TEXT-BOOK OF BACTERIOLOGY. 353 



ready been rejected as untenable. We might rather find fault with 

 investigators for using only liquid media and exposing them to in- 

 cubator temperature, because all the micro-organisms that do not 

 grow at all at higher temperatures (and they constitute quite a 

 large number) are lost by such procedure. 



Certain measures of precaution should be taken in examining 

 water. One point in particular should be carefully considered, 

 since the success of the procedure absolutely depends on it; the 

 bacteriological examination of water must be made as early as 

 possible, directly or, at the latest, a few hours after having been 

 collected, because an incessant and extremely extensive increase 

 of the germs will begin at a very early period. 



The fact has already been stated, in the discussion of some kinds 

 of bacteria usually found in water, that some of them are im- 

 measurably reproduced even in the purest water imaginable. On 

 being placed in surroundings differing from their natural condi- 

 tions, and especially under the influence of the higher temperature 

 nearly always found in our investigation-rooms, they make use of 

 this capacity of reproduction. After finding, for instance, 300 

 germs in a cubic centimetre of water, there will be 5,000 on the 

 second day, 20,000 on the third, and actually innumerable quanti- 

 ties on the succeeding days. This process usually reaches its climax 

 after a short time; the nourishing substances are appropriated to 

 a certain degree, and the number of living micro-organisms in the 

 water slowly diminishes. This shows, at any rate, that investiga- 

 tion should be undertaken as soon as possible, and that the report 

 of the examination of waters sent or transported from a distance 

 should be given with great reserve. 



The samples obtained should, of course, be at once placed in 

 vessels free from germs and well closed (as in Erlenmeyer's flasks) 

 and transferred to the liquid nutrient gelatin by thoroughly ster- 

 ilized pipettes. Before doing so, the water must be vigorously 

 shaken to obtain an accurate mixture of the germs, it having been 

 noticed that the majority of micro-organisms will very soon sink 

 to the bottom and easily escape observation. 



The water having been added to the gelatin, the tube is slowly 

 inclined up and down for a few times. The medium should then be 

 poured directly upon the plate. This plate must not be too small; 

 for, the greater the area, the more surely we shall succeed in separ 

 rating the germs and obtaining well-defined colonies. 



The details of the procedure to be applied in our examination of 

 water will now be readily understood. 



The water is placed in sterile Erlenmeyer's flasks, specially 

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