CHAPTER XI 



THE EFFECT OF THE SURFACE TENSION OF THE MEN- 

 STRUUM UPON BACTERIA AND TOXINS 



W. P. LARSON 



University of Minnesota 



INTRODUCTORY 



Bacteriologists have been slow to recognize the influence of the surface tension of 

 the menstruum upon bacteria and toxins, although surface energy and the phe- 

 nomenon of adsorption have been studied by physical chemists, and even biologists, 

 for many years. It is not our purpose here to discuss the fundamental problem of 

 surface energy, since a complete treatment may be found in works devoted specifically 

 to that subject.' 



It may be recalled that the surfaces of all liquids (and solids) are in a state of 

 stress or tension due to the play of intramolecular forces, the degree of tension being 

 determined by the nature of the fluid and the solutes which it contains. 



Surface energy may be defined as the product of the surface area and the surface 

 tension. Since free energy constantly strives toward a minimum, a liquid will assume 

 the form giving it the smallest possible surface area, as the sphere of a raindrop, and 

 many solutes which it contains will be forced — adsorbed — into the surface, provided 

 the attraction of the molecules of the liquid is greater for each other than for the 

 molecules of the solute. 



Surface tension may be defined as the force per centimeter, in the plane of the 

 surface, required to overcome the tendency of a liquid to maintain a minimum surface 

 area. The unit of measurement is the dyne. Several methods for measuring the sur- 

 face tension of a liquid are in use, the best known of which are: 



The drop-weight method, the capillary-rise method, the jet method, and the method of 

 measuring the force required to pull a disk or ring from the surface of a liquid. 



The physicist, who usually works with pure liquids, seems to prefer the capillar3'-rise 

 method in preference to others. After having tried out all the foregoing methods I have found 

 the drop-weight method best suited to the work of the bacteriologist who never, or seldom, 

 works with pure liquids but rather with a solution of several substances in water. 



Morgan^ and Harkins and Brown' have perfected the drop-weight method to a point 

 where most of the errors have been eliminated. In fact, their method is more delicate than 

 the routine bacteriologist requires. Green' has recently developed a torsion balance which is 

 rapid and yet sufficiently accurate for bacteriological work, where a high degree of accuracy 

 is not important. When hundreds of measurements are required, I have found Green's 



• Harkins, W. D., in Alexander, J.: Colloid Chemistry, New York, i, 192. 1926. See chap, x by 

 Dr. Harkins in this volume. 



' Morgan, J. L. R.: Jour. Amer. Chem. Soc, 33, 349. 191 1. 



J Harkins, W. D., and Brown, F. E.: ibid., 41, 499. 1919. 



* Green, R. G.: Indus, and Eng. Chem., 15, 1024. 1923. 



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