It is observed that when a mercury in a glass thermometer is put in a flame the column of murcury first descends and then rises explain

Mercury expands when heat comes into contact with it. at first, the thermometer touches the flame and gains heat, expanding. hence, more space inside the thermometer. the mercury has more space, so descends. finally, the heat passes through the glass after a while, as glass is a poor conductor of heat. the mercury gains heat and expands, rising.
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