Gosh, Don, now I realize how inadequate my explanations must be. It's not
the bottle that expands, but the bladder connected to it by the aquarium
tubing (the little white balloon, although you don't want to use a balloon).
In the first link, you can see me holding our prototype
blackbottle.http://home.earthlink.net/~clanl/pic-001.jpg and http://home.earthlink.net/~clanl/pic-002.jpg