-like
tube and break this across. This hair-like extremity will permit the
passage of air but is too fine for metallic mercury to pass.

3. From a standard graduated pipette deliver 5 c. mm. clean mercury into
the upper wide portion of the pipette.

4. Adjust a rubber teat to the pipette and by pressure on the bulb
gradually drive the mercury in an unbroken column down the capillary
tube until it is stopped by the filiform extremity.

5. Cut off the capillary tube exactly at the upper level of the column
of mercury, invert it and allow the mercury to run out.

6. Snap off the remainder of the capillary tube from the broad upper
portion of the pipette which is now destined to form the covering tube
or air chamber, or what we may term the "barrel." This barrel now has
the lower end in the form of a truncated cone, the upper end being cut
square. Remove the teat.

7. Introduce the capillary tube into this barrel with the filiform
extremity uppermost, and the square cut end projecting about 0.5 cm.
beyond the tapering end of the barrel.

[Illustration: FIG. 18.--Throttle pipette--small capacity.]

8. Drop a small pellet of sealing wax into the barrel by the side of the
capillary tube and then warm the tube at the gas flame until the wax
becomes softened and makes an air-tight joint between the capillary tube
and the end of the barrel.

9. Fit a rubber teat to the open end of the barrel, and so complete a
pipette which can be depended upon to always aspirate and deliver
exactly 5 cm. of fluid.

Slight modification of this procedure is necessary in making tubes to
measure larger volumes than say 75 c. mm. Thus to make a throttle
pipette to measure 100 c. mm.:

1. Take a short length of quill tubing and draw out one end into a roomy
capillary stem, and again draw out the extremity into a fine hair point,
thus forming a small Pasteur pipette with a hair-like capillary
extremity.

2. With a standard pipette fill 100 c. mm. into the neck of this
pipette, and make a scratch with a writing diamond at the upper level
(a) of the mercury meniscus (Fig. 19, A).

[Illustration: FIG. 19.--Making throttle pipettes--large capacity]

Now force the mercury down into the capillary stem as far as it will go,
so as to leave the upper part of the tube in the region of the diamond
scratch empty (Fig. 19, B).

3. Heat the tube in the region of the diamond scratch in the blowpipe
flame, and removing the tube from the flame draw it