278 Prof. Norton on Molecular Physics. 



that no effective action, either attractive or repulsive, can be 

 transmitted to other more distant particles on the same line. 

 Under these circumstances, one molecule, in receiving the action 

 of another, intercepts the action that would otherwise take effect 

 upon other more distant molecules. This being admitted, it 

 may be perceived, on examining Table I., that the attractive 

 actions of particles which lie beyond the second order of distance 

 from a given particle, will be in a great measure intercepted by 

 intervening particles. In what has now been stated with respect 

 to the solid condition, we have had in mind a homogeneous mass 

 of molecules only. We cannot here enter upon the considera- 

 tion of the case in which the molecules are aggregated into groups. 

 In the liquid state, the contiguous particles repel each other ; 

 and particles more distant exert no sensible action, or a feeble 

 attractive one. Here, as in the case of a solid, the sensible 

 action is confined chiefly to particles that lie at the first and 

 second orders of distance. These remarks apply to the general 

 mass of the liquid. The molecular atmospheres are in an ex- 

 panded condition from the effect of the heat of fluidity ; and it 

 is from this fact that the peculiar properties of the liquid state 

 result. As we draw near the surface of the liquid, the atmo- 

 spheres are in a condition of greater and greater expansion as 

 the necessary result of the process of liquefaction, and therefore 

 their proper attractive actions are less and less. From this 

 cause it happens that each particle near the surface is more 

 effectively attracted by those below it, beyond the first order of 

 distance, than by those above it, and thus each layer of parti- 

 cles is compressed upon that immediately below it ; also to a 

 certain depth more particles will exert their attraction from below 

 than from above. As a consequence, the density must increase 

 from the surface to a certain small depth below it, and a force of 

 compression be exerted throughout the whole liquid mass. This 

 force determines, and is in equilibrium with, a mutual repulsion 

 between the particles of the liquid. From the essential nature 

 of a liquid, as we shall soon see, this increasing molecular repul- 

 sion, from the surface downward, operates in all directions from 

 each molecule, and so tends to neutralize the attractive actions 

 between molecules separated by the second order of distance ; 

 as the final result, therefore, at the depth at which the density 

 ceases to increase, and all greater depths, the action between two 

 such moleules should be either feebly attractive, or altogether 

 evanescent *. 



* The theory of the existence of a contractile force at the surface of a 

 liquid, as the result of molecular action, was advocated by Young and 

 Poisson, and employed by them in explanation of the phenomena of capil- 

 larity. It has also been ably sustained and illustrated by Professor Henry 

 by many ingenious experiments. 



