EFFECT OF PHYSICAL ENVIRONMENT 123 



dried condition than the same organisms when moist. It 

 has already been noted that steam is more effective in 

 sterilization than is dry heat. If the thermal death point 

 is to be determined it should be defined as that temperature 

 which in a given period of time will destroy all of a definite 

 number of bacteria, noting particularly the composition of 

 the medium and its hydrogen ion concentration. Another 

 method of determination is to give under definite conditions 

 the length of time required at ar definite temperature to 

 reduce the number of bacteria by one half. 



EFFECT OF ELECTRICITY 



Bacteria are usually little affected directly by the pas- 

 sage of electricity, (However, the passage of the electric 

 current may cause electrolysis and the consequent forma- 

 tion of substances which act as chemical disinfectants. ' 

 When a current of sufficient strength is passed through a 

 solution containing sodium chloride, for example, chlorine 

 gas appears at one electrode and alkali at the other. The 

 free chlorine and hypochlorites which are formed act as 

 powerful disinfectants. This method has been utilized to 

 some extent in the sterilization of sewage and water. 



EFFECT OF X-RAYS AND RADIUM RAYS 



The rays given off by the Rontgen tube and by radium 

 have been studied in their bactericidal effect, but they 

 have not proved to be sufficiently powerful to be of eco- 

 nomic importance. 



EFFECT OF PRESSURE 



Bacteria are in general resistant to relatively high pres- 

 sures. In one series of experiments a pressure of three 

 thousand atmospheres (an atmosphere is about 15 pounds 

 per square inch) was shown not to kill the Bacterium 

 typhi, Bacterium coli, or many other species of bacteria. A 



