STEAM AND VAPORISATION. 



We must apply cold to the steam instead of 

 heat to the water. 



By this arrangement it is the heat-energy 

 of the water itself instead of that of the flame 

 that is doing the boiling. But if the heat- 

 energy of the water is used up in converting 

 many of its own molecules from the compara- 

 tively quiet state of liquid to the extremely 

 energetic condition of vapour, there must be 

 much less heat left in the water when the opera- 

 tion is over. And this is, in fact, bound to be 

 the case. When the process has been carried 

 on till boiling has very nearly ceased, the water 

 is cool enough to be handled without the least 

 discomfort. 



This result, however, is open to the remark 

 that, since cold water has been liberally poured 

 over the outside of the flask, running down 

 over and under it, no wonder the water in 

 it is cold ! That this criticism is unmerited 

 can be made clear by arranging the experi- 

 ment in a new way, which has other advantages 

 also. 



Fig. 34 shows an arrangement of two similar 

 flasks which can be closed by one double-ended 

 stopper pierced by a communicating tube. This 

 tube can have its long end kept above the surface 

 of the water in either of two positions by being 

 bent in the manner shown in the diagram, and 

 kept bent, if it be an indiarubber tube fitting 

 over the projecting end of a glass or metal tube 



147 



