EMULSION-STRUCTURE OF PROTOPLASM 285 



tension of the interface between the drop and the medium in which it 

 is suspended, it is evident that the tension of the exposed gap in the 

 surface will be greater than the tension of the covered portions of the 

 surface. The two portions of the surface will be pulling unequally, 

 therefore, and unbalanced excess of tension will exist at the gap in the 

 sense indicated by the arrows, and the tendency of this tension in the 

 case of a gap of molecular dimensions will obviously be to draw together 

 the edges of the enveloping film and reduce the tension of all parts of 

 the surface to a uniform value. Any molecules of such a substance 

 coming into contact with the surface will therefore tend to be held or 

 "trapped" there, and since, in the course of the fortuitous motions of 

 the dissolved molecules, a very large number must repeatedly come into 

 contact with any surfaces exposed to a solution, the accumulation of 

 adhering molecules will continue until the droplet becomes covered by 

 a layer of such thickness that the molecular attraction between the 

 underlying molecules of the drop and those of the surrounding medium 

 becomes inappreciable owing to the distance through which it has to 

 be exerted. This will occur when the thickness of the film is of the 

 order of that of a soap-bubble just before it bursts, namely about one 



two millionth of a millimeter. 



A 



FIG. 14. Illustrating the tendency of a lipoidal layer at the interface of two aqueous 

 phases to repair itself when broken. 



The extent to which the Surface-tension of water is reduced by 

 lipoidal substances and soaps may be inferred from the following 

 results reported by Lord Rayleigh. The measurements refer to an 

 air-water interface. 



Dynes per 

 linear c.m. 



Tension of pure water 1 74 



Tension of greasy water 33 



Tension of water saturated with olive oil 41 



Tension of water saturated with sodium oleate 25 



The trace of Olive Oil which dissolves in water, therefore, reduces 



the tension of the air-water interface to one-half its normal magnitude. 



The lipoids in the cell are present partly in soluble forms, such as 



^The tension of the air- water interface is usually considered to be 81 dynes per 

 square centimeter which is the estimate of Quincke. The estimate of Lord Rayleigh 

 who employed very refined method of measurement is more probably correct. In any 

 case, all of the estimates having been made by the same method, they are comparable 

 with one another. 



