1900.J on the Effect of Physical Agents on Bacterial Life. 453 



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 favour, 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 

 ceils 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 favour or hinder their growth, or 

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

 the first instance as a favouring agent, very striking differences 

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

 temperature under which their growth is possible, extending from 

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

 organisms in 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 

 species that the optimum temperature for their development be 

 ascertained, and that this temperature be maintained. In this 

 respect we can distinguish three broad groups. The first group 

 includes those for which the optimum temperature is from 15-20° C. 

 The second group includes the parasitic forms, viz. those which grow 

 in the living bodv and for which the optimum temperature is at 

 blood heat, viz. 37° C. We have a third group for which the 

 optimum temperature lies as high as 50-55° C. On this account 

 this latter group has been termed thermophilic on account of its 

 growth at such abnormally high temperatures — temperatures which 

 are fatal to other forms of life. They have been the subject of 

 personal investigation in conjunction with Dr. Blaxall. We found 

 that there existed in nature an extensive group of such organisms to 

 which the term thermophilic bacteria was applicable. Their growth 

 and development occurred best at temperatures at which ordinary 



