STEAM AND VAPORISATION. 



some cold water over it, and before long there 

 is a loud bang and a cloud of steam, the flask 

 is shattered, and hot water and broken glass 

 fly for yards in all directions. An audience 

 would unhesitatingly pronounce it "another 

 good explosion/' 



The experiment in question is really no ex- 

 plosion at all, though it so well simulates one. 



A little water is boiled in a glass flask, until 

 the steam has been briskly issuing for a few 

 moments. We saw in the last section how the 

 rapid movement of the molecules of steam 

 constitutes a source of powerful energy. The 

 steam is able to drive out all the air from the 

 flask, overcoming the atmospheric pressure 

 of fifteen pounds to the square inch. 



When the air has been all expelled, close the 

 flask with a rubber or good cork stopper. But 

 do not make the mistake of doing this while 

 still holding the flask over the flame, otherwise 

 steam will continue to be generated, and soon 

 there may be a very real explosion, and at a 

 very inconvenient moment. The time to close 

 the mouth of the flask is just after removing it 

 from the flame, while the heat of the glass is 

 still causing the formation of steam. Then the 

 closing should be done quickly, before conden- 

 sation of steam allows any air to re-enter. The 

 glass will safely stand the moderate pressure of 

 any steam generated after removal from the 

 flame. 



143 



