Transportation on Land 301 



High pressure for cooking. The application of high pressure 

 to raise the boiling temperature is familiar to those who make 

 use of vessels for cooking at high altitudes. Such vessels are 

 equipped with covers to be secured with clamps, with a pres- 

 sure gage and a safety valve. They are tested to withstand a 

 pressure of one hundred pounds per square inch. This will 

 give a boiling temperature of 338 F. (See Table of Pressures 

 and Temperatures, page 312.) 



131. Boiling temperature and low pressure. What change 

 is effected in the boiling temperature of a liquid by the reduction 

 of the pressure below that of ordinary atmospheric pressure? 

 There is an ancient puzzle which has to do with the difficulty 

 of cooking eggs in boiling water at an altitude of ten thousand 

 feet or more. ' The problem has to be met by those who dwell 

 upon high mountains or plateaus, anywhere from five thousand 

 to fifteen thousand feet above sea level. Does the boiling 

 temperature of water at such reduced pressure fall in a manner 

 corresponding to its rise under increased pressure ? 



A very interesting way to demonstrate the influence upon 

 the boiling point of water of pressure below that of the atmos- 

 phere is the following. The demonstration should be started 

 at the beginning of a class period, for it will continue, with a 

 little attention, for an hour or more. It is much more striking 

 than the usual experiments for this purpose. 



Exercise. Arrange the apparatus as shown in Fig. 95. Pour 

 into beaker A about a pint of water at 70 C. or 158 F., and an equal 

 amount of cold water into beaker B. Put into each beaker a thermom- 

 eter. The U tube should be eight or more inches long, three fourths 

 of an inch in diameter, and of heavy, strong glass. After closing one 

 end of the tube securely with a rubber stopper, pour into it about 

 three inches of water. Let the water run to the closed end of the 

 tube. Holding it carefully over a flame, bring the water to boiling. 

 When the steam escapes freely, close the end loosely with a rubber 

 stopper and when steam fills the entire space, close it tightly. The 

 steam expels almost all the air from the tube. Put the tube, with 

 the water all in one end, into the beakers as in Fig. 95. The cold 



