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A boiling tube is a small cylindrical vessel used to strongly heat substances in the flame of a Bunsen burner.
A boiling tube is essentially a scaled-up test tube, being about 50% larger. Boiling tubes are made from good quality borosilicate glass (Pyrex) or sometimes even fused quartz, that can withstand high temperatures, and thus can be directly exposed to the hot flame of a burner without risk or breaking.
Boiling tubes are designed to be wide enough to allow substances to boil violently. This is because a test tube is too narrow, which means that the boiling liquid can burst out of the end of test tubes when heated, as there is no room for the gas bubbles to escape independently of the surrounding liquid.
Boiling tubes can be bought from lab suppliers and online. Make sure the tube it's made of good quality heat resistant glass.