(καίδα pure air, and μέτρον a measure), the name of an instrument or of any apparatus subservient to the chemical analysis of the atmospheric air. The first instrument for this purpose was introduced under the idea that the salubrity of the atmosphere depended upon its relative quantity of oxygen; but it is now known that the amount is not subject to any appreciable fluctuation, but that atmospheric air, when it has perfect freedom of motion and mixture, contains in all situations 20 volumes of oxygen in 100. The practice of eudiometry has been extended to other gaseous mixtures, chiefly for the purpose of determining the amount of oxygen they contain. This method of analysis is described under Chemistry, vol. vi., p. 464.