaline SPIRITS, solutions of volatile alkaline salt in water, sometimes with a portion of empyreumatic oil; in which case they receive different appellations, as spirit of hartshorn, of urine, &c. If the volatile alkali contained in these solutions be united with a considerable quantity of fixed air, the spirit effervesces with an acid, and is called a mild spirit; but if the alkali is deprived of its fixed air, the spirit is said to be caustic, and raises no effervescence. The mild spirits, though to the smell they appear much weaker than the caustic, are in reality stronger, as containing a larger proportion of alkali than the latter commonly do. However, there is no alkaline spirit perfectly mild, or it would be entirely neutralised by the fixed air, so that no alkaline smell or taste could be perceived. All mild spirits therefore are partly caustic, to which they owe that degree of smell and pungent taste which they retain. However, being already partly neutralized, they are much less apt to imbibe fixed air than such as have none at all; the solution of the neutral salt composed of volatile alkali and fixed air also is much more apt to retain the alkaline air or caustic salt than pure water; and hence the mild alkaline spirits preserve a much more equable degree of strength than the caustic ones, which partly by the attraction of fixed air, and partly by evaporation, unless very firmly secured, very soon become sensibly weaker both in taste and smell than when originally made.