510 RICE 



AKT. L 



justification be really considered as a surface tension since it 

 resembles a tension in an elastic membrane in most respects), 

 gives a bias towards an explanation of the phenomena at the free 

 surface of a simple liquid, or at the interface between two such 

 liquids, in terms of the same concept. As already hinted, most 

 elementary texts of physics deal with the "surface tensions" of 

 liquids as if there did exist in their surfaces lateral pulls, tan- 

 gential in direction, between the surface molecules, of an order 

 of magnitude much greater than that exerted between these 

 molecules and those immediately under them in the interior. 

 At times one reads accounts of suspended drops of water which 

 imply that the main body of water in the drop is contained in 

 an "elastic" bag made of molecules which cohere together very 

 powerfully like the molecules in a rubber sheet. 



Now it is true that the mathematical form of the results de- 

 duced from such an assumption is precisely the same as that 

 which can be deduced from a physically more real picture of 

 the situation at a liquid surface; and it is also true that this 

 assumption provides an easier mathematical route to these 

 results then does the alternative hypothesis, which when 

 worked out in detail involves rather troublesome analysis of a 

 type first developed by Laplace. However, the course of that 

 analysis and its outcome can be quite easily indicated without 

 going into the purely analytical steps. 



An analysis of the situation requires us first of all to be very 

 careful concerning the interpretation of the word "pressure" in 

 connection with a liquid. When we speak of the pressure of a 

 gas we are thinking of the integral effect of the bombardment 

 of the swiftly moving molecules on unit area of the enclosing 

 vessel, or of the rate of transfer of normal momentum across 

 unit area in the interior. The notion will be quite familiar 

 to those who have some acquaintance with the kinetic theory of 

 gases, and everyone recognizes that pressure arising from weight 

 is usually an entirely evanescent quantity in a gas. Theoreti- 

 cally, of course, the pressure at a point in a gas increases as the 

 point descends in level, but the difference of pressure between 

 the top and bottom of an ordinary-sized vessel is negligible. 

 On the other hand, the pressure in a liquid arising from the 



