RADIATIONS 59 



In a medium of greater density diffusion of water is 

 outward from the cell and this will continue until an equi- 

 librium is established between cell contents and medium. 

 Food for the organism must be in solution and ejiter the cell 

 by diffusion. Therefore, growth ceases in a medium too 

 'dense, since water to carry food in solution does not enter 

 the cell. 



ELECTRICITY. 



Careful experimenters have shown that the electric cur- 

 rent, either direct or alternating, has no direct destructive 

 effect, on bacteria. In a liquid medium the organisms may 

 be attracted to or repelled from one or the other pole or 

 may arrange themselves in definite ways between the poles 

 (galvano taxis), but are not injured. However, electricity 

 through the secondary effects produced may be used to 

 destroy bacteria. If the passage of the electric current 

 increases the temperature of the medium sufficiently the bac- 

 teria will be killed, or if injurious chemical substances are 

 formed (ozone, chlorine, acids, bases, etc.), the same result 

 will follow (see Ozone, pages 57 and 143). (Thunderstorms, 

 see page 73). 



RADIATIONS. 



Roentgen or a:-rays and radium emanations when prop- 

 erly applied to bacteria will destroy them. The practical 

 use of these agents for the direct destruction of bacteria in 

 diseases of man or animals is restricted to those cases where 

 they may be applied directly to the diseased area, since they 

 are just as injurious to the animal cell as they are to the 

 bacteria, and even more so. Their skilful use as stimuli to 

 the body cells to enable them to resist and overcome bacteria 

 and other injurious organisms or cell growths is an entirely 

 different function and will not be considered here. 



PRESSURE. 



Hydrostatic pressure up to about 10,000 pounds per 

 square inch is without appreciable effect on bacteria, as has 



