ANIMAL LIFE AND ANIMAL PKODUCTS. FOODS 443 



other of which enables it to remove dust and dirt (largely minute, 

 solid particles of carbon soot) . The former is its emulsifying 

 power, the latter is probably connected with its nature as a col- 

 loidal suspension. 



The Emulsifying Action of Soap. How Soap Removes 

 Qrgase.-xr- When an insoluble oil, such as kerosene or lubricating 

 oil, is shaken with water it is divided into minute droplets sepa- 

 rated by water from one another. When the shaking ceases, how- 

 ever, the droplets begin to run together and soon the oil and 

 water have separated once more into two layers of transparent 

 liquid. When very dilute soap solution and oil are shaken to- 

 gether, however, the droplets do not run together, but remain 

 permanently suspended. The mixture is opaque and more or 

 less viscous. Such a permanent mixture of two insoluble liquids 

 is called an emulsion (compare p. 110). 



Soap solution, when rubbed on oily or greasy goods, emulsifies 

 the grease, converts it into droplets, surrounded by soap solution 

 and separated from the cloth, afid so permits it to be washed off. 



In mayonnaise dressing, which is a thick, almost solid, per- 

 manent emulsion, the olive oil is emulsified by the colloidal matter 

 of the yolks of the eggs which have been dissolved in the vinegar. 



How^ap_Removes Dirt. When a solution containing 

 colloidal substances, such as many dyestuffs and organic coloring 

 matters, is shaken with finely pulverized charcoal, the colloidal 

 substance adheres to the surface of the powder (is adsorbed) and 

 the liquid is consequently decolorized (see p. 422). 



When dilute soap solution is shaken with infusorial earth and 

 the mixture is filtered, the clear liquid is found to have been de- 

 prived of the soap. The soap is evidently precipitated (adsorbed) 

 on the surface of the particles of the solid. 



Clearly there is a tendency to cohesion between the colloid in 

 a soap solution and the particles of a fine powder. When there is 



