EMULSION-STRUCTURE OF PROTOPLASM 287 



be limited, however, for no matter how tightly packed the particles of 

 oil may be, the thickness of the layer of water between them cannot be 

 of less than molecular dimensions at its thinnest, and must of course 

 be much greater in the interstices of the emulsion. In other words 

 a limited quantity of water cannot emulsify an unlimited quantity of 

 oil and, as a matter of fact, when a given quantity of oil is shaken with 

 varying proportions of alkaline water, if the volume of water is below 

 a certain critical fraction of the volume of the oil, the character of the 

 emulsion which is obtained is altered altogether, and we now have 

 emulsions of Water in Oil. A convenient method of symbolically 

 representing these differing types of emulsions is to enclose the Internal 

 Phase of the emulsion in brackets. Thus an emulsion of oil in water 

 would be designated: 



water (oil) 



while an emulsion of water in oil would be distinguished by the symbol : 



(water) oil 



When the proportion of water to oil is in the neighborhood of the 

 critical ratio, complex intermediate forms of emulsion may be encoun- 

 tered, such as emulsions of oil in water emulsified in oil, thus : 



((oil) water) oil 



It is, in fact, highly probable that the majority of emulsions of water 

 in oil are really of this more complex type. 



The character of an emulsion obtained by shaking together olive 

 oil and alkaline water may very readily be ascertained without micro- 

 scopical examination by the simple device of sprinkling upon the surface 

 of the emulsion a little finely powdered Sudan III or Scarlet R. These 

 dyes are soluble in oil, but insoluble in water. Hence if they are 

 sprinkled upon the surface of an emulsion of oil in water, the dye 

 simply dissolves in and stains the drops of oil with which it comes into 

 actual contact, leaving the remainder of the emulsion unstained. If, 

 however, the emulsion is one in which water is the internal phase and 

 oil the external, the dye dissolves in the interstitial oil and spreads over 

 the surface of the emulsion. 



The following are illustrative results obtained by shaking together 

 olive oil and water- at an approximately uniform rate of shaking, and 

 in the presence of a fixed proportion of alkali : 



Components of emulsion. 

 Oil Water 5N.NaOH 



c.c. c.c. c.c. Character of the emulsion obtained. 



(Water) oil; fluid, yellow. 



(Water) oil; fluid, yellow. 



(Water) oil ; fluid, yellow. 



(Water) oil; fluid, creamy. 



(Water) oil; fluid, creamy. 



Water (oil) ; white, very viscous. 



98 1 



96 3 



92 7 



91 8 



90 9 



89 10 



Water (oil) ; white, very viscous. 



The critical ratio was in this instance: 



^ 



oil 90.5 



