APRIL, 1949 Also in this instance the misunderstanding seems to be due to looking a t a physical system through one mechanical and one molecular glass. Laplace (5)was quite clear as to the nature of the repulsive force which prevents surface molecules from flying into the liquid. In his vocabulary it was la fmce repulsive de la chaleur; we now speak of molecular movement. If the student (or teacher) remembers that the density of a liquid is determined by competition between molecular attraction and molecular movement and that molecular at-
this density diierence will be possible when we know more about intermolecular forces. I could not find in the twomemoirs by ~~~l~~~ (6, 8) any definite parison between the structure of surface layers and that of the atmosphere, but I think this comparison would be helpful in teaching Laplace's views.
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In my recent book (7), I used both the notion of surface tension and the theory of molecular attraction. The above remarks may serve as defense of my treatment. LITERATURE CITED (1) ADAM,N. K., "The physics and Chemistry of SnrIaces," 1st ed., OxfordUniversityPress, 1930, p. 3. (2) GVTH. E., H.M. JAMES, AND H. MARK, Advances in Colbid
,- - ~ , Paris, 1807, p. 69. (This is the 2d ' Supplhent a; dixi6me livre du Trait6 de m6canique c6leste). (6) LAPLACE, P. S., "Thborie de l'action capillaire," Paris, 1806. (This is the first supplement to the book mentioned in \-,
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