332 Lord Rayleigh : Investigations in Capillarity. 



a small disk (6 mm. in diameter) in contact with the surface, 

 measuring the force necessary to detach it. In my own 

 experiments I have employed the method of Wilhelmy, 

 which appears to be better adapted to the purpose. A thin 

 blade is mounted in a balance, its plane being vertical and its 

 lower horizontal edge dipping under the surface of the liquid. 

 If absolute measures are required, the edge of the blade should 

 lie at the general level of the surface when the pointer of the 

 balance stands at zero. If in be the mass in the other pan 

 needed to compensate the effect of the liquid, I the length 

 of the blade, the surface-tension (T) may be deduced from 

 the equation 



2lT = mg (1) 



When only differences of tension are concerned, the precise 

 level of the strip is of no consequence. As regards material, 

 glass is to be preferred and it should be thin in order not 

 unduly to diminish the sensitiveness of the balance by the 

 displacement of water. I have used a small frame carrying 

 three parallel blades, the total length being 27 cm., while the 

 thickness may be considered nearly negligible. Before use 

 the glass is cleaned with strong sulphuric acid, and the angle 

 of contact with the water when the balance is raised appears 

 to be zero. The total value of m for a clean surface may then 

 be calculated from (1), taking T at 74. We find ra = 4 , l gms. 

 The balance could be read without difficulty to "01 gm., 

 giving abundant accuracy. 



The position of the barrier, giving the length of the surface 

 to which the grease is confined, is measured by a millimetre- 

 scale, but is subject to a correction needed in order to take 

 account of the additional surface operative when the sus- 

 pended strip is raised. This amounts to about 3 cm., and is 

 to be added to the measured length. In a set of experiments 

 where the grease is successfully confined, the density is 

 proportional to the reciprocal of the above corrected length. 

 It sometimes happens that continuity is lost by the passage 

 of grease across the barrier. This is of course most likely to 

 happen when the tensions on the two sides differ considerably, 

 and the danger may be mitigated by the use of a second 

 barrier, so manipulated that the densities are nearly the 

 same on the two sides of the principal barrier. 



In commencing a set of observations the first step is to 

 secure the cleanness of the surface. To this end the surface 

 is scraped, if the expression may be allowed, along the whole 

 length by one of the movable partitions, and, if thought 

 necessary, the accumulated grease at the far end may be 



