14 



COLLOIDS IN BIOLOGY AND MEDICINE 



like an elastic skin which surrounds the drop. If we drop water 

 from a tube, the drop develops in size and is retained by its skin 

 until the increasing weight tears it away. 1 We call this force which 

 determines the tension of the surface, the surface tension (o-). A flat 

 surface of water of 1 square cm. endeavors to contract with a o- of 

 about 0.075 gin., when it is spread out like a soap bubble with air 



Mr interface 



Fluid 



FIG. 1. 



pressure. Every change in shape of the sphere of water, i.e., every 

 increase of the surface, presupposes work; this depends upon the 

 surface area (co) and the surface tension (a). Surface energy = er w 



or a 



surface energy 



expressed in 



dynes 

 cm. 



There are many methods for determining the surface tension. 

 Some of them depend upon the shape (distortion) which the sur- 

 face of a fluid takes; some, upon the height to which a fluid 

 ascends a capillary; some, upon the determination of the maximum 

 weight attained by a falling drop. Detailed descriptions are to be 

 found in every large treatise on Physics, such as H. FREUNDLICH'S 

 " Kapillarchemie." 



A number of values of a are given as examples. The a of water is 

 determined most frequently; it has the highest a of all the sub- 

 stances which are fluid at room temperature (mercury excepted). 

 The various methods give quite divergent values. The values given 

 here are in each case the surface tension towards air. 



(fluid /air) 



<T 



n-Butyric acid 26 . 3 



Chloroform 26 



Olive oil 32.7 



Resin (melted and solidified) 37 . 2-33 . 1 



Glue 48.3 



Shellac.. 36.7-30.4 



(fluid/air) 

 a 



Water 71.7-76.8 



Mercury 436 



Benzol 28.8 



Ethyl alcohol 22 



Ethyl ether 16.5 



Glycerin' 65 



Acetic acid 23 . 5 



1 The comparison to an elastic membrane, at best, has only a limited applica- 

 bility, since it is true that the surfaces are enlarged or diminished, yet they are 

 not stretched. The particles in the surface neither separate nor approach each 

 other but more particles are forced into the surface (when the surface enlarges) 

 or removed from it when it diminishes. 



