n. (Chem.) An instrument for the volumetric measurement of gases; - so named because frequently used to determine the purity of the air. It usually consists of a finely graduated and calibrated glass tube, open at one end, the bottom; and having near the top a pair of platinum wires fused in, to allow the passage of an electric spark, as the process involves the explosion and combustion of one of the ingredients to be determined. The operation is conducted in a trough of mercury, or sometimes over water. Ure's eudiometer has the tube bent in the form of the letter U.