BACTERIA IN WATER 69 



3. The " cholera red reaction." It is necessary that the 

 culture be pure for successful reaction. 



4. Isolation from water is, according to Dr. Klein, best 

 accomplished as follows: A large volume of water (100-500 

 cc.) is placed in a sterile flask, and to it is added so much 

 of a sterile stock fluid containing 10 per cent, peptone, 5 per 

 cent, sodium chloride, as will make the total water in the 

 flask contain i per cent, peptone and . 5 per cent. salt. Then 

 the flask is incubated at 37" C. If there have been cholera 

 vibrios in the water, however few, it will be found after 

 twenty-four hours' incubation that the top layer contains 

 actively motile vibrios, which can now be isolated readily 

 by gelatine-plate culture. 



5. To demonstrate in a rapid manner the presence of 

 cholera bacilli in evacuations, even when present in small 

 numbers, a small quantity must be taken up by means of a 

 platinum wire and placed in a solution containing i per cent, 

 of pure peptone and . 5 per cent, sodium chloride (Dunham). 

 This is incubated as in the case of the water, and in twelve 

 hours is filled with a turbid growth, which when examined 

 by means of the hanging drop shows characteristic bacilli. 



NATURAL PURIFICATION OF WATER 



We have already noticed that rivers purify themselves as 

 they proceed. There are many excellent examples of this 

 self-purification. The Seine as it runs through Paris be- 

 comes highly polluted with every sort of filthy contamina- 

 tion. But twenty or thirty miles below the city it is found 

 to be even purer than above the city before it received the 

 city sewage. In small rivers it is the same, provided the 

 pollution is less in amount. Whilst authorities differ with 

 regard to the mode of self-purification, all agree that in 

 some way rivers receiving crude sewage are able in a mar- 

 vellous degree to become pure again. 



