84 HEAT. 



depth of mines. Owing to various causes, the atmo- 

 sphere at the same elevation is a little heavier some 

 days than others, so that the height of a mountain or 

 the depth of a mine, as thus measured, would not be 

 always precisely correct. 



200. ARTIFICIAL CHANGE OF BOILING 

 POINT - I* is obvious, from what has already 



of liquids be been stated, that all it is necessary to do to 



changed? , ... 



change the boiling point, is to change the 

 pressure of the atmosphere, on the surface of the water 

 to be boiled. To produce this change of pressure, it is 

 not necessary to ascend mountains, or to descend into 

 mines ; it may be done by removing the atmosphere 

 by artificial means. This would be done by attaching 

 a tube, air-tight, to the mouth of a test-tube or 

 flask and drawing off the air by means of an J^ 

 air pump. Cold water may thus be caused to 

 boil. So by pumping more air into the flask, the 

 pressure would be increased, and the boiling point 

 elevated ; and by this means boiling water 

 would be prevented from further boiling. This 

 subject is further considered in paragraph 204. 



201. CULINARY PARADOX. Boil some wa- 



Descnbe the , . . 



culinary par- ter m a test-tube, and then cork it tightly, 



while steam is still issuing from its 

 mouth. Though removed from the fire, the wa- 

 ter will still continue to boil. This will behest 

 observed by inverting the tube, as the bubbles of 

 steam form more rapidly from the cork surface 

 than from the glass. A few drops of cold water 

 sprinkled on the tube will occasion a more violent 



