WEIGHING 21until the acid rises and barely touches the solid in the generator
tube. The generation of gas will now begin and proceed auto-
matically as fast as the gas is allowed to flow from the delivery
tube H.
Place the solution to be saturated with the gas in the flask G,
insert the stopper and delivery tube, but let the stopper remain
loose until the air is entirely expelled. Then make the stopper
tight; the gas will pass in as rapidly as it can be absorbed by the
solution. Shaking the receiving flask will greatly increase the
rapidity of absorption, but this should be done with a good deal
of caution at first, because if the undiluted acid is drawn too sud-
denly up in the tube E, the violence of the reaction may either blow
out the stoppers or drive foam through the glass wool filter F and
into the delivery tube H. After the solution is partly saturated
the flask may be shaken continuously and the reservoir A raised
to a higher level.
- WEIGHING
Two types of balances are available for weighing: - Rough Balances or Platform Scales. These are to be used
for weighing out materials approximately for preparations and
qualitative experiments and for weighing heavy objects of more
than 100 grams. - Analytical balances, which will weigh accurately to a centi-
gram. These balances are enclosed in glass cases. They are not
as sensitive as the best analytical balances, which weigh to one-
tenth of a milligram, but they are sufficiently accurate to do some
kinds of work extremely well and are so sensitive that they require
careful and intelligent handling. Hence before using one of these
balances apply to an instructor for individual instruction as to its
manipulation. The balances must not be used until permission
is obtained. The following general rules must always be ob-
served : - No load of more than 100 grams should be put on the sensitive
balances. No object heavier than this will need to be weighed
with a greater precision than can be obtained on the platform
scales. - The material to be weighed, unless it is in a single, clean,
dry piece, should never be placed directly on the scale pan.