and Repulsions of Small Floating Bodies. 51 



therefore assumed that the moistened body B or B' will be 

 drawn away from A or A! by the excess of the weight of the 

 exterior above the interior meniscus, due to the difference of 

 height o n or o' n' '. In like manner, the non-moistened body 

 A or A' will be pushed away from B or W by the excess of 

 interior hydrostatic pressure due to the difference of level k r 

 Hence there is apparent mutual repulsion. 



Defects of the foregoing explanations. — Leaving out of consi- 

 deration those physicists who have adopted, more or less com- 

 p^tely, the mathematical methods of Laplace and of Poisson, 

 the foregoing seem to be the generally received popular expla- 

 nations of this class of capillary phenomena. They are those 

 given by Brewster, Daguin, Silliman, Snell, and other writers 

 on elementary physics, and are essentially identical with the 

 explanations originally proposed by Monge*. The most cur- 

 sory examination will serve to show their unsatisfactoriness. 



In the first place, each case requires a special explanation: 

 there is no common physical principle coordinating the three 

 cases under consideration. Thus, in case 1 the weight of the 

 intervening elevated column of liquid draws the bodies toge- 

 ther, without reference to the modification of hydrostatic 

 pressure due to the elevation. On the other hand, in case 2 

 the bodies are pushed together by the excess of the exterior 

 hydrostatic pressures. Finally, in case 3 it will be noticed 

 that the excess of hydrostatic pressure due to the difference of 

 height equivalent to o n or o' n ( is made a pulling force, urging 

 B or W to the right ; while the excess of hydrostatic pressure 

 due to the difference-height equivalent to h r is made a push- 

 ing force urging A or A / to the left. Now, why this difference 

 in the direction of action of the excess of hydrostatic pressures ? 

 Why not regard the excess of pressure on the right of B or W 

 (equivalent on or o' n f ) as a pushing force urging B or B / 

 towards A or A 7 ? — a result which is evidently at variance with 

 experiments. 



In the second place, it is very clear that the laws of hydro- 

 statics are so seriously modified by the action of capillary 

 forces (the disturbances of level being in fact due to them), 

 that it is very questionable whether hydrostatic pressure can 



* Brewster, Encyc, Britannica, 8th ed., art." Hydrodynamics," chap. iii. 

 u On Capillary Attraction and the Cohesion of Liquids." Daguin, Traite 

 Elementaire de Physique, 3rd ed. (Paris, 1867), tome i. art. 226, pp. 209, 

 210. Silliman, l Principles of Physics,' 2nd ed., revised and rewritten, 

 (Philad. 1861), art. 242, pp. 195-196. Snell's Olmsted's ' Nat. Phil.' 2nd 

 revised ed. (N. Y. 1870), art. 229, pp. 150, 151; also Kimball's 3rd re- 

 vised ed. (N. Y. 1882), art. 202, p. 135. Monge, Mem. de VAcad. cit. ante. 

 Of the above, Daguin gives the most explicit and clear statement of these 

 explanations. 



E2 



