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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?)

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    $\begingroup$ Definitely a candidate for "What-if(xkcd) we tried moar power?" Presumably the vapor pressure is not all that high and enough heat is dissipated for the vapor to liquefy very quickly; but swap in a 400W bulb for the 1W (or whatever the standard is) and see what happens! $\endgroup$ Jul 1, 2014 at 14:58

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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!).

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In mine, the bubble tube is glued directly to the top of the bulb for maximum heat transfer. The bubbles form there and rise to the surface of the liquid, cooling as they go up further from the heat source. I think that is why they don't explode; the vapor in the bubbles condenses back to liquid in the length of the tube. It's a delicate balance between just enough heat to make them work and too much heat and catastrophe. Probably, if you had a long tube, the bubbles might condense before reaching the top. Or, on the other hand, they might not ever get hot enough to form bubbles. That's a good experiment for someone.

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