Theory of Surface Forces. 427' 
The gradient of the hydrostatic pressure p x normal to the 
surfaces of equal density may be deduced from the equation 
(16), when we change R into — R. Hence 
dh ' R 
(20)*" 
Eta. 6. 
Volum - aocJs 
The departure from the law of Laplace or the difference 
Pl -p 2= U-A being positive, the gradient -^ is 
always negative. That the j>y— v-curve has a point of in- 
flexion may be demonstrated in the same manner as above 
in the case of a capillary layer, which envelops a spherical 
bubble of vapour. Integrating (20) we have the well-known 
equation of Lord Kelvin : 
2H 
The p 2 — ti-curve mav ne studied in the same manner as- 
above. 
§ 5. The physical meaning of the unstable part of the 
Isotherm of James Thomson. 
We will now firstly resume the considerations with regard 
to the p x — v-curves in one figure. In the case that the 
vapour envelops the capillary layer, and that therefore the 
liquid is in the interior of the capillary layer, the maximum 
value of the pressure p v in the vapour corresponds with the 
* This equation is the same as the "relation (17). Above we have 
found this equation as a particular case of the more general relation (16«). 
