How Methylene Chloride bubble tubes work.04:33

You’ve probably seen these tubes without realising it. They are used in Christmas lights (not so much in the UK) and for the bubble effect in old Wurlitzer jukeboxes.
They consist of a glass tube with one end closed, some porous material like pumice or rock salt to hold pockets of the liquid and allow it to heat to the point it vapourises, and the liquid itself, which is normally methylene chloride, a common solvent that is not particularly flammable.
A vacuum is drawn on the tube to lower the pressure to the point that the liquid will readily boil at room temperature, and the glass tube is then sealed off by melting it.
When even the slightest amount of heat is applied in the area where the liquid is trapped in the crystals, it will boil and create a stream of bubbles that travel up the tube and recondense back into the liquid again.