322 VAPORIZATION. 



of the three tasks can not be varied ; either of the other 

 two easily may. If we increase the pressure we increase 

 the work to be done, and therefore increase the necessary 

 amount of heat, the only form of energy competent to do 

 the work. If we lower the pressure we lessen the work to 

 be done, and therefore lessen the necessary amount of 

 heat. This means., in the first case, raising the boiling 

 point; in the second case, lowering the boiling point. 



5O4. Franklin's Experiment. The boiling of 

 water at a temperature below 100 C. may be shown as 

 follows: Half fill a Florence flask with water. Boil the 

 water until the steam drives the air from the upper part 

 of the flask. Cork tightly, remove the lamp and invert 

 the flask. The exclusion of the air may be made more 

 certain by immersing the corked neck of the flask in water 

 that has been recently boiled. When the lamp was re- 

 moved the temperature was not above 100 C. By the 



time that the flask is in- 

 verted and the boiling 

 bas ceased the tempera- 

 ture will have fallen be- 

 low 100 C. When the 

 boiling stops, pour cold 

 water upon the flask ; di- 

 rectly the boiling begins 

 again. 



(a.) The cold water poured 

 upon the flask lowers the 

 temperature of the water in 

 the flask still further, but it 

 also condenses some of the 

 steam in the flask or reduces 

 FIG. 247 its tension (494). This re- 



