Eudiometer (Gr. pure air, andmeasure), an instrument invented by Priestley for determining the proportion of oxygen in the air, in the belief that on this depended its salubrity. Many other instruments have since been invented for estimating the amount of oxygen in gaseous mixtures, and the name is retained for these, though it has no longer its original significance. In the application of the instrument for estimating oxygen, the gas is made to unite with some substance, as phosphorus, introduced into the gaseous mixture, which is contained in the upper end of a graduated glass tube inverted over mercury. The diminution of bulk caused by the absorption of the oxygen indicates its quantity. In other forms a known quantity of hydrogen is introduced, and a mixture fired by an electric spark produced by means of two wires being melted into the sides of the tube and nearly meeting each other within. In this case the tube is made very thick to withstand the explosion.

Every two volumes of hydrogen consume one of oxygen, whence the quantity of the latter may be estimated.