Walsh — On the Surface Tension of Liquids. 485 



tlie value of the fraction in (3) depends upon the value of S\ 

 The volume of water displaced, therefore, varies as V" and as <>'. 



The following Table, taken from the memoir of M. Quincke, 

 gives in grammes weight per lineal metre, the tension of the 

 surface at 20° 0., separating water from air, olive oil, and petro- 

 leum : — 



Superficial Tension in Grammes Weight per Lineal Metre. 



Between water and air, . . . 8*253 



Between water and olive oil, . . 2096 



Between water and petroleum, . . 2 - 834 



Specific Gravities. 



Olive oil, '915 



Petroleum, -840. 



When olive oil is poured on the surface of water upon which 

 a needle is floating, the high specific gravity of the oil, and 

 the weakness of the surface separating it from water, combine 

 together to sink the needle. 



In order that a needle should be able to float between olive 

 oil and water, it would be necessary for the weight of water dis- 

 placed to be twice as great as the weight of water displaced when 

 the same needle floats between petroleum and water. 



With a certain needle, the volume of the water displaced was 

 found to be thirty-five times the volume of the needle. 



In order that the same needle might float between olive oil 

 and water, the volume of water displaced would require to be 

 seventy times the volume of the needle. 



It is possible for a very small needle to float on water which is 

 covered by olive oil ; for by halving the volume of the needle, the 

 volume of the water displaced is at the same time halved. 



