72 METHODS OF CULTIVATION .OF BACTERIA. 



medium, and the latter plated and incubated. An ordinary 

 plate should 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. 26 is employed. 

 This consists of a sheet of glass ruled into squares as in- 

 dicated, and supported 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 medium is then 

 taken, and by a simple calculation the total number of 

 colonies present can be obtained. Plate-cultures in Petri's 

 dishes are sometimes employed for purposes of counting. 

 The bottoms of such dishes are, however, never flat, and the 

 thickness of the medium thus varies in different parts. If 

 these dishes are to be used, a circle of the same size as the 

 dish can be drawn with Chinese white on a black card, the 

 circumference divided into equal arcs, and radii drawn. 

 The dish is then laid on the card, the number of colonies 

 in a few of the sectors counted, and an average struck as 

 before. In counting colonies it is always best to aid the 

 eye with a small hand lens. 



The Bacteriological Examination of Water. This may 

 be undertaken with a view to finding either the number of 

 bacteria present or the varieties present. In either case a 

 small quantity (J to i c.c.) is taken in a sterile pipette and 

 added to a tube of gelatine, which is then plated and 

 incubated at the room temperature. In case of water taken 

 from a house tap, the latter should be made to run for 

 several hours before the sample is taken, as water standing 

 in pipes in a house is under very favourable conditions for 

 multiplication of bacteria taking place, and if this precau- 

 tion be not adopted, an altogether erroneous idea of the 

 number present may be obtained. In the case of the 

 examination of river water, the gelatine plates ought to be 

 prepared on the spot; at any rate the time elapsing 

 between the sample being taken and the plates being pre- 



