PHYSICAL AGENTS AND BACTERIAL LIFE. 243 



as typhoid or cholera, their transmission to man by water may be 

 excluded by simple boiling or by an adequate nitration. The 

 freezing of water, whilst stopping the further multiplication of or- 

 ganisms, may conserve the life of disease germs by eliminating the 

 destructive action of commoner competitive forms. Thus the typhoid 

 bacillus may remain frozen in ice for some months without injury. 

 Employment of ordinary cold is not, therefore, a protection against 

 dangerous disease germs. 



As regards electricity, there is little or no evidence of its direct 

 action on bacterial life, the effects produced appear to be of an indirect 

 character, due to the development of heat or to the products of elec- 

 trolysis. 



Ozone is a powerful disinfectant, and its introduction into polluted 

 water has a most marked purifying effect. The positive effects of 

 the electric current may, therefore, be traced to the action of the 

 chemical products and of heat. I am not aware that any direct action 

 of the X-rays on bacteria has up to the present been definitely proved. 



Mechanical agitation, if slight, may favor, and if excessive, may 

 hinder bacterial development. Violent shaking or concussion may 

 not necessarily prove fatal so long as no mechanical lesion of the 

 bacteria is brought about. If, however, substances likely to produce 

 triturating effects are introduced, a disintegration and death of the 

 cells follows. Thus Eowland, by a very rapid shaking of tubercle 

 bacilli in a steel tube, with quartz sand and hard steel balls, produced 

 their complete disintegration in ten minutes. 



Bacteria appear to be very resistant to the action of pressure. At 

 300-450 atmospheres putrefaction still takes place, and at 600 

 atmospheres the virulence of the anthrax bacillus remained unim- 

 paired. Of the physical agents that affect bacterial life, tempera- 

 ture is the most important. Temperature profoundly influences 

 the activity of bacteria. It may favor or hinder their growth, or it 

 may put an end to their life. If we regard temperature in 

 the first instance as a favoring agent, very striking differences 

 are to be noted. The bacteria show a most remarkable range of tem- 

 perature under which their growth is possible, extending from 

 zero to 70° C. If we begin at the bottom of the scale we find organ- 

 isms in the water and in soil that are capable of growth and 

 development at zero. Amongst these are certain species of phosphor- 

 escent bacteria, which continue to emit light even at this low tempera- 

 ture. At the Jenner Institute we have met with organisms growing 

 and developing at 34-40° F. The vast majority of interest to us find, 

 however, the best conditions for their growth from 15° up to 37° C. 

 Each species has a minimum, an optimum and a maximum tempera- 

 ture at which it will develop. It is important in studying any 'given 



