The earliest spirit‐in‐glass thermometer and a comparison between the earliest CET and Italian observations

The earliest device to detect temperature changes was a vertical glass tube, with a ball blown on the upper extremity and the bottom end open and immersed in a cup filled with liquid (Figure 1). When the air pocket in the ball is warmed, it expands and some air is expelled forming bubbles in the cup; when the ball is cooled, a column of water enters the tube and the level moves upwards or downwards following any temperature change. However, this device was not properly an air thermometer because, in addition to temperature, it was sensitive to the external air pressure exerted on the surface of the liquid in the cup; it was called a thermoscope. Some experiences based on the principle of water displaced with the thermal expansion of air as well as some peculiar devices based on the above principle were published in a book by Philo of Byzantium (third century BC) and Heron of Alexandria (first century BC). Philo was translated into Latin in the fourteenth century and a codex is preserved at the Bayerische Staatsbibliothek in Munich; it was published in Rome in 1640, after the invention of the thermometer. Heron was translated and published four times, first in Latin and then three times in Italian, from 1575 to 1595. In 1593 Galileo Galilei, probably inspired by Heron, made experiments with the thermoscope in his lessons at the University of Padua (Italy), and explained the theory of the water column displaced by the action of expanding air. For this reason he claimed to be the Father of the air thermometer. In 1612 Santorio Santorii, another teacher at the Padua University, added a scale to the thermoscope, so making possible quantitative readings, although in arbitrary units useful to his medical investigations. In 1638 Robert Fludd, an Englishman who travelled in Italy and Germany for six years, and Cornelius Drebbel, a Dutchman who lived in the United Kingdom, were inspired by the earlier work and they too claimed to be Fathers of the thermoscope. This instrument was therefore studied by several investigators, more or less independently of each other (Middleton, 1966; Frisinger, 1983). However, every thermoscope had its own particular scale and, as described above, the readings were conditioned by atmospheric pressure; this difficulty was partially overcome in 1702 by the French scientist Guillaume Amontons, who added barometric pressure to the thermoscope readings. Another solution was found in 1707 by the Italian scientist Vittorio Francesco Stancari who sealed the end of the capillary tube in order to make the readings independent from the atmospheric pressure. Nevertheless, in the seventeenth century, the air thermometer was not yet fully developed and the thermoscopes were unable to provide quantitative temperature observations. The first thermometer independent from atmospheric pressure and useful for objective observations was the liquid-in-glass thermometer, with a sealed tube. The spiritin-glass thermometer was invented in Florence in 1641, or earlier, by the Grand Duke of Tuscany (Ferdinand II de’ Medici), Evangelista Torricelli and Galileo. Several types were built and tested; the best instrument had the scale divided into 50 degrees Galileo (°G) and is known as the Little Florentine Thermometer (LFT). It was made completely of glass: on the bottom a hollow sphere constituted the bulb, and a capillary tube was sealed to the sphere. The scale was marked on the tube with glass enamel beads (Figure 2). The thermometer was filled with refined spirit and then the top was hermetically sealed by flame, leaving some air in the upper part of the tube. Its history and characteristics, including calibration, have been extensively described elsewhere (Middleton, 1966; Borchi and Macii, 1997; Camuffo and Bertolin, 2012).