The Galileo thermometer consists of a sealed glass tube that is filled with water and several floating bubbles. The bubbles are glass spheres
filled with a colored liquid mixture. This liquid mixture may contain alcohol, or it might simply be water with food coloring. Attached to each
bubble is a little metal tag that indicates a temperature. A number and degree symbol are engraved in the tag. These metal tags are actually
calibrated counterweights. The weight of each tag is slightly different from the others. Since the bubbles are all hand-blown glass, they are not
exactly the same size and shape. The bubbles are calibrated by adding a certain amount of fluid to them so that they have exact same density. So,
after the weighted tags are attached to the bubbles, each differs very slightly in density from the other bubbles, and the density of all of them
is very close to the density of the surrounding water.
The basic idea is that as the temperature of the air outside the thermometer changes, so does the temperature of the water surrounding the
bubbles. As the temperature of the water changes, it either expands or contracts, thereby changing its density. At this changed density, some of
the bubbles will float and others will sink. HowStuffWorks or Wikipedia.