THE THERMOMETER. 143 



have been first suggested by Cornelius Drebbel, a resident at Alkmaer, in Hol- 

 land. He is said by Boerhaave and Muschenbroek to have invented thermom- 

 eters about the year 1600. Some Italian writers also assign this honor to 

 Drebbel, but others give the credit of the invention to Galileo : while it is as- 

 serted by other Italian authorities, including Borelli and Malpighi, that the 

 merit of the invention is due to Sanctorio, a well-known medical professor at 

 Padua. Sanctorio, indeed, claims the invention himself, and the Florentine aca- 

 demicians, Borelli and Malpighi, are witnesses not likely to be biased in favor 

 of the Patavinian professor. 



The thermometer of Sanctorio was formed of a glass bulb and tube, in which 

 the air was first rarefied in a slight degree by the application of heat. The end 

 of the tube was then plunged in a colored liquid, which, when the air contract- 

 ed by cooling, was forced up into the tube by the atmospheric pressure. The 

 tube was divided into a number of equal parts, called degrees. When the tem- 

 perature of the medium surrounding the bulb was raised, the air included in 

 it expanded, and the colored liquid was forced downward in the tube. When 

 the temperature surrounding the bulb, on the other hand, was lowered, the air 

 losing some of its elasticity, the liquid was forced higher in the tube by the 

 atmospheric pressure. The number of degrees on the tube through which the 

 colored liquid moved were taken as the indication of the changes of tempera- 

 ture. Thus the thermometer of Sanctorio was, in fact, an air thermometer. Its 

 indications, however, were necessarily affected by the changes in the atmo- 

 spheric pressure, as well as by change of temperature. At the same tempera- 

 ture, an increase in the atmospheric pressure would cause the column to rise 

 in the tube, and a decrease would cause it to fall. Such an instrument, there- 

 fore, when used as an indicator of the variations of temperature, should always 

 be corrected with reference to the changes in the thermometric column. This 

 thermometer has no fixed points of temperature, nor could the indications of 

 one instrument be compared with those of another, nor with itself, after any de- 

 rangement or change of circumstances. 



About fifty years subsequently to this, the Florentine professors constructed 

 thermometers of spirits of wine, and excluded from them the air in the upper 

 part of the tube by the manner already explained with reference to the mercu- 

 rial thermometer. The tube was divided into one hundred parts, called degrees ; 

 but still no fixed points of temperature were adopted. 



About the year 1725, Fahrenheit, a thermometer-maker of Amsterdam, first 

 substituted mercury for spirits of wine in thermometers, and by this means con- 

 siderably reduced their magnitude. The instrument was thus capable of meas- 

 uring much higher degrees of temperature than thermometers of spirits of wine, 

 because mercury does not boil until it attains a very high temperature. Still, 

 however, thermometers labored under defects arising from the want of fixed 

 points of temperature, the nature of which have been already fully explained. 

 Various attempts were made to insure the correspondence of the scale of differ- 

 ent thermometers employed in different parts of the world, but as yet no effect- 

 ual method was suggested. 



Late in the seventeenth century Dr. Hook discovered the fact, that water 

 during its conversion into ice, and ice during its conversion into water, main- 

 tained a fixed temperature ; and also that water, during the process of boiling 

 under the same circumstances, retains the same temperature. These two tem- 

 peratures, depending upon fixed phenomena not affected by change of time or 

 place, furnished convenient standards by which the fixed points upon thermom- 

 eters might be determined ; and as* such they were first recommended and 

 adopted by Newton. As the process of fusion and evaporation of all bodies 

 are attended with the same peculiar effects as those of water, their temperatures 



