Transportation on Land 299 



comes more nearly equal to that of the lower layers, the steam 

 bubbles expand as they approach the surface. The fact is 

 explained thus : the heated water and the steam bubbles at 

 that part of the flask where heat is applied, are under the pres- 

 sure of the water above as well as the pressure of the atmosphere 

 exerted upon the surface. As the bubbles rise to the surface, 

 they meet with less resistance or pressure than that exerted 

 upon them at the bottom, and they expand. Their rate of 

 movement increases at the same time, because by their expan- 

 sion they displace more water which in turn forces them more 

 rapidly toward the surface. At the surface the bubbles break 

 the film of water enveloping them and are what we generally 

 call steam. 



130. Boiling temperature and high pressure. In your ex- 

 periment water is made to boil in the open air, at ordinary 

 atmospheric pressure. Steam is generated, however, in boilers 

 at a pressure much greater than that of normal atmosphere. 

 The steam gage may show a pressure from one pound to two 

 hundred fifty pounds per square inch. What happens as re- 

 gards the boiling temperature of water when the pressure is 

 increased ? Does it remain the same as at normal air pressure, 

 or, if not, what change is effected ? The answer to the question 

 may be obtained by the aid of an experiment. 



Exercise. The apparatus illustrated in Fig. 94 is designed to 

 allow increase in pressure to be made at will and thus to indicate 

 immediately what change occurs in the boiling temperature of water 

 under more than atmospheric pressure. The experiment is safe, if 

 performed with reasonable caution. 



A laboratory boiler is the best apparatus for the demonstration. 

 But if such is not available, set up the apparatus shown in Fig. 94. 

 The flask should be of large capacity, round bottom, and very strong, 

 tough glass. Make a pressure gage of a twelve-inch piece of glass 

 tubing, T V of an inch bore, bent into the form of a double U, but with 

 one arm at least six inches long. Insert tubing in stopper as in dia- 

 gram. Put in mercury as in diagram. Put a thermometer in the 

 middle hole so that the bulb will be well covered with water. Into 

 the third hole insert a piece of glass tubing to which is attached a piece 



