How do you fix spirit thermometers where the alcohol has separated?
Place the thermometers in a beaker of vegetable oil and heat the oil using a Bunsen or preferably a hot plate which is safer. The oil will heat to over 100 degrees C which means all the spirit will then join up again at the very top section of the thermometer.
Keep a watch on them and remove them from the oil when the spirit rejoins.
You must keep a very close eye on the thermometers because of the pressure build up. Wear safety glasses etc because the spirit has been known to burst through, most likely around the bulb at the bottom, as the glass is thinner there. If done carefully, the job can be managed without breakage. Do not be overly-alarmed at potential breakage. It is not violent, just messy and with appropriate eye-protection and lab coat you should be perfectly safe.
This method does not fix them permanently so you may find that some separate out again after numerous uses.