THE COLLOIDAL STATE 113 



As the progress of aggregation goes on, the colloidal droplets 

 coalesce to form larger drops of rain or dew. (Coalescence 

 follows condensation which takes place on dust or other particles 

 in the air. Both condensation on particles and subsequent 

 coalescence are colloidal phenomena.) A miniature cloud is 

 formed when steam is set free in a cool room. 



Liquid-in-liquid systems are emulsions; they hold a prominent 

 place in our everyday life. The most familiar example is milk, an 

 emulsion of butterfat dispersed in water (with salt, sugars, 

 casein, etc.). Medicinal emulsions of vegetable and mineral oils 

 have now become common. Emulsions may be of oil dispersed 

 in water or water dispersed in oil. The former are usually of 

 relatively low consistency, as is milk, while the latter are usually 

 firm enough to hold their shape unsupported. Cold cream and 

 mayonnaise are water-in-oil emulsions. 



Liquid-in-solid systems arise when the pores of gas-in-solid 

 systems (dry clay, charcoal, silica gel) become filled with liquid. 

 Wet clay is generally regarded as a liquid-in-solid colloid, but 

 Freundlich calls attention to the fact that in clay, the water may 

 be the continuous phase and the clay discontinuous, which 

 is the condition characteristic of solid-in-liquid systems. This is 

 a likely point of view, as clay particles are separated from each 

 other by thin films of water which surround them. Knowledge 

 of the colloidal behavior of clays is the means toward a solution 

 of the problems of soil moisture. Possibly more truly of the 

 liquid-in-solid type are the pearl and opal. The opal is chiefly 

 silica (sand) and water. Pearls are mostly calcium carbonate 

 (marble) and water. If pearls are kept in a very dry place, as in 

 a safe deposit box, for a long time, they lose their "life," or 

 luster, owing to the loss of their colloidal water. They retain 

 their beauty best when worn frequently next to the skin, because 

 they are then in a moist atmosphere. 



Solid-in-gas systems are of interest because of atmospheric 

 problems. The precipitation of smoke and other air impuri- 

 ties, such as fumes in cities, is a colloidal problem involving solid- 

 in-gas systems. Smoke that quickly settles is a coarse dispersion 

 of carbon in air; the "blue haze" of forest fires and of tobacco 

 smoke is more truly colloidal, as it remains long in suspension. 

 Atmospheric dust, miles above the earth, is a solid-in-gas colloidal 

 system. To it has been ascribed the blue color of the sky, but, 



