"The effervescence or non-effervescence of alkalis with acids was proved by Black to accompany the presence or absence of fixed air (). From this time a distinction was clearly drawn between alkalis, which dissolved in acids without effervescence, and carbonated alkalis, the solution of which in acids was accompanied by the escape of carbonic acid gas. It was recognised that whether a caustic or a carbonated alkali dissolved in an acid, the body which remained in solution, and which had no close resemblence either to the acid or the alkali, was one and the same."