Agglutinin is the name given to a substance present in the blood-serum of an animal that has successfully resisted inoculation with a certain micro-organism. This substance possesses the power of collecting together in clumps and masses, or agglutinating watery suspensions of that particular microbe. ~Dilution of the Specific Serum~: _Apparatus required_: Sterile graduated capillary pipettes to contain 10 c. mm. (Fig. 17). Sterile graduated capillary pipettes to contain 90 c. mm. (Fig. 17). Small sterile test-tubes 5 × 0.5 cm. Normal saline solution in flask or test-tube. Pipette of specific serum. Glass cutting knife, or three-square file. Glass capsule, nearly full of dry silver sand, or roll of plasticine. Grease pencil. METHOD.-- 1. Take three sterile test-tubes and number them 1, 2 and 3. 2. Pipette 0.9 c.c. sterile normal saline solution into each tube, and stand tubes upright in the sand in the capsule, or in the plasticine block. 3. Make a scratch with the glass cutting knife on the blood pipette above the upper level of the clear serum, and snap off and discard the empty portion of the tube. 4. Remove 0.1 c.c. of the serum from the blood pipette tube, and mix it thoroughly with the fluid in tube No. 1; and label ~s.s.~, (specific serum), 10 per cent. 5. Remove 0.1 c.c. of the solution from tube No. 1 by means of a fresh pipette, and mix it with the contents of tube No. 2; and label ~s.s.~, 1 per cent. 6. Remove 0.1 c.c. of the solution from tube No. 2 by means of a fresh pipette, and mix it with the contents of tube No. 3; and label ~s.s.~, 0.1 per cent. When the yield of serum from the specimen of blood which has been collected, or is available, is small, the above method of diluting is not practicable, and the dilution should be carried out by Wright's method in a capillary teat pipette. ~Dilution of Serum by Means of a Teat Pipette.~ _Materials required:_ Blood pipette containing sample of specific serum after centrifugalisation. Capsule of diluting fluid--normal saline solution. Supply of Pasteur pipettes (Fig. 13a). India-rubber teats. Small test-tubes. A block of plasticine to act as a test-tube stand. Grease pencil. METHOD: 1. Mark three small test-tubes 10 per cent., 1 per cent. and 0.1 per cent. respectively, and stand them upright in the plasticine block. 2. Take a Pasteur pipette, nick the capillary stem just above the sealed end with a glass cutting knife, and snap off the sealed end with a quick movement so that the fracture is clean cut and at right angles to the long axis of the capillary stem--cut "square", in fact. Prepare several, say a dozen, in this manner. 3. Fit a rubber teat to the barrel of each of the pipettes. 4. Make a mark with the grease pencil on the stem of one of the pipettes about 2 or 3 cm. from the open extremity. [Illustration: FIG. 193.--Filling the capillary teat pipette.]