Air-Traps.=--In Fig. 33, p. 66, an air-trap (_J_) is shown. An air-trap
is a device for preventing the mercury supplied to Sprengel pumps, etc.,
from carrying air into spaces that are exhausted, or are for any reason
to be kept free from air. Figs. 36 and 37 give examples of air-traps. In
the simpler of the two (Fig. 36) mercury flowing upwards from _C_ that
may carry bubbles of air with it passes through the bulb _A_, which is
_filled_ with mercury before use.[16] Any air which accompanies the
mercury will collect at _a_, the mercury will flow on through _b_. So
long as the level of the mercury in A is above _b_, the trap remains
effective.

[16] This may be done by clamping the tube which supplies mercury below
_C_, exhausting _A_, and then opening the clamped tube and allowing the
mercury to rise.

[Illustration: FIG. 36.]

[Illustration: FIG. 37.]

In the trap shown by Fig. 37, the tube _d_, which corresponds to _b_ in
Fig. 36, is protected at its end by the cup _E_. _E_ prevents the direct
passage of minute bubbles of air through _d_. This trap, like the other,
must be filled with mercury before it is used, and it will then remain
effective for some time.

CHAPTER V.

_GRADUATING AND CALIBRATING GLASS APPARATUS._

Although the subjects to which this concluding chapter is devoted do
not, properly speaking, consist of operations in glass-blowing, they are
so allied to the subject, and of such great importance, that I think a
brief account of them may advantageously be included.

=Graduating Tubes, etc.=--It was formerly the custom to graduate the
apparatus intended for use in quantitative work into parts of equal
capacity; for example, into cubic centimetres and fractions of cubic
centimetres. For the operations of volumetric analysis by liquids this
is still done. But for most purposes it is better to employ a scale of
equal divisions by length, usually of millimetres, and to determine the
relative values of the divisions afterwards, as described under
calibration. It rarely happens that the tube of which a burette or
eudiometer is made has equal divisions of its length of exactly equal
capacities throughout its entire length, and indeed, even for ordinary
volumetric work, no burette should be employed before its accuracy has
been verified. An excellent method for graduating glass tubes by
hand[17] has been described in Watts's _Dictionary of Chemistry_, and
elsewhere. Another excellent plan, which