SURFACE TENSION 



161 





= i. 



capillary is slightly more than two-sevenths of that to which 

 water will climb. 



The most precise way to measure surface tension is with the 

 tensiometer. If a ring of known circumference is attached to 

 the end of a balanced rod on 

 the opposite end of which is a 

 scale pan, and if the ring is in 

 contact with the surface of a 

 liquid, the surface tension of 

 the liquid can be measured by 

 adding weights to the pan until 

 the contact between the sur- - 

 face film of the liquid and the ; 

 ring is broken. Searle con- ^ be 



verted such a simple tensi- Fig. 95.— (a) Meniscus formed against 



Ometer into a torsion balance, f^^^^ by water; (6) the rise of water in a 



large, (c) in a small, capillary. 



wherein the twist or torsion 



of a steel wire is the force that pulls the contact ring. Lecomte 

 duNoliy carried the improvement further. With his tensiom- 

 eter (Fig. 96), direct readings in dynes per centimeter can be 



Fig. 96. — The duNoiiy tensiometer. (Arthur H. Thomas Company.) 



made. He has also developed an interfacial tensiometer for 

 reading the tension of membranes between liquids. 



Soaps and fatty substances lower the surface tension of water 

 when added to it. The surface tension of a (saturated) aqueous 

 solution of sodium oleate is 25 dynes per centimeter. Salts 



