92 THE ROYAL SOCIETY OF CANADA 



The curves in Fig. 1 show that — has been found by some ex- 



du 



perimenters to be constant, by others to change, hence it is likely 

 that 5 also changes. 



It is surprising that Lord Rayleigh in all his work on surface 

 forces apparently does not develop Waterston's theories. He works 

 more on the theories of Young, Laplace and Dupré, using the idea 

 of Intrinsic Pressure. An elementary treatment of this part of the 

 subject is given in Poynting and Thomson's Properties of Matter, 

 although even these writers apparently make an error (See pp. 174, 

 175, 1902 edition). Following up Laplace's conception of the internal 

 pressure they equate the latent heat of evaporation of unit volume 

 of the liquid to the intrinsic pressure. Thus in the case of water at 

 20°C. they get 



iC = 550X4.2X10^ dynes per sq. cm. 

 = 23100 atmospheres, 

 while their own text on p. 175 show that to bring unit volume from 

 the interior to the surface necessitates work equal to K and an equal 

 amount is required to tear the unit volume off the surface layers and 

 to disintegrate the films into a gas. This would make X= 11500 

 atmospheres a value agreeing much better with that obtained by 



a 

 VanderWaals from the term ~ of his celebrated equation. Young 



in 1805 got 23,000 atmospheres and apparently Lord Rayleigh 

 (Phil. Mag. XXX, 1890, Collected Works, Vol. III., p. 423) agrees 

 in the method, for he says "The view (viz. K = L per unit volume) 

 appears to be substantially sound" and by direct equation as above 

 gets for water, K = 25000 atmospheres. 



University of Toronto. 



