THE COUNTING OF COLONIES. 77 



Method of use : Fix up the apparatus as shown above, 

 the screws being just tight enough to keep the parts in 

 position, and sterilise in the steam steriliser. Screw up 

 more firmly so as to make the rubber bulge slightly. Fill 

 a hypodermic syringe with some sterile glucose bouillon, 

 push the needle through the rubber d and, tilting the point 

 of the needle against the glass <r, slowly inject enough to 

 form a drop on the under surface of c. Withdraw the 

 syringe and inoculate its point with the bacterium, again 

 introduce and inoculate the drop. Pass hydrogen through 

 one of the tubes for fifteen minutes, close the ends of the 

 tubes, and incubate at the required temperature. The 

 apparatus can be put on the stage of a microscope and 

 examined from time to time. 



The Counting of Colonies. An approximate estimate of 

 the number of bacteria present in a given amount of a 

 fluid (say, water) can 

 be arrived at by 

 counting the number 

 of colonies which 

 develop when that 

 amount is added to 

 a tube of suitable 

 medium, and the 

 latter plated and 

 incubated. An or- 

 dinary plate should FIG. 28. Apparatus for counting colonies, 

 be used in such a 



case, and the medium poured out in as rectangular a 

 shape as possible. For the counting, an apparatus such 

 as is shown in Fig. 28 is employed. This consists of a 

 sheet of glass ruled into squares as indicated, and sup- 

 ported by its corners on wooden blocks. The table to 

 which these blocks are attached has a dark surface. The 

 plate -culture containing the colonies is laid on the top of 

 the ruled glass. The numbers of colonies in, say, twenty 

 of the smaller squares are then counted, and an average 

 struck. The total number of squares covered by the 



