304 Dr. G. Quincke on the Formation of Emulsions, 



of the liquid, and is generally found by multiplying together 

 the height at which the fluid will stand in a capillary glass 

 tube, the radius of that tube, and half the specific gravity of 

 the fluid. 



In consequence, however, of the difficulty of thoroughly 

 moistening the interior of the tube with the fluid*, this method 

 is not quite accurate. It is a better way to determine the 

 tension by measuring the vertical distance between the apse 

 and vertex of a large flat bubble of air lying in the fluid un- 

 derneath a horizontal glass plate ; the square of this distance 

 into half the specific gravity of the fluid gives the surface- 

 tension directly, independent of the nature of the plate under 

 which the bubble lies. The shape of such a bubble is the same 

 as that of an inverted dewdrop in air. By forming a flat 

 bubble of some other fluid in the heavier fluid (for example a 

 bubble of oil in water) the surface-tension of the common 

 surface of the two fluids may be found in a similar way — for 

 instance, in this case by multiplying the square of the vertical 

 distance between the bubble's apse and vertex into half the 

 difference of the specific gravities of oil and water. 



We find by this method the tension at the common surface 

 of air and water to be =8*25 mgr., of air and olive oil =3*76 

 mgr., of water and olive-oil =2*30 mgr. Other fatty oils, 

 rape-oil, almond-oil, castor-oil, cod-liver oil, &c, give similar 

 results. Fluids which mix with water in all proportions, like 

 alcohol or dilute salt solutions, form neither bubble nor glo- 

 bule in water ; the tension of the common surfaces of such 

 fluids and water is =0f. 



3. The Dispersion of Soap Solutions and of other Fluids over 

 the common Surface of Oil and Water, 



When some other fluid is applied to an air-bubble in water, 

 and it disperses itself over the surface of the bubble, the ver- 

 tical distance between the vertex and apse of the latter is di- 

 minished ; in other words, the tension at the common surface 

 of air and water has been reduced. For instance, when olive 

 oil is dispersed over the common surface of air and water, the 

 air-bubble is coated with a thin film of oil ; and the surface- 

 tension is now compounded of the tension at the surface of air 

 and oil, and of the tension at the surface of oil and water. 



* Compare G. Quincke on the Cohesion of the Solutions of the Salts , 

 Poggendorfl's Annalen, clx. p. 369, 1877. 



t The more detailed explanation of these physical laws, as well as of 

 the theory of the dispersion of one fluid substance over the surface of 

 another, I have given with mathematical and experimental illustrations 

 in PoggendorfTs Annalen, cxxxix. p. 1, 1870 ; ib. clx. pp. 337 and 560, 

 1877 5 and in Wiedemann's Annalen, ii. p. 144, 1877. 



