Solid Sphere in contact with a Liquid Surface. 153 



and the pull is therefore a maximum when the vertex o£ the 

 sphere is very slightly below the level of the " free" hori- 

 zontal surface of the liquid. 



It is obvious that equation (ix.) might be made the basis 

 of a practicable method for the determination of surface- 

 tensions ; but in point of accuracy the method of which 

 equation (vii.) is the expression is distinctly preferable. 



A simplified form of the experiment, involving the use of 

 a plate instead of a sphere, forms an interesting piece of 

 laboratory practice. Although very simple and straight- 

 forward, I have not seen the experiment mentioned elsewhere, 

 and it may not be out of place to outline it briefly here. 



A plate of glass is taken — a microscope slide does well — 

 and suspended by a thread from the underside of a balance- 

 pan with its surface vertical and lower edge horizontal. 

 The balance being equilibrated and left free to move, a 

 basin containing the liquid to be examined is placed under- 

 neath the plate on an adjustable table. The table is raised 

 till the liquid barely touches the plate. Weights are now 

 added to the balance to restore equilibrium, when we have 

 the well-known equation 



mg = 2(l + b)T, (xi.) 



where m is the mass of the added weights, I the length and 

 b the breadth of the plate (the contact-angle is of course 

 assumed to be zero). 



Now remove the added weights, and, first noting the 

 position of the adjustable table, raise it until the balance- 

 pointer is again in its zero position. If d Y is the distance 

 through which the table has been raised, we have 



%?<?!= 2 (Z + 6)T, (xii.) 



since in this position the " buoyancy " just balances the 

 surface-tension pull. The distance d^ can be measured with 

 fair accuracy, and the comparison of the values of T given by 

 (xi.) and (xii.), and the discussion of the relative accuracy of 

 the two methods, affords an interesting extension of the 

 usual experiment in which equation (xi.) alone is used. 



University College of North Wales, 

 Bangor. 

 April, 1914. 



