184 MICRO-OHaANISMS IN WATER 



or ordiiiai-y tap water, and plate-cultivated after expo- 

 sure to different degrees of lieat for varying periods 

 of time. It was found that a temperature of 100° C, 

 maintained, during ten minutes, was required to destroy 

 the spores (?) of the tubercle bacilli, whilst the spores 

 of anthrax were killed off in five minutes. Other 

 pathogenic microbes are even more sensitive to high 

 temperatures. Pyogenic micrococci, the typhoid and 

 diphtheria bacilli, as well as the microbes of malaria ^ 

 were destroyed when the water was heated to boiling 

 and then allowed to cool, whilst the Comma bacilli in 

 water could not survive even a momentary exposure 

 to 70° C. 



For the complete sterilisation of water by heat 

 Currier states that a temperature of 100*^ C. maintained 

 for fifteen minutes, except in the case of most extra- 

 ordinarily resistant microbes, is sufficient, whilst boil- 

 ing for five minutes already eliminates the risk of 

 using water containing those pathogenic forms, like 

 the cholera and typhoid germs, which are usually re- 

 garded as a source of danger in water. 



THE PURIFICATION OF WATER BY MEANS OF SEDIMENTATION 



In the practice of water-engineering there is no 

 method of improving the quality of surface-waters which 

 has been so much taken advantage of as that of causing 

 them to remain at rest for a longer or shorter period of 

 time in large reservoirs. In this manner waters which 

 are turbid and unpleasing to the eye not only become 

 comparatively clear and bright by the subsidence of 

 mechanically suspended particles, but if the storage be 

 sufficiently j)rolonged, a very considerable reduction iii 

 the amount of dissolved organic matter may also take 



^ These were the bacilli at that time believed by som6 to be 'the 

 inducing cause of malaria. 



