Mercury
A so-called "glass" thermometer has a small bore-hole in the center of the glass that has some liquid in it. It's the activity of the liquid in the narrow hole that makes the thermometer a...
its called "Mercury"
If the liquid is silver, it is liquid mercury (Hg). If the liquid is red, it is coloured ethyl alcohol (C2H5OH).
Actually anything that won't freeze or boil at the operating range of the thermometer. The most common are mercury and alcohol.