MEASUREMENTS OF HEAT 131 



man scientist, who introduced the use of mercury in ther- 

 mometers in 1720, and established the scale which bears 

 his name. The Centigrade instrument was introduced 

 by Anders Celsius, a Swedish astronomer, about 1742; 

 it is sometimes called the Celsius thermometer. 

 F. is used as abbreviation for Fahrenheit, and 

 C. for centigrade. 



At 38.8 C. mercury freezes. Obviously, 

 then, a mercury thermometer cannot be used for 

 reading temperatures lower than this. Some 

 substances with a still lower freezing-point must 

 be substituted. Alcohol does not freeze until 

 in C. is reached. So alcohol thermometers 

 are used on arctic expeditions or whenever very 

 low temperatures are to be recorded. 



The thermometer which Galileo invented in 

 J 593 (see page 124) was quite a different instru- 

 ment. It depended on air-pressure to maintain the col- 

 umn from which the reading was made. The bulb was 

 at the top of the tube instead of at the bottom, and 

 was large instead of small. The tube was open at the 

 lower end, and that end was immersed in some colored 

 liquid (see Fig. 56). To start this apparatus the air in the 

 bulb was heated. Then as this air cooled down to the tem- 

 perature of its surroundings, it contracted, and the force of 

 atmospheric pressure caused the ascent of the liquid in the 

 tube. (Compare with the action of a suction-pump.) Evi- 

 dently, the liquid in the tube would rise as the air above 

 it cooled, and would descend as this air became warm, thus 

 reversing the action of a mercurial thermometer. The tem- 

 perature was read by means of a graduated rule placed 

 alongside the tube. Such a thermometer is unsatisfactory 



