132 PROTOPLASM 



more stable oil-in-water (0 W) emulsion than the smaller 

 metal Na. The theory is better supported by the divalent 

 metals. In water-in-oil (W O) emulsions, where the metal 

 forms the inside of the curve, the smaller the metal the sharper 

 are the wedge and the bend, the smaller the (water) globule, 

 and the more stable the emulsion. The larger metal calcium 

 forms a poor wedge and an unstable emulsion; the smaller 

 magnesium forms a finer and more stable emulsion; and zinc, 

 the smallest of the three, yields the best emulsion because it 

 forms the sharpest wedge. 



The oriented-wedge hypothesis has much in its favor, but 

 stabilization membranes are not always just one molecule thick, 

 and there is no reason to believe that the molecules are actually 

 and sufficiently wedged to make a sharp curve, nor can we be 

 certain that the metal end of the wedge will "stay put"; it may 

 ionize. The English chemist Clayton, who has brought together 

 all-important work on emulsions, warns us not to become over- 

 enthusiastic about monomolecular films. There is experimental 

 evidence to indicate the presence of films many molecules thick 

 on emulsion globules. Saponin when used as a stabilizer may 

 form films 40 ji thick. The irregular shape of oil globules, so 

 often seen in artificial and natural emulsions, is due to plastic 

 films of colloidal dimensions. The globules of latex are pear- 

 shaped, their form being maintained by a stiff membrane 

 (Fig. 89). Blood corpuscles are surrounded by a delicate but 

 resistant membrane which encloses the liquid hemoglobin. In 

 spite of a nonrigid content which should give the droplet a 

 spherical shape, the red blood cell of Amphibia is a flattened disk, 

 and of human beings an invaginated disk. The shape is 

 maintained by the rigidity of the membrane. 



Membranes, when first formed in an experimental emulsion, 

 are too delicate to be seen microscopically, but often, especially 

 when electrolytes have been added, they thicken and stiffen 

 and may, when the emulsion breaks, separate from the globule 

 and maintain their identity like so much crumpled tin foil. 



Solids as Emulsifying Agents. — Solid matter tends to enter a 

 liquid that wets it and to remain at the surface of one that does 

 not wet it. If particles of clay, which are more readily wet by 

 water than by oil, are shaken with oil and water, they will enter 

 the water and line up at the surface of the oil. The result is an 



