The thermometer consists of a steel bulb, a steel tube and Bourdon tube.
The bulb is partly filled with a volatile liquid, evacuated and sealed off. With rising temperature, the average velocity of the molecules in the liquid will increase. As a result, more liquid molecules will acquire enough energy to escape from the surface of the liquid and saturate the evacuated area.
The result will be an increase in vapour pressure in the capillary tube and Bourdon tube. The Bourdon tube, sensitive to the changes in pressure, will record the temperature via the pointer that is linked to the moving end of the Bourdon tube.