PHYSICAL PARADOXES. 



flask while steam is briskly issuing, so that when 

 the steam is condensed there will be no air in 

 the flask. 



By the time that the flask is placed in the 

 holder, the water will have left off boiling 

 briskly. Pour water freely over the steam 

 space, and brisk boiling will re-commence. The 

 boiling will cease or re-commence as we cease 

 or resume the pouring of cold water over the 

 flask ; and this will go on for some time, till the 

 water is too cold to boil at all. 



This paradoxical effect is due to the fact 

 that the temperature at which liquids will boil 

 depends upon their pressure. When first the 

 flask is closed the pressure inside it is more than 

 equal to that of the atmosphere, since the steam 

 has been able to drive the air out and keep it 

 out. Against this atmospheric pressure the 

 molecules on the surface of the water cannot 

 jump off into the space above, taking the form 

 of steam, without constantly receiving energy 

 of vibration from the hot flame ; and so, when 

 the open flask is removed from the flame, boil- 

 ing ceases. But when the flask is closed and 

 the steam is condensed by pouring cold water 

 over the flask, the pressure inside is much re- 

 duced, and against this reduced pressure the 

 vibration energy of the molecules is still great 

 enough to enable them to jump off without the 

 assistance of external energy. In other words, 

 the^water can boil without a flame. 



146 



