Surface Forces in Fluids. 353 



the molecules of the liquid near the newly exposed surface 

 would at once readjust themselves to the new conditions of 

 equilibrium, receding from each other and absorbing heat. 

 But if we did the same to a solid, it is very probable that the 

 readjustment of the intramolecular distances would not take 

 place. The experimental evidence that at the surface of solids 

 there exists a layer of condensed gas is very strong; and 

 just as a gas may he liquefied by an external pressure, so 

 it is possible that the molecular pressure caused by the 

 attraction of the solid may condense on its surface in a liquid 

 form some or all of the gases contained in the atmosphere to 

 which it is exposed. It is to be expected that the gases of 

 our atmosphere which are most readily liquefied by pressure, 

 viz. water-vapour and carbon dioxide, are those which will 

 be most condensed at the surface of solids exposed to it; and 

 in experiments in which the liberation of the condensed gases 

 is effected, it is these which may be expected to appear in 

 greatest quantity. With respect, however, to those gases 

 whose critical temperature is below that at which capillary 

 experiments are usually made, it is to be observed that, 

 although they will be condensed at the surface of a solid, 

 yet the outer surface of the condensed stratum will not 

 exhibit the phenomena of a surface-tension. For the experi- 

 ments of iindrews and Ramsay, which are confirmed by 

 Cailletet and Jamin, show that above the critical temperature 

 the densities of the gas and the liquid are identical. Hence 

 in the case of such a gas there will be a continuous though 

 very rapid diminution of density in the condensed stratum 

 as we pass outwards from the solid surface into the un- 

 modified gas beyond. Nowhere can the condensed film 

 be in a state of tension, for we have seen that the state 

 which we call the surface-tension is due to the molecules 

 being in the condition to which we could reduce the molecules 

 of the interior of the liquid by stretching. But a liquid 

 which is only kept liquid by pressure above its critical tem- 

 perature cannot sustain a tension. For let us imagine a 

 portion of such a liquid enclosed within solid walls, and then 

 removed for experiment, and let the experiment consist in 

 mechanically enlarging the space enclosed by the walls. 

 The liquid will expand, and will pass without breach of con- 

 tinuity into a gas which will continue to expand, but will 

 continually exert a pressure, and never a tension, on the 

 enclosing walls. For this statement of the continuity of 

 liquid and gaseous conditions above the critical temperature 

 we have experimental evidence. 



lo. Now it is interesting to observe that our diagram of 



