Surface Forces in Fluids. 335 



Corps," that it may be said to be founded upon the ideas 

 which are there so lucidly set forth. 



Before the development of the modern theory which 

 ascribes the phenomena of heat to molecular motion, it was 

 customary to regard a solid or liquid at rest as composed of 

 an indefinitely large number of discrete molecules, themselves 

 also at rest; and physicists, such as Young and Poisson, did 

 not hesitate to frame theories which should account for the 

 statical equilibrium of such molecules. The phenomena of 

 cohesion and adhesion required that the molecules should be 

 endowed with an attractive force which was a function of the 

 distance between them, and which vanished when the distance 

 became sensible; while the phenomena of elastic resistance to 

 compression, and of expansion with a rise of temperature, led 

 to the description of a repulsive force of heat which was sup- 

 posed to be exerted between adjacent molecules. The mole- 

 cules of a homogeneous solid were supposed to be of equal 

 mass and of equal efficiency as centres of force, and to be 

 uniformly distributed throughout the interior of the solid. A 

 varying state of stress corresponded to variations from uni- 

 formity in the arrangement of the molecules, while fluids were 

 distinguished from solids by the supposition that the internal 

 equilibrium of the latter was influenced by the shape as well 

 as by the situation of the molecules ; while in the case of the 

 former, owing either to the sphericity of the molecules or to 

 their relatively great distance of separation, consideration of 

 their shape was unnecessary. 



The properties ascribed to the molecules were those which 

 seemed simplest, or which were most obviously suggested by 

 analogy with phenomena in which finite masses of matter 

 were concerned. No attempt was made to explain how the 

 molecules acquired these properties of attracting or repelling 

 each other at a distance. 



The properties must not, however, be regarded as so many 

 additional hypotheses about the molecules, but rather as a 

 part of the conception of stationary molecules, which obser- 

 vation of phenomena proved to be necessary. 



A large number of phenomena, however, especially those 

 belonging to gaseous pressure, diffusion of gases and liquids, 

 and to evaporation, were not explainable by the supposition of 

 stationary molecules, but seemed to demand that they should 

 be regarded as endowed with motion; and this opinion, which 

 received so great an encouragement from the discovery of the 

 mechanical equivalent of heat, is now almost universally 

 accepted. 



But the change of the point of view has, as is often the 



