302 



Introduction to the Study of Science 



water in B causes the steam to condense rapidly, leaving a partial 

 vacuum, thus reducing considerably the pressure upon the free surface 

 of the water. The water begins to boil, large bubbles of steam pass- 

 ing over to the other end. If the boiling is too violent, withdraw the 

 tube partly from the beakers. The boiling will moderate as the tem- 

 perature of the water in the tube falls. When all the water has 



passed over into the other end 

 of the U, the tube may be re- 

 versed to continue the process. 

 The water in beaker B will 

 soon be too warm to condense 

 the vapor. Replace it with 

 colder water, and continue to 

 cool gradually until you have it 

 as cold as a mixture of salt and 

 ice will make it. The tempera- 

 ture of the water in the tube 

 may fall to 68 F. (20 C.) or 

 lower during the class period, 

 and the water will continue to 

 boil vigorously. To maintain 

 ebullition longer, it is necessary 

 only to have the water in B 

 FIG. 95. At lower than atmospheric cold enough to cause condensa- 



pressure water boils at low temperatures. 

 This simple apparatus demonstrates the 

 fact. 



tion. 



The behavior of the bubbles 



indicates that there is very little, 

 if any, pressure upon the free surface of the water. The steam volumes 

 rising from the stopper continue unbroken until they reach the sur- 

 face where they are as large as the diameter of the tube. If the pres- 

 sure were uniform at all depths of the water, the size of the bubbles 

 would of necessity remain unchanged. 



Boiling temperature lowered by reduction of pressure. By re- 

 ducing pressure or resistance to the expansion and escape of 

 the vapor or steam, the boiling temperature of a liquid is caused 

 to fall below that at ordinary pressure. A piece of ice- in a 

 vacuum as complete as can be made would produce steam ; or 

 water at a normal freezing temperature would boil under the 

 same conditions. This may be demonstrated with suitable 

 apparatus. But a fact of fundamental importance is that 



