CAPILLARY ACTION. 543 



But the idea of surface-tension introduced by Segner had a most important 

 effect on the subsequent development of the theory. We may regard it as a 

 physical fact established by experiment in the same way as the laws of the 

 elasticity of solid bodies. We may investigate the forces which act between 

 finite portions of a liquid in the same way as we investigate the forces which 

 act between finite portions of a solid. The experiments on solids lead to certain 

 laws of elasticity expressed in terms of coefficients, the values of which can 

 be determined only by experiments on each particular substance. Various 

 attempts have also been made to deduce these laws from particular hypotheses 

 as to the action between the molecules of the elastic substance. We may 

 therefore regard the theory of elasticity as consisting of two parts. The first 

 part establishes the laws of the elasticity of a finite portion of the solid sub- 

 jected to a homogeneous strain, and deduces from these laws the equations of 

 the 'equilibrium and motion of a body subjected to any forces and displace- 

 ments. The second part endeavours to deduce the facts of the elasticity of a 

 finite portion of the substance from hypotheses as to the motion of its con- 

 stituent molecules and the forces acting between them. 



In like manner we may by experiment ascertain the general fact that the 

 surface of a liquid is in a state of tension similar to that of a membrane 

 stretched equally in all directions, and prove that this tension depends only 

 on the nature and temperature of the liquid and not on its form, and from 

 this as a secondary physical principle we may deduce all the phenomena 

 of capillary action. This is one step of the investigation. The next step 

 is to deduce this surface-tension from an hypothesis as to the molecular 

 constitution of the liquid and of the bodies that surround it. The scientific 

 importance of this step is to be measured by the degree of insight which 

 it affords or promises into the molecular constitution of real bodies by the 

 suggestion of experiments by which we may discriminate between rival molecular 



theories. 



In 1756 Leidenfrost * shewed that a soap-bubble tends to contract, so that 

 if the tube with which it was blown is left open the bubble will diminish 

 in size and will expel through the tube the air which it contains. He attri- 

 buted this force, however, not to any general property of the surfaces of liquids, 

 but to the fatty part of the soap which he supposed to separate itself from 



* De aquae communis nonnullis qualitatibus tractalus, Duisburg. 



