WOLFFIIUGEL'S APPARATUS 125 



sterile condition, only containing two or three germs per 

 cubic centimeter. 



Examination of water. For purposes of examination 

 i to 1 c.cm. is taken with a sterilised pipette and mixed 

 with sterile melted gelatine, which is then poured upon a 

 plate, and the development of the colonies carried on at the 

 temperature of the room. The number of islets formed is 

 then ascertained with the aid of a counting apparatus, and 

 in this way the relative value in micro-organisms of various 

 samples of water is determined. 



Supporting slab 



Glass plate engraved 



in divisions 

 Frame 



FIG. 45. WOLFFHUGEL'S COUNTIXG-PLATE. 



The counting apparatus (Wolffhiigel's counting-plate, 

 fig. 45) consists of a black slab upon which the plate with 

 the gelatine culture is laid, and over this is arranged a pane 

 of glass on which squares of uniform size have been 

 engraved. The islets in the individual squares are then 

 counted with the help of a lens, and an average struck, when 

 the number so obtained multiplied by the total number of 

 squares on the plate gives approximately the total number 

 of colonies for a certain area, a number which varies with 

 different kinds of water. The water to be used in this 

 experiment must not be kept, but must be examined imme- 

 diately after collection. In examining water presumably 

 rich in germs for example, that from rivers or ponds the 



