Although water is the most abundant and important component of the body, many of its relations to disease have until recent years failed of recognition. In the blood alone, for example, acute changes in concentration are associated with conditions of shock or collapse and their treatment, while chronic hydremia (relative, at least) accompanies the anemias. Perhaps the favorite method of studying blood concentration heretofore has been by estimation of the hemoglobin content. This cannot be achieved with any degree of accuracy in less time than required for the complete development of the acid hematin color (ten minutes). Nor can any hemoglobin method justly claim a sensitivity of better than 1 or 2 per cent; Leake, 1 in fact, places the average error of the best methods at ± 5 per cent. The other body fluids do not lend themselves to very rapid and accurate determination of changes in the water