CORRESPONDENCE OF MOLECULAR STATES 225 



with its liquid is increased there will be a condensation 

 unless the molecular k.e. is correspondingly increased, 

 that is, unless the temperature of the liquid (and vapor) 

 is raised to the new boiling point. Now we have seen 

 that when the molecules of a substance are in the liquid 

 state the potential energy of the systems they form 

 with each other has been reduced. Obviously then, 

 if an attempt is made to increase the pressure exerted 

 upon a liquid and its vapor, the condensation which 

 occurs will cause a reduction of this molecular potential 

 energy. What becomes of the energy thus released? 

 It is available for increasing the molecular kinetic energy 

 and thus for opposing, by the pressure of these mole- 

 cules, the very cause, namely increased external pres- 

 sure, which induced the effect. 1 An increase in molecu- 

 lar pressure thus occurs to meet the increase in the 

 externally applied pressure. 



Suppose, however, that as fast as energy is liberated 

 by the condensing molecules of the vapor it is with- 

 drawn from the system. That is, suppose the change 

 in pressure is made isothermally. The slightest in- 

 crease in pressure results in the condensation of a part 

 of the vapor. The pressure exerted by the vapor does 

 not rise because the energy released by condensation 

 is immediately subtracted. Condensation, therefore, 

 occurs at constant pressure and temperature until all 

 the vapor is condensed. This phenomenon is usually 

 shown in the laboratory by using a barometer tube, 

 I, as in Fig. 26. A drop of liquid is released into the 

 vacuum at the top by inserting the point of a medicine 



1 This reminds us of "action and reaction." Later we shall dis- 

 cuss the principle of Le Chatelier which has to do with such cases. 



Q 



