Cartesian aqualungist

Overview:
A fun way to demonstrate the fact that liquids are nearly incompressible while gases are quite easy to compress.

Equipment:
a) Glass bottle with a wide mouth, or a glass jar, as tall as possible

b) *RUBBER* party balloon (not the plasic one !!!)

c) a small glass vial (the only requirement is that it fits through the opening of the jar or bottle.

d) tap water.

Safety:
If you break glass, don't chew on the shards.

How to do the experiment:
Goes as follows :

Fill the jar or widemouth bottle with water almost to the top, but leaving 2-3 cm of air above water level. Put the small vial in the bottle or jar in such a manner that the mouth points down, and carefully flood the vial with water almost but not completely to the top so that the vial *barely* floats.

Cut the party balloon open and close the mouth of the jar or bottle with a piece of rubber, securely attach the rubber by tying the rubber around the neck of the jar or bottle with a string (tightly !) so that the mouth is completely covered with a rubber diaphragm.

Now, press on the rubber with a finger and observe. If everything has been done properly, the little vial will gently sink to the bottom on the bottle as you press on the rubber and will immediately float up as you release the pressure. Experiment is repeateable ad infinitum or, at least, while the rubber holds.

Here is a scheme to illustrate how the finished set should look, if you didn't get it from the above explaination :

              ========  <---- rubber membrane
              |      |
              |      |
              |      | 
          -----      -----
         |      ___       |
         |#####!   !######| <---- water level and the floating vial
         |#####!###!######|
         |#####!###!######| 
         |#####!###!######|
         |################|
         |################| <- the bottle with water.
         |################|
         |################|
          ----------------

Useful References:

Further comments: