ALLEGRA BOVERMAN/Staff photo. Gloucester Daily Times. Gloucester: Watching the effect of Alka Seltzer tablets on the oil, water and food coloring in a water bottle as children create a lava lamp during the monthly science workshop held at the Sawyer Free Library on Tuesday. They were learning about the density of objects and liquids. The Alka Seltzer’s fizzing caused the liquids to bubble around inside and rise and fall just like in a lava lamp.