BACTERIA IN WATER 41 



means of a sterilised rod stroked into a thin, even film all 

 over the glass. It is then covered with a bell-jar and left 

 at rest to set. The level stage prevents the gelatine running 

 over the edge of the plate ; the iced water under the stage 



MOIST CHAMBER IN WHICH KOCH'S PLATES ARK INCUBATED 



expedites the setting of the gelatine into a fixed film. When 

 it is thus set the plate is placed upon a small stand in a 

 moist chamber, and the whole apparatus removed to the 

 room temperature incubator. A moist chamber is a glass 

 dish, in which some filter paper, soaked with corrosive sub- 

 limate, is inserted, and the dish covered with a bell-jar. By 

 this means the risks of pollution are minimized, and moisture 

 maintained. In all cases at least two plates must be pre- 

 pared of the same sample of water, and it is often advisable 

 to make several. They may be made with different media 

 for different purposes, and with different quantities of water, 

 though the same method of procedure is adopted. In a 

 highly polluted water extremely small quantities would be 

 taken, and, vice versa, in pure water a large quantity. 



When we come to discuss the relation of disease organ- 

 isms to water, particularly those causing typhoid fever, we 

 shall learn that they are both scarce and intermittent. This 

 point has been dwelt upon frequently by Dr. Klein, and it 

 is clear that such a state of things greatly enhances the diffi- 

 culties in detecting such bacteria, and he has proposed a 

 simple procedure by which the difficulty of finding the 

 Bacillus typhosus in a large body of water may be met. 



