880 Mr. Wilson Taylor on the Coalescence of 



The latent heat of evaporation is commonly thought to be 

 the energy due to the attraction of molecules for one another 

 at close ranges in the liquid. In the interior, where the 

 attractions on a particular molecule are balanced, this energy 

 is not apparent or is unavailable. It is apparent only in 

 regions near the surface where these attractions are un- 

 equally distributed. The latent heat, then, is the work done 

 in bringing the molecules of a gram mass from the interior 

 through the surface region of unequal attraction and 

 separating them from one another beyond the range of 

 their attraction. Surface energy is regarded as the energy 

 belonging to the molecules in the surface layer only, and, 

 consequently, is only a part of the latent heat. Various 

 attempts to define the relation between these which assume 

 a general form, E. V* = &L, have so far been unsatisfactory, 

 the experimental data giving no consistent values for the 

 quantities involved. Other attempts to connect the whole 

 internal molecular energy with the latent heat have failed 

 for the same reason. These consist of seeking to verify 

 relations of the form N . E,„ = Hj, where ~E m is the energy 

 of one molecule and k is some constant. 



A more successful attempt, however, was recently made 

 li to give precision " to the generally recognized relationship 

 between surface energy and internal latent heat by Mr. D.. 

 L. Hammick in an article on "Latent Heat and Surface 

 Energy," which he published in the ' Philosophical Magazine," 

 August 1919 and January 1920. -He conceives the true mole* 

 cular surface energy belonging to all the molecules in a gram 

 molecule to be the potential energy they acquire in reaching 

 their positions in the surface layer. He imagines the gram 

 molecule spread out in a layer one molecular diameter in 

 thickness on a surface of excess of the liquid. This energy 



VI. 



is equal to E . -j . j calories, where E is the surface energy 



per sq. cm., d is the diameter of a molecule, and V is the 

 volume of the liquid gram molecule. His argument to show 

 that this surface energy is one-sixth of the internal latent 

 heat is as follows : — The work done upon a molecule to bring 

 it into the surface layer is not; as is commonly supposed to- 

 be the case, one-half of that required to bring it altogether 

 out of its liquid state. For, since the molecule in reaching 

 the surface layer moves perpendicularly to the surface, the 

 work is done against only " one of the three components of 

 internal pressure," and is, therefore, only one-third of this- 

 amount, or one-sixth of the work to bring it altogether from, 

 the liquid. 



