58 HEAT. 



and fill the tube with its own vapor. As the metal 

 cools, it contracts and collects in the "bulb and lower 

 part of the tube, leaving a vacuum above. The 

 end of the tube being then closed by fusion, the 

 instrument is complete, with the exception of grad- 

 uation. Used in this condition, the mercury would 

 be observed to rise and fall with changes of tempe- 

 perature, but we should not be able to say how much 

 or how little. 



129. GRADUATION OF THERMOMETERS. 

 thermometers To obtain a fixed point from which to 

 graduated/ count, the instrument is immersed in melt- 

 ing ice, and the point to which the mercury sinks 

 scratched on the glass. This point is called zero. 

 Another fixed point is obtained by immersing the 

 thermometer in boiling water, and when the 

 mercury has risen, noting this height also on 

 the glass, and marking it 100. The space be- 

 tween the two points is next divided into one 

 hundred equal parts, by scratches on the glass, 

 and numbered from one up to a hundred. The 

 upper and lower portions of the tube are marked 

 off into divisions of the same length, for very 

 high and low temperatures. 



CENTIGRADE THERMOMETER. 



Describe the 



Centigrade A thermometer graduated as above 

 is called a centigrade thermometer, 



from th e fact that the space between < ' boiling ' ' and ^ 

 " freezing" is divided into one hundred degrees. This 

 is by far the most rational method of graduating, 

 and these thermometers are in general use on the 



