162 PROTOPLASM 



raise the surface tension of water; the surface tension of a calcium 

 chloride solution (of density 1.35) is 95 dynes per centimeter. 

 Such behavior means that the surface composition of a solution 

 differs from that of the solvent and in such a way that soaps and 

 fats, which lower surface tension, are of higher concentration in 

 the surface than in the interior, while of salts, which raise tension, 

 the converse is true. This is the law of Willard Gibbs, the Amer- 

 ican physical chemist whose mathematical and physical deduc- 

 tions anticipated so much, though at the time (1878) they passed 

 unnoticed. Thus does the surface of a solution or heterogeneous 

 mixture differ both physically and chemically from the interior — 

 physically by having a more compact arrangement of the surface 

 molecules and chemically by having a different relative propor- 

 tion of the ingredients at the surface (with the possible absence of 

 some) . 



Studies in surface tension have found application in theoretical 

 physics and chemistry. Langmuir and later duNoiiy determined 

 the size of organic molecules from their tension at surfaces. 

 DuNoiiy observed the gradual reduction in tension of the surface 

 film of a soap solution with increase in concentration and con- 

 cluded that the lowest tension value obtained of the soap solu- 

 tion indicated a monomolecular layer of soap at the surface. 

 With the area of the surface and the specific gravity of the 

 substance known, it was possible to compute the thickness of 

 the layer and therefore the length of the soap (sodium oleate) 

 molecule, which was found to be 12.3 X 10~^ cm. From this 

 value and the molecular weight, the width and depth of the 

 sodium oleate molecule were found to be 6.8 X 10~^ cm. and 

 7.56 X 10~^ cm. Thus may such fundamental physical prob- 

 lems as molecular size be attacked by quite different methods 

 from those ordinarily employed and with corroborative results. 



Surface Tension in Living Matter. — Surface tension has 

 been regarded as the cause of many vital phenomena. Cell 

 division, amoeboid movement, protoplasmic streaming, gastrula- 

 tion, permeability changes, nerve impulses, muscular contraction, 

 excretion, and even memory itself have been interpreted as 

 involving, if not primarily due to, changes in surface tension. 

 That some of these speculations are, in part, true we must 

 acknowledge, because there is no physical force more universally 

 present and operative in liquid systems than surface tension. 



