The flask is filled with highly soluble alkaline ammonia gas.
It dissolves in water causing a reduction in pressure that draws more water into the flask.
The demonstration is tricky but the result worthwhile.
The fountain takes place in this apparatus - a 2 decimetre cubed flask
fitted with a bung with a piece of glass tubing with a jet at one end
similar to the end of a pipette
and a syringe attached to a short length of flexible tubing.
The tip of the jet should end up
about two-thirds of the way up the flask.
This flask and indeed all the other apparatus must be absolutely dry.
If necessary rinse it with propanone
and allow it to evaporate.
The ammonia is generated in this apparatus.
880 ammonia is heated in here with a couple of boiling stones.
As it passes through here there's some potassium hydroxide in there which will dry it,
passes through here and is collected by downward displacement of air because
the ammonia is lighter than air.
Heat the ammonia solution gently for several minutes.
When you think that the flask is full
test the bottom with damp indicator paper, which will go purple,
showing that the flask is full.
When you're confident that the flask is full of ammonia, stopper it.
The flask can now be left for some time.
and you may prefer to prepare the flask of ammonia, and perhaps a spare, before the demonstration.
Have ready a 2 decimetre cubed trough full of water,
and add to it some universal indicator
and you want enough to give a clearly visible colour,
far more than you'd use for a titration, for example.
Add to it some acid. You want just enough to make the water turn red.
Then you need a stand and a clamp or a ring to hold the ammonia.
Quickly so that no ammonia escapes,
you need to remove the bung, and replace it with the jet assembly
keeping the rubber end of the tubing in the water.
Squirt some water from the syringe into the flask
and this may be enough to start the fountain.
If nothing happens immediately
then swirl the flask keeping the rubber end
of the tubing in the water.
As the water emerges it turns from red to purple
due to the alkaline nature of the ammonia.
The fountain will continue for some time until all the ammonia in the flask has been displaced by water.
Should the fountain fail to start the most likely reasons
are either the apparatus being wet or leaks in the apparatus.
This apparatus does have a small leak, but it has still worked, but it is probably
the reason why this hasn't gone through to completion.
You may find a competent assistant helpful in manoeuvering the flask
and you may need to practice it a few times before you become confident.
Preparing the ammonia flask beforehand with one or more spares may help on the first few occasions.