SURFACES 15 



The gas particles on the surface between fluid/air are affected by 

 different forces than in the center of the gas space; they receive a pull 

 in the direction of the fluid phase, they are condensed at the inter- 

 face, and in the surface film, fluid is constantly changing into gas. 

 Analogous phenomena occur wherever interfaces are formed, that is, 

 at the interface between fluid/gas, fluid/fluid (in the case of non- 

 miscible fluids), gas/solid, fluid/solid, solid/solid. At the fluid/gas 

 and fluid/fluid interfaces we recognize these forces by the shape 

 of the surface of contact (concave or convex), which is formed 

 under the influence of the surface tension. At the interface 

 between solid/gas we recognize the condensation of the gaseous 

 phase at the surface by the fact that it is almost impossible to 

 remove the layer of gas. This is the reason why it is so difficult 

 to create high vacua, and why exhaustion of gas is undertaken in 

 the presence of charcoal which by reason of its still greater surface 

 removes that layer of gas which clings to the glass vessel. Spongy 

 platinum acts as an igniter by condensing on its surface gases which 

 then unite chemically with the liberation of heat. 



The surface tension at the interfaces between two fluids can be 

 determined by the same methods that are employed in the case of 

 fluid/gas. In this case also, a limiting surface develops at the 

 interface of the two phases, which changes with every variation of 

 surface tension. 



A few values of <r for fluid/fluid are now given: 



<T 



Water /benzol 32. 6 



Water /oil of turpentine 12.4 



Water/chloroform 27.7 



Water /ethyl ether 9. 69 



Water /isobutyl alcohol 1 . 76 



Water/rosin 19.8 



Water/olive oil 22 . 9 



Alcohol/olive oil 2. 26 



Rape seed oil/egg albumen 7 . 10 



Olive oil/ox gall (9 per cent) 7. 21 



Olive oil/Castile soap (1/4000) 3.65 



Olive oil/rubber solution 14.9-10.2 



A method for measuring the surface tension of the dispersed phase 

 in an emulsion is here described in some detail, because it is too 

 new to be found as yet in any textbook on Physics, and particularly 

 because on account of its general applicability it promises to be of 

 especially great importance for colloid chemical research. 



E. HATSCHEK * separated an oil emulsion into water and oil 

 by means of ultrafiltration. From these experiments HATSCHEK 

 deduced the diameter of drops of oil and the size of pores of the 



