350 Mr. A. M. Worthioffton on the 



& 



a pressure on the layers immediately below ; but the down- 

 ward action is accompanied by an equal and opposite upward 

 attraction exerted by the new molecules on those below, so 

 that no additional pressure (other than that due to gravitation) 

 will be transmitted to a sensible depth, and the only effect will 

 be to shift upwards the condition of surface-extension. What 

 were previously the first, second, third layers, &c, are now 

 each at a greater depth below the surface, and the molecular 

 distances are in each case diminished, but are still greater than 

 in the interior of the liquid, unless the number of new layers 

 added be so great that the original surface lies now at a sen- 

 sible depth within the liquid. Then, and not till then, the 

 condition of rarefaction will disappear. Now the mechanical 

 pressure, such as that which we may conceive to be produced 

 by the impacts of a gas above the liquid surface, differs from 

 the pressure Ave have described in not being attended by an 

 equal upward attractive action, and consequently its pressure 

 will be transmitted to the whole liquid mass below, but the 

 amount of diminution which it produces in the rarefaction at 

 any point near the surface w r ill be less than that produced in 

 the way described, if the mechanical gaseous pressure is less 

 than the increase in cohesive action at that point due to the 

 addition of the new layers. 



Now our knowledge of the value of the surface-tension, and 

 of the smallness of the depth to which it extends, obliges ua 

 to conclude that the extension to which, as we have seen, it is 

 due is such as must be attended by a diminution of cohesive 

 action so considerable as to be far greater than any gaseous 

 pressure to which liquids are exposed in our experiments on 

 surface-tension. Consequently the rarefaction of the surface- 

 layers, even in a direction normal to the surface, will be 

 diminished by such pressure, but by no means destroyed. 

 Thus we shall have the pressure of a gas above transmitted 

 through rarefied layers to the liquid below, just as, conversely, 

 a solid wall may condense a rare liquid at its surface, and yet 

 a mechanical tension may be transmitted through the wall 

 and condensed layers to the liquid beyond. 



Let us now examine what would happen if the outermost 

 molecules of a liquid, with a vacuum above its plane surface, 

 were at such a distance from the next layer that the least 

 oscillation outward would carry them beyond the range of 

 molecular attraction of the rest of the liquid. The position is 

 one of unstable equilibrium with respect to such oscillations, 

 and the molecules, unless absolutely stationary, will escape 

 into the space beyond, where they will, considered from a 

 statical point of view, exert only a repulsive force, will arrange 



