352 



On the Surface Forces in Fluids. 



[Feb. 28, 



distances apart, at which, they may be supposed to be situated in the 

 interior of the liquid, at some given temperature in nature. They 

 are then supposed to be endowed with the attractive forces which (on 

 a statical theory) they may be supposed to possess in nature, and the 

 value of the repulsive heat-force or internal pressure is sought which 

 is requisite to maintain the existing uniform density. Tt is found that 

 this value increases with the depth up to the limit of the radius of 

 molecular activity, after which it remains constant, and that con- 

 sequently if such a uniform temperature be assigned to the substance 

 as shall maintain the existing density of the interior molecules, a 

 readjustment of the surface molecules will take place, involving an 

 expansion of the surface liquid and an absorption of heat by it, from 

 which it is seen that the surface liquid is in a condition to which the 

 interior liquid would be reduced by stretching it, and may be regarded 

 as a portion of the liquid on which heat energy has been spent in 

 doing work against the molecular attractions. 



The same treatment is then extended to the more general case, first 

 of a liquid in contact with an incompressible and inextensible sub- 

 stance ; then to two liquids in contact ; and then to the contact of a 

 liquid with its own vapour ; and finally, to the contact of a fluid and 

 a solid ; with the result that at the surface of contact of two fluids the 

 surface layers of each differ from the interior fluid by being in a state 

 either of tension or pressure, and that the forces which are the subject 

 of measurement in experiments on capillarity are the algebraic sum of 

 the two sets of forces belonging to the two fluids respectively. Jf 

 this resultant sum is a pressure, then the surface is unstable and its 

 extension involves a dissipation of energy. 



In the case of a liquid in contact with a gas or its own vapour, it is 

 shown that the liquid is rarefied at the surface while the gas or 

 vapour is condensed, but that there is a limit, for temperatures below 

 the critical point, to the rarefaction possible to the one, and to the 

 condensation possible to the other, and that the two limits are not 

 coincident, and that consequently up to this point the phenomena of 

 a surface-tension are presented, while above the critical temperature 

 the two limits disappear simultaneously. The relation of this inter- 

 pretation of the critical temperature to the suggestion of Professor 

 James Thomson, made in his paper, " Proc. Hoy. Soc," vol. 20, p. 1, in 

 reference to the continuity of the fluid state of matter, is pointed out. 



In the case of the contact of a solid with a fluid it is only the 

 surface forces of the latter that can produce any motion, and when 

 one of the fluids is a gas above its critical temperature, it is shown 

 that the surface force must be a pressure, whence its follows that a 

 liquid which spreads over a solid in air must really be condensed on 

 its surface, and the surface force must be a pressure, and not a 

 tension, as usually supposed— a result which is arrived at in a different 



