Acid bottles like these can be found inside the canisters of soda/acid fire extinguishers. Inside these bottles would most often be sulfuric acid, primed to mix with sodium bicarbonate to create a chemical reaction that would generate pressure within the fire extinguisher. These acid bottles would come with a plug that was meant to fall from the bottle when the extinguisher was turned on its end, allowing the two chemicals to mix. Removing the acid bottles from the containers allowed the bottle to be refilled with acid so that the extinguisher was always ready to be used in case of emergency.