The Galilean thermometer has a series of fluid filled glass bulbs (different colors) in a tube filled nearly to the top with another fluid. The manufacturer warns that this fluid is not water and is potentially toxic. The glass bulbs are arranged in terms of decreasing density with increasing height. The density of the bulbs is very near the density of the surrounding fluid. When the temperature changes the fluid expands or contracts slightly changing its density, enabling a different number of bulbs to float. Is it always binary, either to the top or to the bottom? Doesn't the fluid inside a bulb expand too? Lots to sort out! The color is a dye that glows in dark light.