288 Lord Rayleigh on the 



sents to the mind a good picture of capillary phenomena, and, 

 as it probably contains nothing not needed for the further 

 development of the subject, labour spent upon it can hardly 

 be thrown away. 



Upon this view the pressure due to the attraction measures 

 the cohesive force of the substance, that is the tension which 

 must be applied in order to cause rupture. It is the quantity 

 which Laplace denoted by K, and which is often called the 

 molecular pressure. Inasmuch as Laplace's theory is not a 

 molecular theory at all, this name does not seem very appro- 

 priate. Intrinsic pressure is perhaps a better term, and will 

 be employed here. The simplest method of estimating the 

 intrinsic pressure is by the force required to break solids. As 

 to liquids, it is often supposed that the smallest force is 

 adequate to tear them asunder. If this were true, the theory 

 of capillarity now under consideration would be upset from 

 its foundations, but the fact is quite otherwise. Berthelot* 

 found that water could sustain a tension of about 50 atmo- 

 spheres applied directly, and the well-known phenomenon of 

 retarded ebullition points in the same direction. For if the 

 cohesive forces which tend to close up a small cavity in the 

 interior of a superheated liquid were less powerful than the 

 steam-pressure, the cavity must expand, that is the liquid 

 must boil. By supposing the cavity infinitely small, we see 

 that ebullition must necessarily set in as soon as the steam f 

 pressure exceeds that intrinsic to the liquid. The same method 

 may be applied to form a conception of the intrinsic pressure 

 of a liquid which is not superheated. The walls of a mode- 

 rately small cavity certainly tend to collapse with a force 

 measured by the constant surface-tension of the liquid. The 

 pressure in the cavity is at first proportional to the surface- 

 tension and to the curvature of the walls. If this law held 

 without limit, the consideration of an infinitely small cavity 

 shows that the intrinsic pressure would be infinite in all 

 liquids. Of course the law really changes when the dimen- 

 sions of the cavity are of the same order as the range of the 

 attractive forces, and the pressure in the cavity approaches a 

 limit, which is the intrinsic pressure of the liquid. In this 

 way we are forced to admit the reality of the pressure by the 

 consideration of experimental facts which cannot be disputed. 



The first estimate of the intrinsic pressure of water is doubt- 

 less that of Young. It is 23,000 atmospheres, and agrees 



* Ann. de Chimie, xxx. p. 232 (1850). See also Worthington, Brit. 

 Assoc. Report, 1888, p. 583. 



t If there be any more volatile impurity (e. y. dissolved gas) ebullition 

 must occur much earlier. 



