olume of any weight of water at a known
temperature can be readily calculated.

Pipettes which are to be used as measuring instruments should also have
the relation one to another of the volumes of liquid which they deliver
determined, and also the proportions these bear to the values found for
the divisions of the burettes in conjunction with which they will be
employed.

=To Calibrate Tubes for Measuring Gases.=--Prepare a small glass tube
sealed at one end and ground at the other to a plate of glass. The tube
should hold about as much mercury as will fill 10 mm. divisions of the
graduated tube. Fill this tube with mercury, removing all bubbles of air
that adhere to the sides by closing the open end of the tube with the
thumb, and washing them away with a large air-bubble left for the
purpose. If any persistently remain, remove them by means of a fine
piece of bone or wood. Then completely fill the tube with mercury,
removing any bubbles that may be introduced in the operation, and remove
the excess of mercury by placing the ground-glass plate on the mouth of
the tube, and pressing it so as to force out all excess of mercury
between the two surfaces. Clean the outside of the tube, and place it on
a small stand (this may be a small wide-mouthed glass bottle), with
which it has been previously weighed when empty, and re-weigh. Repeat
this operation several times. From the mean of the results, which should
differ one from another but very slightly, the capacity of the tube can
be calculated.

The purest mercury obtainable should be used. Since the density of pure
mercury at 0 deg. C. is 13.596, the weight of mercury required to fill the
tube at 0 deg. C., taken in grams, when divided by 13.596, will give the
capacity of the tube at 0 deg. C. in cubic centimetres. If the experiment be
not made at 0 deg. C., and if a very exact determination of the capacity of
the tube be required, the density of mercury must be corrected for
expansion or contraction.

Having now a vessel of known capacity, it can be employed for
ascertaining the capacities of the divisions of a graduated tube in the
following manner:--The graduated tube is fixed perpendicularly, mouth
upwards, in a secure position. The small tube of known capacity is
filled with mercury as previously described, and its contents are
transferred to the divided tube. The number of divisions which the known
volume of mercury occupies is noted after all air-bubbles have