Temperature. Thermometers 



145 



the liquid ; evaporation takes place at all temperatures 

 though much less rapidly at lower temperatures. 



92. To show how the boiling-point of a liquid is 

 affected by variation of the pressure to which its vapour 

 is subjected, two simple experiments may be performed. 



EXPERIMENT 49. Fit up the apparatus illustrated 

 in Fig. 44, and attach the outlet tube to the end of the 

 long bent tube which dips under the surface of the 

 mercury in a jar to a depth of about 20 cm. as shown 

 in Fig. 45. Heat the water in the flask to boiling-point, 

 and note the reading of the thermometer when the 

 water is boiling freely. It will be found to be some 

 6 to 7 degrees higher than that previously observed 

 when the water was boiling under atmospheric pressure. 



Exercise for Student. 



If the atmospheric pressure at the time of the experiment be 

 750 mm., and the end of the tube is at a depth of 20 cm. below 

 the surface of the mercury, under what pressure is the water boiling ? 



EXPERIMENT 50. Obtain a well-made round-bot- 

 tomed flask. Half fill it with water and heat the 

 water till it boils. While still boiling, remove the 

 source of heat and quickly 

 insert a well-fitting stopper. 

 Invert the flask upon a tripod 

 stand as shown in the accom- 

 panying figure. When no more 

 bubbling takes place pour cold 

 water over the outside of the 

 flask. This will cause vigorous 

 boiling to take place. When 

 no more bubbles appear, show- 

 ing that boiling has again 

 ceased, pour a further supply 

 of cold water over the flask. 



Fig. 46. 



H. D. S. 



10 



