SURFACE TENSION 163 



but biologists now feel that there has been undue enthusiasm 

 for surface tension as the causative agent of certain vital proc- 

 esses. That surface tension is a very significant force in both the 

 living and nonliving world is evident from the fact that if 1 cc. 

 of water is sprayed into spherical droplets 0.01 lu, (0.00001 mm.) 

 in diameter, the total area of the droplets will be 6,000,000 sq. 

 cm. The free surface energy of this area, at room temperature 

 (20°C.), would be 218,000,000 ergs, or 10.5 cal. The extra- 

 ordinary activity of colloidal systems is due to this tremendous 

 increase in surface over the solid state. 



S. Mudd has developed a neat technique, involving an inter- 

 facial film, for the study of the passage of bacteria and blood cells 

 through surface-tension membranes. Water and oil are brought 

 into contact on a microscope slide; the cells to be studied are 

 suspended in the water. The red blood cell passes from the water 

 into the oil through the interfacial film. The tension of the 

 latter stretches the blood cell into the shape of a lens. Normal 

 bacteria, belonging to the so-called acid-fast or lipoid-coated 

 group, pass readily into the oil and hence are hydrophobic 

 because of a fatty coat. After the fatty layer from the bacteria 

 is removed, they no longer pass from the water into the oil, 

 for their surface is now presumably protein in nature. Bacteria 

 or red blood cells when coated (sensitized) with so-called anti- 

 bodies from immune serums do not pass into the oil ; their coating 

 (antibody) is now globulin. 



An interesting and somewhat amusing application of surface 

 tension in the living world is the use to which water insects put 

 the surface film of water. Ramsden cites the case of various 

 small aquatic animals which rest upon or cling to the surface 

 even though they are heavier than water. The black, hairy 

 insect Podura frisks about on the surface of a pond; the larvae 

 of the gnat hang head downward suspended from the water sur- 

 face by means of their breathing tubes. Entomostraca, which 

 normally live within water, are unable to get back into it when 

 by accident they are caught in the surface; they float helplessly 

 on their side until they starve to death or are blown ashore. 



When an amoeba puts out a pseudopod (Fig. 24), there 

 possibly takes place a reduction in the surface tension of the 

 protoplasm at that point, due to some internal reaction or (less 

 likely) an external change. The internal (imbibition or turgor) 



