As a child, I loved to watch the bubble lights at Christmas time. The colorful bubbling was hypnotic. However, there was always something about them that bothered me. With all of that boiling happening within the glass vial, why didn't the entire thing explode? I knew how explosive steam boiling could be, why was the bubble light able to withstand the constant boiling, and pressure changes that were going on inside it? Also, how was the fluid able to boil within an enclosed space (where the volume could not change?)
The boiling point of methylene chloride (mentioned in the Wikipedia article in the question) is 40C (see, say, the Dow product data sheet). This is above room temperature in most houses, and not far off of what the normal light bulb temperature is (you don't want it too hot or else people get burned and houses burn down). So, as long as the tube has enough surface area to keep the top below the boiling point the cycle will continue. Certainly, you could put so much power in the bottom that you would heat the entire device above the boiling point and then bad things would happen (boom!).