and Repulsions of Small Floating Bodies. 53 



prevail, and the two bodies are drawn apart by the superior 

 tensile reactions directed toward the centres of concavity at 3 

 and at 4 (fig. 3). 



It will be noticed that in the preceding explanations of this 

 class of capillary phenomena I have referred the apparent 

 attractions and repulsions exclusively to the elastic reactions 

 of the tense surface-film, whose form is modified by the proxi- 

 mity of the partly immersed solid bodies. For the reasons 

 previously assigned, I have left out of consideration the modi- 

 fications of hydrostatic pressure, which are, after all, really 

 due to these elastic reactions. To those physicists who prefer 

 the mathematical methods of Laplace and of Poisson, my 

 explanations may seem to be less complete and exhaustive; 

 but in such general explanations it is of primary importance 

 to keep steadily before the student the fundamental physical 

 principle which constitutes the fons et origo of these phe- 

 nomena. 



There is, however, an objection to the explanation which 

 refers this class of capillary phenomena to the contraction of 

 the tensile film of liquid adjacent to the sides of the partly 

 immersed solids, which it is proper to notice. Since the 

 capillary force thus developed is inversely proportional to the 

 radius of curvature and directed toward the centre of conca- 

 vity, it has been urged that when an isolated vertical plate that 

 has its two parallel faces of different substances is partly im- 

 mersed in the liquid under such conditions that the radii of 

 curvature of the meniscuses on the opposite faces are unequal, 

 there should result a difference of pressure; so that such an 

 isolated body floating on an indefinite surface of a liquid would, 

 under the mutual action of the fluid and solid, take on a hori- 

 zontal and perpetual motion of translation. There are obvious 

 mechanical difficulties in the way of the admission of such a 

 result; for, as suggested by Poisson, in such a movement the 

 centre of gravity of the entire system is not displaced. Laplace 

 seemed to think that there would be some difference of pres- 

 sure in such cases, but that it would be so small that it might 

 be neglected*. It is evident that, however small it might be, 

 a motion of translation would be the consequence; and this it 

 seems difficult to admit. Dr. Thomas Y oung, in a letter to 

 Poisson, insists upon this as a most serious objection to Laplace's 

 theory of Capillarity f. On the contrary, Poisson shows that 

 his own modified theory does not lead to this mechanical dif- 

 ficulty; for it indicates that, under the foregoing conditions, 

 the horizontal pressures on the opposite faces exactly counter- 



* Supplement a la Theorie de V Action Capillaire, p. 43. 

 t Poisson, op. cit. ante, art. 128, p. 265. 



