STEAM AND VAPORISATION. 



copper diaphragm, as arranged for the last 

 experiment, will do very well. When the water 

 is heated its expansion will burst the copper 

 covering, just as that of the forming ice did, 

 and the rapid formation of steam will be harm- 

 less if the opening is properly directed. 



If, however, the chamber be only three- 

 quarters filled with water, the diaphragm will 

 then be burst, not by the expanding water, 

 but by the expansive force of steam, and both 

 forms of explosion can be shown by the same 

 apparatus. In the latter case, to ensure a 

 good explosion if the heat be weak or if the 

 quantity of water in the chamber be by acci- 

 dent very small, a diaphragm of thin india- 

 rubber sheet may be used instead of copper. It 

 will also save the confidence of a nervous experi- 

 menter, while making hardly less noise. 



How, then, does heat, applied to water, pro- 

 duce explosive force ? 



Heat is mechanical energy developed in 

 extremely small portions of matter, and closely 

 associated with the other forms of minute 

 mechanical energy which we know as electricity 

 and light. The smallest particles into which 

 we can divide substances without altering their 

 chemical nature are called molecules, and heat 

 may be briefly described as molecular vibra- 

 tions. That some of the heat vibrations 

 are internal to the molecules, and are to be 

 classed as atomic vibrations, is shown both by 



'39 



