(4) Hammerschlag Method.--This is an indirect method which depends upon the fact that the percentage of hemoglobin varies directly with the specific gravity of the blood. It yields fairly accurate results except in leukemia, where the large number of leukocytes disturbs the relation. Mix chloroform and benzol in a urinometer tube so that {147} the specific gravity of the mixture is near the probable specific gravity of the blood. Add a drop of blood by means of a pipet of small caliber. If the drop floats near the surface, add a little benzol; if it sinks to the bottom, add a little chloroform. When it remains stationary near the middle, the mixture has the same specific gravity as the blood. Take the specific gravity with a urinometer, and obtain the corresponding percentage of hemoglobin from the following table: SPECIFIC HEMOGLOBIN SPECIFIC HEMOGLOBIN GRAVITY. PER CENT. GRAVITY. PER CENT. 1.033-1.035 25-30 1.048-1.050 55-65 1.035-1.038 30-35 1.050-1.053 65-70 1.038-1.040 35-40 1.053-1.055 70-75 1.040-1.045 40-45 1.055-1.057 75-85 1.045-1.048 45-55 1.057-1.060 85-95 For accurate results with this method, care and patience are demanded. The following precautions must be observed: (_a_) The two fluids must be well mixed after each addition of chloroform or benzol. Close the tube with the thumb and invert several times. Should this cause the drop of blood to break up into very small ones, adjust the specific gravity as accurately as possible with these, and test it with a fresh drop. (_b_) The drop of blood must not be too large; it must not contain an air-bubble, it must not adhere to the side of the tube, and it must not remain long in the fluid. (_c_) The urinometer must be standardized for the chloroform-benzol mixture. Most urinometers give a reading two or three degrees too high, owing to the low surface tension. Make a mixture such that a drop of distilled water will remain suspended in it (_i.e._, with a specific gravity of 1.000) and correct the urinometer by this. (5) Tallquist Method.--The popular Tallquist hemoglobinometer consists simply of a book of small sheets of absorbent paper and a carefully printed scale of colors (Fig. 68). Take up a large drop of blood with the absorbent paper, {148} and when the humid gloss is leaving, compare the stain with the color scale. The color which it matches gives the percentage of hemoglobin. Except in practised hands, this method is accurate only to within 10 or 20 per cent. [Illustration: FIG. 68.--Tallquist's hemoglobin scale.] * * * * *