PURPOSE: To demonstrate differential thermal expansion between two liquids, and to take us all back to the 1960s.

DESCRIPTION: Globs of petrolium-based fluids expand at a different rate from the water bath in which they are placed. After the mixture gets up to its equilibrium temperature differential expansion occurs, leading to the rising and falling of the globs as they become more or less dense than the liquid bath. It's also kind of pretty.

SUGGESTIONS: Must warm up for at least three hours, so call us and warn us that you have requested it so we can warm it up.

REFERENCES: (PIRA unknown.) See the Demonstration Reference File for an article on the inventor of the lava lamp written at the time of his death.

Here are some interesting web sites with information bout these lamps:

• How Things Work: How Liquid Motion Lamps Work Nice general explanation.
• oozinggoo.com: The Lava Lamp Syndicate Includes history and sales.
• The Science Hobbyist: MATERIALS FOR MOVING-GLOBULE LAMPS How to make one!

EQUIPMENT: Lava lamp, as photographed.

SETUP TIME: Three hours.