1 70 PHYSIOLOGY OF MICROORGANISMS 



within by the addition of alcohol. This pull of the water without may 

 be broken by the addition of a trace of alcohol. In this case the rubber 

 band again resumes its former shape (Fig. 1040). 



a. 



FIG. 104. Illustrating surface pull. 



In the case of an oil drop on water the oil runs to a ball because of 

 the cohesive forces within the oil and the lack of sufficient gravitational 

 and molecular forces or pulling forces within the water film. Mercury 

 for the same reason distributes itself in many small globules when split. 

 On the other hand if the forces below or upward attraction has a 

 stronger pull than the cohesive forces, then the oil would spread out as 

 on a clean glass. 



The definite reactions resulting from experiments as employed in 

 demonstrations of the above nature at once establish the possibility 

 of accurate quantitative measurements. It has been found that 

 substances vary very materially in their surface tensions. Kimball* 

 gives the following table: 



SURFACE TENSIONS IN DYNES PER CENTIMETER 



Air Water Mercury 



Water ................ 73.5 412 



Mercury ................... ............ 539 . o 412 



Olive oil ............................... 34.3 20.6 335 



Alcohol ................................ 24 . 5 ..... 



Ether ................................. 17.6 ..... 



* "College Physics." For Method of Measurement, also consult Kimball. 



