ontents 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.]

5. Compress the teat between the finger and thumb (Fig. 193) to such an
extent as to drive out the greater part of the contained air.

6. Maintaining the pressure on the teat pass the stem of the pipette
into the capsule holding the saline solution, until the open end of the
pipette is below the level of the fluid.

7. Now cautiously relax the pressure on the teat and let the fluid enter
the pipette and rise in the stem until it reaches the level of the
grease pencil mark. As soon as this point is reached, check the movement
of the column of fluid by maintaining the pressure on the teat, neither
relaxing nor increasing it.

8. Withdraw the point of the pipette clear of the fluid, and again relax
the pressure on the teat very slightly. The column of saline solution
rises higher in the stem, and a column of air will now enter the pipette
and serve as an index to separate the first volume of f