Effects of Physical Forces on Bacteria 171 



wall. As the frequency increases, the death rate rises. Spheres 

 are more difficult to kill than are rods, with the large rods being 

 most easy to kill. This type of cell disruption is useful because it 

 does not appear to denature the protein so readily as application 

 of chemicals and heat. It has particular usefulness in immuno- 

 logical studies of such materials as endotoxins which are intimately 

 bound with the cell's protoplasm. Cells that are not visibly dis- 

 rupted by ultra-sonics are undoubtedly killed by a combination of 

 physical and chemical forces. Cavitation by dissolved gases aids 

 in cell disruption. 



Short exposures to sub-lethal doses of high frequency sound 

 waves are capable of inducing interesting genetic changes in plants. 

 There is some evidence that orchids can be shaken out of their 

 lethargy and germination speeded up when subjected to ultra- 

 sonics. Chemicals that normally settle out of solution can be 

 made to remain in suspension after exposure to high frequency 

 sound waves. Here is another field for fruitful investigation. 



SURFACE TENSION 



Surface tension is the force that operates in each square centi- 

 meter of cross section of the surface to hold it together, while 

 surface energy is the wo,rk required to increase the surface area 

 when measured at constant temperatures. Liquids are always 

 bounded by surfaces which have some of the characteristics of 

 membranes. Liquids tend to maintain a minimum surface area, 

 and this is why a drop of water in the air tends to be spherical. 

 The surface is under tension and the sphere has the least surface 

 area for a given volume. The drop becomes spherical for the 

 same reason that a toy balloon becomes round when it is inflated- 

 surface forces. 



Surface tension may be measured by means of a Du Noiiy 

 Tensiometer, which determines the force required to separate a 

 platinum ring from the surface of a liquid. This force is measured 

 in dynes, and a dyne is defined as the force which will produce 

 a velocity of 1 centimeter per second in gram mass. 



