The liquid is almost always methylene chloride, a solvent that is toxic and possibly carcinogenic. It is generally sealed in a glass vial or capsule to prevent its release; if it is broken, the area should be evacuated until the fumes have dissipated. Some early bubble lights instead used a lightweight oil or camphor (a white substance used in some moth balls) to create the low boiling point. In these older lamps, one can often see a white piece floating at the top of the vial, until the heat of the lamp dissolves it and it starts to bubble.