lloon has about all it will
hold, by pressing gently with your fingers. If the rubber
feels tight, do not blow any more. Let the air out of the
balloon again.

Now get some hydrochloric acid (HCl) diluted with three parts
of water. Find the bottle marked "HCl, dilute 1-3," in which
the acid is already diluted. Before you open the bottle,
get some solution of soda, and keep it near you; if in this
experiment or any other you spatter acid on your hands or
face or clothes, wash it off _immediately_ with soda solution.
_Remember this._ Ammonia will do as well as the soda solution
to wash off the acid, but be careful not to get it into your
eyes.

Pour the hydrochloric acid (HCl) on the zinc shavings in the
bottom of the flask, until the acid stands about an inch deep.
Then quickly put the rubber stopper with its attachments into
the flask, so that the gas that bubbles up will blow up the
balloon.

If the bubbles do not form rapidly, ask the teacher to pour a
little strong hydrochloric acid into the flask; but this will
probably not be necessary. Let the balloon keep filling until
it is as large as you blew it. But if the bubbles stop coming
before it gets as large as that, close the neck of the balloon
by pinching it tightly, and take the stopper out. Let some
one add more zinc shavings and more acid to the flask; put the
stopper back in, and stop pinching the neck of the balloon.
_In this and all other experiments when you use strong acids,
pour the used acids into the crockery jar that is provided
for such wastes. Do not pour them into the sink, as acids ruin
sink drainpipes._

When the balloon is full, close the neck by slipping the
rubber band up from the part of the neck that is over the
glass tube on to the upper part of the neck. Pull the balloon
off the glass tube and pinch the neck firmly shut. Take the
stopper out and rinse the flask several times with running
water. Any zinc that is left should be rinsed thoroughly,
dried, and set aside so that it may be used again. Now tie one
end of a long thread firmly around the mouth of the balloon
and let the balloon go. Does it rise? If it does not, the
reason is that you did not get it full enough. In that case
make more hydrogen and fill it fuller, as explained above.

[Illustration: FIG. 162. Filling a b