72 METHODS OF CULTIVATION OF BACTERIA 



may be placed under the microscope. If necessary, it may be first 

 incubated and then examined on a warm stage. (2) The sterile 

 cover-glass is placed on a sterile plate (an ordinary glass plate 

 used for plate cultures is convenient). The drop is then placed 

 on its upper surface, the details being the same as in the last 

 case. The edge of the cell in the slide is then painted with 

 vaseline, and the slide, held with the hollow surface downwards, 

 is lowered on to the cover-glass, to the rim of which it of course 

 adheres. The slide with the cover attached is then quickly 

 turned right side up, and the preparation is complete. 



In the case of B, the drop of fluid is placed on the centre of 

 the table x. The drop must be thick enough to come in contact 

 with the cover-glass when the latter is lowered on the slide, and 

 not large enough to run over into the surrounding trench y. 

 The cover-glass is then lowered on to the drop, and vaseline is 

 painted along the margin of the cover-glass. It is sometimes 

 convenient for the observation of the growth of bacterial colonies 

 or of fungi to make hanging-drop cultures with a solid medium. 

 This can be done by substituting a drop of melted gelatin or 

 agar for bouillon. The method of microscopic examination is 

 described on page 92. 



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 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 appa- 

 ratus such as is shown 

 in Fig. 28 is employed. 

 This consists of a sheet 

 of glass ruled into 

 squares as indicated, and 

 supported by its corners 

 on wooden blocks. The 

 table to which these 

 FIG. 28. Apparatus for counting colonies, 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 



