354 TEXT-BOOK OF BACTERIOLOGY. 



closed with a rubber cap above the cotton plug- for greater 

 safety. 



An accurately -measured quantity is now taken from the flask, 

 which has previously been well- shaken, and transferred into liquid 

 gelatin. This is done by means of graduated sterile pipettes, a dif- 

 ferent one being used for each portion. The quantity of water 

 to be tested is generally placed in two test-tubes, one having a 

 capacity of 1 c.cm. and the other -J c.cm. 



A double purpose is thus gained. Given a large quantity of 

 micro-organisms in the water, the colonies on the first plate may 

 develop in such dense masses that enumeration and examination is 

 out of the question, while the plate with half the quantity may still 

 give an available result. 



Besides, the second plate will serve to control the first, as it were, 

 for there will appear only half as many colonies; but even if this 

 should not always prove to be absolutely correct, striking devia- 

 tions from the rule will point to some mistake in the procedure. 



The water having been poured into the gelatin, the tube is 

 tipped to and fro and its contents at once poured out upon as large 

 a plate as possible. 



The germs will have grown into colonies after a few days and 

 examination may now be commenced. Should the number be 

 small, they may be counted with the naked eye. But the quan- 

 tities are sometimes so great that this simple procedure must 

 be abandoned and a special counting apparatus resorted to. A 

 glass plate, divided by a diamond pencil into small quadrangles, is 

 placed over the gelatin plate which rests upon a dark background 

 of black glass. 



The number of colonies developed within such a square is ascer- 

 tained by a magnifying glass; repeat this observation in six or 

 more squares, take the average number per square, and multiply 

 it by the number of the quadrangles corresponding to the extent 

 of the gelatin area. 



The number of germs, of course, varies, according to the water 

 investigated. River-water, especially if near thickly populated 

 districts, contains sometimes so many micro-organisms that even 

 one drop (=$ c.cm.) gives rise to several thousand colonies on the 

 plate. Temporal conditions are also of some influence, the figures 

 being higher in summer than in winter, etc. 



The species of micro-organisms found in water belong mainly to 

 the class of bacteria. Some of these have already been considered; 

 most of them are insignificant. Even pathogenic bacteria have 

 been ascertained in some cases by immediate investigation, as, 



