2. Heat the capillary portion near the free extremity in the by-pass flame of the bunsen burner and draw it out into a very fine hair-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 out so that the diamond scratch now occupies a position somewhere near the centre of this new capillary portion (Fig. 19, C). 4. Heat the tube in this position in the peep flame of the Bunsen burner, and draw it out into a hair-like extremity. Snap off the glass tube, leaving about 5 mm. of hair-like extremity attached to the upper capillary portion (Fig. 19, D). Allow the glass to cool.