The idea is prevalent among physicians that hydremia or hydremic plethora is the rule in cases of edema. This impression is based on (1) the pallor or so-called pasty appearance of the skin, (2) the decrease in urinary output, (3) the accumulation of water in the subcutaneous tissues or in the serous cavities, and (4) the decrease, in certain cases, in the percentage of solids, hemoglobin, albumin, erythrocytes, and so forth, in the blood and plasma. Although these findings are suggestive of an edematous or watery state of the blood, that is, of dilution of the blood by water which has failed of excretion, there has long been a wish that this general impression might be supported by more tangible and conclusive evidence. Edema, although a familiar clinical phenomenon and the subject of innumerable investigations, is at present far from being understood. Since its experimental production is difficult, indeed practically