( o ^v u x-j 



Surface Energy, Latent Heat, and Compressibility . 21 



also vary gradually in extent, as is observed. If the rotation 

 were proportional approximately to the fractional change in 

 length at each point, the dark extinction-band would be a 

 straight line ; and its slope would vary but little with the 

 radius of curvature and width of the crystal. Further, the 

 rapidity ot* the initial bend on applying a load might be 

 expected on a theory involving rotational local change. 

 And, on the other hand, any theory based on a simple slip 

 would have to explain the rotatioual effect shown by the 

 optical properties. 



II. Surface Energy, Latent Heat, and Compressibility . 

 By D. L. Hammick, Oriel College, Oxford*. 



ACCORDING to the classical researches of Young, the 

 relation between surface energy and cohesion or in- 

 ternal pressure takes the form 



K. 



where p = surface energy, 

 K = internal pressure, 



r= radius of action of the cohesive attraction. 

 Young's result is independent of molecular views as to 

 the nature of liquids ; if, however, we adopt the view that 

 internal pressure is a molecular phenomenon and that the 

 radius ot action of the inter-molecular attraction is most 

 probably equal to the radius of the molecule itself, we have 



v d 



where d — molecular diameter. 



If K can be represented as a function of the specific 



volume v of a liquid by — 2 , we get 



a d 

 ' it b 



(i.) 



It has b3en shown (Phil, Mag. xxxix. p. 32) that the van 

 der Waals expression for the internal pressure in the form 



—~, a being taken to be a function of the temperature, gives 



* Communicated bv the Author. 



