80	QUANTITATIVE AGRICULTURAL ANALYSIS
I
!    ;	Place the bulbs in position, close the cock of the dropping fan**el  an(*
I    "	open the pinch cock at e to allow water to flow from the aspirator.     J3u.bbles
{'   <	of air will at first pass through the bulbs but this action will finally cease
;    I   !	unless there is a leak in the apparatus, in which case it must be found and
|     '	closed.    It is important that all glass tubes be brought entirely together
I    e	inside the rubber connections since rubber is slightly permeable to gases.
>    i	After the apparatus has been shown to be free from leaks the   pinch
I    '	cock at / is closed, the cock of the separatory funnel is slowly opened and,
I    r	after equilibrium is established, the clamp k is so adjusted that when clamp
[    \	/is opened air will pass through the bulbs at a rate not greater than three
|    1	bubbles per second.    Clamp k is not thereafter changed.    This  provides
'<	against too rapid flow of gas under any conditions.    Clamp/is now closed,
i	,	the bulbs are removed, the inlet and outlet tubes are closed by short rubber
;	tubes containing glass plugs and the bulbs are Wiped clean and placed in the
;   .	balance case.    A short glass tube is inserted to bridge the gap made by
removing the bulbs.
;	The bulbs should be allowed to stand for 15 minutes before weighing;.     In
the meantime about 1 gm of the carbonate is weighed and brushed into the
generating flask and a small amount of water is added to moisten   the
sample.    The stopcock of the funnel B and the clamp E are now  opened
„{	and 500 cc of air is drawn through the apparatus, measured by the  out-
flowing water from the aspirator. This frees the apparatus from eaxbon
dioxide. After the absorption bulbs have stood for 15 minutes the tubes
carrying the plugs are removed and the bulbs are weighed. The plxip;H llr<}
>	'	then replaced and left so until the bulbs can be connected in the ap pa, rut us.
i   ,	Fifty cubic centimeters of dilute sulphuric acid or hydrochloric acid is placed
'   '         ' I	m *ne dropping funnel, a test having previously been made to determine
: |	whether sulphuric acid will form a clear solution with the carbonate.     If
*	such a solution is not produced, of course hydrochloric acid must be used
and silver sulphate and pumice must be placed in tube E.
Reconnect the apparatus and open all cocks except the stopcock in the
dropping funnel, leaving the clamp k set for the proper rate of gas flow, as
previously determined. Slowly open the cock of the dropping funnel,
allowing acid to drop just fast enough to evolve carbon dioxide at the
!	prescribed rate.    The constant attention of the operator is necessary at this
point, for by causing too rapid evolution of gas some moisture may escape
absorption in the small tube of the absorption bulbs and the experiment
1   •   .	be rendered worthless.
The acid should be allowed to run in until about 1 cc is left above the
:	stopcock, this acting as a seal during the subsequent boiling.    After the
decomposition of the carbonate is complete the solution in the flask is
slowly heated until it boils, always with due regard to the rate at which
the gas is made to flow through the absorption bulbs. The boiling is
continued for one minute, when the flame is withdrawn, the cock of the
dropping funnel being opened at the same time to allow air to enter bo
that no back suction occurs, due to the cooling effect. Air is now drsawn.
through the apparatus until 1000 cc of water has flowed from the suspira-