360 Mr. A. M. Worthington on the 



alone the pressure or tension at any point is proportional to 

 the depth below or above the free horizontal surface*, all the 

 phenomena of capillarity, the constancy of the contact angle, 

 the horizontal motion of floating bodies, the equation to the 

 liquid surface, and the laws which regulate the stability of 

 liquid figures, may be easily deduced. 



It has not been necessary to assume with Maxwell f that 

 the contraction of a liquid surface takes place in conformity 

 with the principle of the conservation of energy, and to de- 

 duce the surface-tension from a combination of this assump- 

 tion with the molecular hypothesis. On the contrary, the 

 fact that the contraction or extension of the surface which 

 takes place is consistent with the principle of conservation of 

 energy has flowed from the investigation itself ; and this, I 

 think, is a considerable advantage, for the assumption of the 

 conservation of energy, on which Maxwell bases his own in- 

 vestigation, does not carry with it in molecular matters the 

 security of previous experimental justification : it appears as 

 an additional hypothesis used in conjunction with the other 

 hypotheses as to the nature of the molecular actions, but it is 

 one whose exact relation to the other hypotheses is not easily 

 perceived, since it is partly implied in them already, and it 



* An insight into the nature of the hydrostatic pressure, due to gravi- 

 tation at the earth's surface, and its connexion with the internal repulsive 

 force of which we have spoken, is, I think, best obtained by the light of 

 such a theory as that of Lesage, which attributes the gravitative action 

 to the momentum of "ultra-mundane corpuscles." 



On such a theory we may regard any horizontal layer of molecules as 

 receiving a momentum from above, which would, in the absence of the 

 earth, be met by an equal corpuscular momentum from below; but, owing 

 to the earth's screening action, this compensating momentum is wanting-, 

 and must be replaced by an increase in the molecular momentum, which 

 in the statical theory is represented by the repulsive force, and which is 

 obtained by a diminution of the distance between the layer under con- 

 sideration and that next below it, to which the effect of the corpuscular 

 momentum is thus transmitted; this next layer thus receives from above, 

 in addition to the corpuscular momentum proper to itself, a repulsive 

 pressure equivalent to that belonging to the former layer; and the sum of 

 these must now again be balanced by an increase in molecular pressure 

 from the layer below, and so on throughout the liquid : thus there is an 

 increase of density and of molecular momentum, i. e. of internal repulsion, 

 as we descend. But the increase of molecular momentum which is thus 

 substituted for the directed corpuscular momentum from below, cannot, 

 from the nature of the motion in fluids, be supplied in one direction only, 

 and is therefore recognized as what we call the hydrostatic pressure 

 equally in all directions. It is thus seen that the hydrostatic pressure at 

 any point due to the Aveight of the liquid is the excess of the repulsive 

 force above the value which it would possess if the liquid were removed 

 from the influence of attracting bodies. 



t Vide article on Capillarity, Encyc. Britannica (present edition). 



