MATHEMATICAL SCIENCESSIXTEENTH CENT. 177 



that he remembered an experiment shown to him by Galileo more 

 than thirty-five years before. He describes Galileo as taking a small 

 glass bottle, which had a neck 22 inches long and as narrow as a 

 straw. Having warmed the empty bulb or bottle by holding it for some 

 time in his hands, Galileo immersed the mouth of the inverted bottle 

 in water, and then allowing it to become cold by removing his hands, 

 the water rose in the neck of the bottle more than 1 1 inches above its 

 level in the vessel containing it. Castelli then expressly states that 

 Galileo applied this principle in the construction of an instrument for 

 measuring heat and cold. In 1613, a friend and former pupil of 

 Galileo's wrote to him in the following words : " I have brought the 

 instrument which you invented for measuring heat into several con- 

 venient and perfect forms, so that the difference of temperature between 

 two rooms is shown by a scale of 100 degrees." The arrangement in 

 Galileo's instrument is identical with that adopted by Drebbel, the 

 Dutch physician, and by Santorio, the Italian physician, who are seve- 

 rally accredited with the invention. Fig. 47, p. 106 represents such a 

 thermometer, which consists of a bulb A prolonged downwards into a 

 narrow tube, the lower end of which is open, but dips into some coloured 

 liquid contained in the vessel B. The bulb A is warmed, when the 

 atmospheric air contained in it becomes expanded, and a portion 

 escapes in bubbles through the liquid from the open end of the tube. 

 When the apparatus is allowed to cool again, the remaining air con- 

 tracts its volume, or rather, diminishes its elastic force, and, as a con- 

 sequence, the pressure of the external atmosphere forces some of the 

 liquid up in the tube until it reaches a point m, where the balance is 

 restored. If now the temperature of the air in A rises from any cause, 

 the enclosed air increases its volume by expelling some of the liquid, 

 the level of which in the tube is therefore lowered. In colder weather, 

 on the other hand, the liquid will stand at a higher level by the con- 

 traction of the air in A. It will be observed by the reflective reader 

 that this instrument is, to a certain extent, a barometer also, inasmuch 

 as a change in the external atmospheric pressure will affect the level 

 of the liquid m independently of any change of temperature. For this 

 reason its indications cannot be relied upon. Galileo's pupil and sub- 

 sequent patron, Ferdinand II., effected some improvements in the ther- 

 mometer ; and that prince's younger brother, Leopold de Medici, in- 

 vented the modern plan of filling the bulb and tube with spirits of wine, 

 which is then boiled in the bulb, so that all the air is expelled ; and, 

 while the instrument contains nothing but the liquid and its vapour, 

 the end of the tube is sealed up. Another improvement, advocated 

 by Lana in 1670, was the employment of quicksilver instead of spirits 

 of wine, and this brought the thermometer into the form which is com- 

 monly used at the present day. 



The seventeenth century is memorable in the annals of science for 

 the establishment of associations having the investigation of nature 



