When a Mercury in a glass thermometer is put on flames the Glass bulb because of high coefficient of expansion expands and the mercury inside it descends( appears to descend). As soon as the heated reaches mercury its expands faster than the glass and hence its level rises in the glass( α for mercury is greater than α for glass , α here is coefficient of thermal expansion)
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. hope this helps!
