The ones I have seen are calibrated to a rather narrow range of typical indoor temperatures, with the floats made of glass with varying amounts of colored liquids and air enclosed. The float density is fine tuned by the addition of small metal weights. Different floats vary by about one degree F in the temperature at which they change from sinking to floating. The lower the temperature, the denser the liquid becomes and the more floats will rise to the top of the cylinder. Individual floats are labeled with the temperature at which the fluid density just equals that of the float.
Gallilean thermometers do have drawbacks.
The large volume of liquid required to contain the floats warms and cools slowly, so the temperature as recorded by the thermometer may not change nearly as fast as the temperature of the surroundings. But they provide an attractive way of monitoring your living room temperature.