The Theory of Surface Forces. 413 
other history of! Earth and Moon, differing materially from 
that which Darwin has given us. 
§ 35. Returning now to § L, an unanswerable question 
occurs': — Were the primordial atoms relatively at rest in the 
most ancient time, or were they moving with velocities, 
relative to fixed axes through the centre of inertia of the 
whole, sufficiently great to give any considerable contribution 
to the present kinetic energy of the universe ? It is con- 
ceivable that all the atoms were relatively at rest in the most 
ancient time, and that " the potential energy of gravitation 
may be in reality the ultimate created antecedent of all the 
motion, heat, and light at present in the universe " *. 
XLI. On the Theory of Surface Forces. — III. The Physical 
Meaning of the Unstable Part of the Isotherm of James 
Thomson. By G. Bakker f. 
§ 1. The Equations of Lord Kelvin for the Pressure of the 
Vapour in tlie neighbourhood of a Curved Capillary 
Layer. 
IN this periodical J I have demonstrated, that in a plane 
capillary layer of a liquid, which is in contact only with 
its vapour, the hydrostatic pressure (p{) perpendicular to its 
surface is equal to the vapour-pressure, but that on the con- 
trary the hydrostatic pressure Q? 2 ) parallel to the surface of 
the capillary layer has a gradient in the direction of the 
normal to the surface. The relation between the pressure p 2 
and -, where p denotes the density in a point of the capillary 
layer, was given by the curve HUWVK of fig. 1 (p. 414) ; the 
curve HRGrFPK presents the theoretical isotherm of James 
Thomson. I will now extend these considerations to a 
capillary layer which has the form of a spherical shell. 
Therefore I consider firstly a spherical bulb of vapour in 
equilibrium in the interior of a liquid. The capillary layer 
round about the vapour is in this case a spherical shell, and 
* Quoted from " On Mechanical Antecedents of Motion, Heat, and 
Light," Brit. Assoc. Rep., Part II., 1854 ; Edin. Xew Phil. Journal, vol. i., 
1855 ; Comptes Rendus, vol. xl., 1855 ; republished in Sir William 
Thomson's ' Math, and Phys. Papers/ vol. ii. p. 84. 
f Communicated by the Author. 
% " On the Theory of Surface Forces— II." : Phil, Mag. Oct. 1907, 
pp. 515 & 526. 
Phil. Mag. 3. 6. Vol. 15. No. SS. April 1908. 2 F 
