The method of Roy for determining the specific gravity of the blood is a very excellent one, and is capable of yielding sufficiently accurate results. Over thirty bottles containing mixtures of glycerine and water of different specific gravity, ranging from 1·030 to 1·070, are used for the estimation, and a drop of blood to be tested is placed in a sample of one of these fluids. If the drop sinks it is heavier, if it floats it has a lower specific gravity, and then another drop of the same blood is tested until by a few experiments the exact specific gravity is determined.