The “frog” thermometer, made of glass in the mid-17th century AD, is attributed to Grand Duke Ferdinand II de’ Medici and is currently in the collection of Museo Galileo, Florence.
This clinical thermometer incorporates small glass spheres of varying densities, once immersed in a liquid known as acquarzente. The glass body is cracked and no longer contains its original fluid. In use, the device was tied to a patient’s wrist or arm with the frog’s head facing upward, and fluctuations in body temperature were observed through the movement of the spheres. These spheres shifted in response to temperature-induced changes in the volume of the acquarzente, earning the thermometer the additional name “infingardo” for its gradual response. The invention is attributed to Grand Duke Ferdinand II de’ Medici.