672 RICE 



ART. L 



cussion on p. 521 of the connection between the external pres- 

 sure on the spherical surface of a liquid and its internal pressure 

 at the surface, the quantity Ci + Ca here replacing the quantity 

 2/R there, R being the radius of the sphere. It is in fact 

 equivalent to the use of equation [500]. The writer, however, 

 feels that the qualification in the text concerning o- being the 

 "true tension of the surface" is uncalled for. If a is the free 

 surface energy per unit area, the same form of proof will hold 

 as before for the statement, and will lead to the same conclu- 

 sion, viz., equation [500]. It is true that in the case of the solid 

 the causes giving rise to free surface energy will include changes 

 in the relative configuration of molecules in the surface arising 

 from surface stretching, as well as the already familiar inward 

 attractions of underlying molecules ; but whatever be the causes, 

 o- has the same meaning in these formulae as before, and 

 p" + (ci + ^2)0- is the internal pressure under all circum- 

 stances. On the same grounds the writer is somewhat critical 

 concerning the remarks at the end of the first paragraph on 

 page 318. He feels that the conclusion there drawn is based 

 on a mistaken view that the surface phenomena resemble in 

 this respect those in a stretched membrane separating two 

 bodies of fluid, and he cannot persuade himself that one should 

 adopt any other view concerning a than those already indicated ; 

 if he is right in this contention and if one introduces the con- 

 ception of an isotropic internal pressure, he fails to see how the 

 familiar proof from energy considerations already used on pages 

 228-229 of Gibbs' work is not as valid as before. In short he 

 cannot satisfy himself that there is any need in these arguments 

 to separate artificially a certain portion of the free surface 

 energy, viz., that arising from stretching apart of the surface 

 molecules, from the whole amount of it, and to introduce it as 

 the sole determining factor in the difference between internal 

 and external pressure. 



In order to convince himself of the truth of the statements 

 made in the second paragraph on page 318, the reader should 

 refer back to the conclusions drawn in Gibbs' discussion of 

 strained solids at the bottom of page 196, which might other- 

 wise not be recalled. The additional argument when gravity 

 is taken into account needs no comment. 



