and Repulsions of Small Floating Bodies. 49 



when the effective forces are considered, the pressure of the 

 atmosphere is practically eliminated from their equations ; so 

 that finally the forces actually in operation which produce the 

 tendency of such bodies to approach or to recede from one 

 another are due exclusively to molecular actions. 



Nevertheless, inasmuch as Laplace's explanation contains 

 the pressure of the atmosphere as a term, while at the same 

 time it makes the effective force equivalent to a modification 

 of hydrostatic pressure, which is negative or positive accord- 

 ing as the surface of the liquid adjacent to the solid bodies is 

 concave or convex; it is by no means surprising that the idea 

 of atmospheric pressure should have been associated with these 

 phenomena. Hence we find that many first-class physicists, 

 such as Lame, Desains, Jamin, Everett*, and others introduce 

 the pressure of the atmosphere as a fundamental element into 

 their explanations of these motions. Indeed, Laplace himself 

 seems to have been impressed with the apparent conflict between 

 theory and experiment; for after giving the result in relation 

 to the case of two solids moistened by the liquid (in which the 

 hydrostatic pressure between them has a negative value), he 

 significantly adds: — " Dans le vide les deux plans tendraient 

 encore a se rapprocher ; l'adherence du plan au fluide, pro- 

 duisant alors le meme effet que la pression de l'atmosphere " t- 

 Doubtless the view which ascribes these apparent attractions 

 and repulsions to the modifications of hydrostatic pressure due 

 to the action of capillary forces is a philosophical one, and 

 capable of being put into a mathematical form ; yet when the 

 physical cause of the disturbance of hydrostatic equilibrium is 

 kept in the background, the student is greatly embarrassed 

 and perplexed in obtaining clear conceptions of negative and 

 positive pressures, complicated as they are with considerations 

 relating to the pressure of the atmosphere, especially when he 

 is assured that the last-indicated pressure must be inoperative, 

 from the experimental fact that the phenomena take place in 

 vacuo. Moreover, from this point of view, he is liable to lose 

 sight of the real physical cause of all capillary phenomena, 

 viz. the reaction of the tense superficial film of the liquid — 

 the true and efficient cause of the disturbance of hydrostatic 



* Lame, Cours de Physique, 2nd ed. (Paris, 1840), tome i. p. 188 etseq. 

 art. 148. Desains, Traite de Physique (Paris, 1857), tome i. pp. 603-606. 

 Jamin, Cows de Physique, 3rd ed. (Paris, 1871), tome i. pp. 229, 230. 

 Everett, translation of Deschanel's Traite Elementaire de Physique (N.Y., 

 1880), part i. p. 136, art. 97 D. Pec let, in his Traite de Physique (4th ed. 

 Paris, 1847, tome i. p. 158, art. 224), leaves out the pressure of the 

 atmosphere, and ascribes the motions exclusively to negative and positive 

 hydrostatic pressures. 



t Mecanique Celeste, Supplement au Livre x. art. 11, p. 44. 



Phil. Mag. S. 5. Vol. 15. No. 91. Jan. 1883. E 



