Question

It is observed that when a mercury is an glass thermometer is put in a flame, the column of mercury first descends and then rises. Explain.
@genius12

Answer 1

I guess because the glass of the thermometer expands first (it is a poor conductor), slightly increasing the internal volume of the thermometer? This would cause the still cooler mercury to descend. Once the mercury starts heating up, it will expand and the column will rise.

Answer 2

@TimSmit thanks.