Section III, 1922 [83] Trans. R.S.C. 



On Surface Tension, Surface Energy and Latent Heat 



By John Satterly, F. R.S.C. 



(Read May Meeting, 1922) 



In the Philosophical Magazine of January, 1858, J. J. Waterston, 

 Esq., wrote a paper on "Capillarity and its relation to Latent Heat." 

 His argument was that "if the capillarity of a liquid is the exhibition 

 of part of the cohesive force of the superficial stratum of its molecules, 

 numerical relations with the latent heat of its vapour ought to be 

 demonstrable if latent heat is the measure of liquid cohesion." 



Experiments to determine the capillary constant are described 

 in the paper. They were of two kinds (1) capillary ascents between 

 plates and in tubes, (2) capillary pulls on plates immersed vertically 

 in the liquids. The effect of change of temperature was also carefully 

 studied. Waterston selected the inch as the unit of length and the 

 weight of the grain as the unit of force. Both Fahrenheit and Centi- 

 grade scales of thermometers were used and the paper must be care- 

 fully watched for the scales of temperature. 



Waterston expressed the capillary constant in a manner very 

 different from that employed at present. In one experiment, where 

 a spiral strip of paper 10 inches long was suspended from one arm 

 of a balance so that the lower edge dipped in water, he found that 

 when "the level of the water surface was adjusted so that the spiral 

 edge of the paper should separate from it at the turn of the beam, 

 the difference of weight just at the separation and immediately 

 after was found by careful observation to be exactly 38 grains. This 

 being the weight of .1505 cubic inch of water at the temperature 

 86°F. (the work was done in the tropics), shows the volume raised 

 by a water line 20 inches long. For an inch the volume = .00752 or 

 1/132.9 of a cubic inch. The value of the quotient of capillarity, 

 Q is 132.9." 



This means that the pull along a wetted-line 132.9 inches long 

 is equal to the weight of a cubic inch of water. 



"The weight of the whole column hangs, as it were, upon the 

 water line and is equilibrated by the cohering energies of one ring 

 line of molecules." 



We should say: 



If 7" = surface tension in grains per inch 



p = density of water in grains per cubic inch 

 20 r = 38 



