[ 484 ] 



XLV.— AN EXPEBIMENT ON THE SUBFACE TENSION OF 

 LIQUIDS. By A. B. WALSH. (Communicated by F. T. 

 Trouton, B.E.) 



[Read, February 16, 1887.] ' 



The fact that an oily needle will float on the surface of water has 

 been known for a long time, and is often referred to as an experi- 

 ment illustrating the tension of the surface between water and 

 air. 



If a medium-sized needle (No. 6) is placed floating on water, 

 and olive oil is gently poured on the surface of the water, until 

 the needle is covered by the oil, the needle sinks to the bottom of 

 the water. 



But if the same experiment be made, using this time petroleum 

 instead of olive oil, the needle will remain floating on the surface 

 of the water. 



The depression formed by the needle and oil resembles a boat, 

 the sides of which are formed by the depressed surface of the 

 water, while the contents consist of the oil and the needle. 



If the needle used in the two experiments be the same, the 

 amount of oil in the boat depends upon the specific gravity of the 

 oil ; while the amount of oil the boat will bear without the sides 

 giving way depends upon the strength of the sides, that is to say, 

 on the superficial tension. 



Let 



V and 8 be the volume and density of the water displaced;. 



V and S' be the volume and density of the oil in the boat ; 

 V" and <T be the volume and density of the needle ; 



then 



VB= VV+ F'S". (1) 



V=V+ V". (2) 



Since S' is always nearly equal to S, and small compared with S" y 



