Effects of Physical Forces on Bacteria 169 



potentially dangerous bacteria emanating from the nose and throat 

 of infected individuals and from healthy carriers must be con- 

 siderable. Applications of ultraviolet light have found their way 

 into biological supply houses where such rays are employed to re- 

 duce possible contamination of vaccines; in banks where ultra- 

 violet "screens" supposedly protect the tellers from the multitude 

 of organisms sprayed at them during the course of a single day; in 

 school rooms; in military barracks; in the treatment of bottles in 

 beverage plants; in infants wards of hospitals, etc. Commercial 

 ultraviolet lamps give off light with a wavelength of about 2600 

 Angstrom units. 



Interesting mutations have been observed in bacterial cultures 

 exposed to ultraviolet radiation for controlled periods. Certain 

 combinations of dyes and light may exert effects on organisms that 

 neither dye nor light can produce alone— a synergism. This is 

 called a photo-dynamic action, and it takes place only in the 

 presence of oxygen. 



ROENTGEN OR X RAYS 

 While X-ray therapy may be used in the treatment of certain 

 skin disorders, particularly deep-seated fungus infections inacces- 

 sible to ordinary topical applications of drugs, just what effect X 

 rays have on bacteria is a rather sketchy bit of our knowledge. 

 They appear to be lethal, however, and interesting mutations may 

 occur in bacteria exposed to X rays. 



RADIOACTIVE ELEMENTS 

 Uranium, thorium, etc., may give off radiations called alpha, 

 beta, and gamma rays. Alpha rays of radioactive substances are 

 positively charged helium atoms given off at a high velocity. The 

 beta rays are negatively charged particles of uncertain nature 

 traveling at high velocities. Beta particles have more penetrating 

 power than do alpha rays, but they tend to pass directly through 

 small objects like bacteria. Five mm. of aluminum or one mm. of 

 lead will hold back beta particles, in contrast to alpha rays which 

 are retarded by a thin sheet of paper, glass, or aluminum. Gamma 



