Measuring the Surface- Tension of Liquids. 393 



if the greatest attainable accuracy is desired, I should advise 

 a different arrangement. A deep, thin-walled glass dish with 

 a close-fitting cover in which are three holes — one in the 

 centre for the arm of the frame, one for the thermometer, and 

 one for the stirrer — should fit loosely into a double-walled 

 box, also partly covered over the top, within which are air- 

 or water-pipes, resistance-coils, or other arrangement for con- 

 trolling the temperature. The large wooden closet of figure 2 

 may then be replaced by a simple subshelf for the box. For 

 volatile liquids it is necessary to keep a slow current of air 

 passing down the tube from the balance, to prevent condensa- 

 tion of the liquid on the rod that carries the frame. 



A small correction is required for the capillarity of the sides 

 n n of each frame. When the film is broken after the first 

 weighing, a capillary elevation remains completely surround- 

 ing each side, n (fig. 1). But our calculation of the tension 

 is based on the assumption that during the second weighing 

 no tension is exerted at all between the sides nn. In effect, 

 the difference between the two weighings gives the tension of 

 a film a little narrower than the frame. The amount of this 

 correction was found by taking the tension of the same liquid 

 with two or more frames of the same thickness but of different 

 widths. An example will explain the method. 



The surface-tension of distilled water at 19°*5 C. was mea- 

 sured with three frames, whose widths «, b, c, and whose ten- 

 sion-weights m, n, r, were respectively 



a = 5*2321 centim. 



m = '7742 gram 



5 = 2-0325 „ 



n = '300l „ 



c=0-2037 „ 



^=•0291 „ 



Disregarding the capillary correction, the resulting values 

 for the surface-tension in grams per centim. are : — 



— = -073985, 

 2a 



£■ = -073825, 

 26 



£=•0714. 



The true value of the tension, however, is 

 T = 



2(a-.«) 2(6 — .a?) 2(c-.r) 



