Measuring the Surface- Tension of Liguids. 395 



The weight required to balance a film of soap solution 

 increased slightly for some time after its formation. The 

 apparent tension of films of glycerine and of strong sulphuric 

 acid decreased, at first rapidly, then more and more slowly for 

 several minutes, finally becoming sensibly constant. The 

 whole amount of increase or decrease after the first few 

 seconds was generally about *02 per cent, of the tension. 



The apparent tension of a film of water was in all cases 

 sensibly constant as long as the film lasted. This seems 

 remarkable when it is remembered that the water-film must 

 have been growing thinner by evaporation, and also that the 

 water under the bar m and along the sides n n was gradually 

 flowing down into the liquid. The first process tended to 

 decrease the weight very little and to increase the surface- 

 tension by lowering the temperature ; the second to decrease 

 the weight. A change of one part in ten thousand could be 

 readily detected. 



The second weight — the weight of the frame plus adherent 

 water after the film is broken — is variable. If the film is 

 broken two or three seconds after its formation, the weight of 

 the frame may be two or three milligrams greater than it is 

 when the film lasts ten seconds. But the differences when the 

 film has lasted more than five or six seconds are very small, 

 amounting to only one or two tenths of a milligram for a 

 frame 10 centim. wide. 



This variability in the second weighing marks the limit of 

 accuracy of the method as about one part in 5000, or '02 per 

 cent., though a change in the tension amounting to less than 

 '01 per cent, may be readily measured. 



It will be shown later that the bar m must be raised from 

 five to ten millimetres above the liquid surface before a true 

 film is formed, and that the tension is not correctly found at 

 a less height. It follows that the tension of alcohol, ether, 

 chloroform, and similar liquids, in which the film breaks 

 almost instantly at such a height, cannot be found by this 

 method. 



B. 



In order to find the surface-tension of such liquids I 

 adopted a modification of Wilhelmy's method. When the 

 edge of a thin vertical plate is touched to a liquid surface, 

 the liquid usually rises along the line of contact, and the 

 weight of the plate is increased by 



to = 2(a + 6)Tcos0, 



where a is the length and b the thickness of the plate in 

 centimetres, T the surface-tension in grams per centimetre, 



