24 DAIRY BACTERIOLOGY 



with vaseline ; a drop of water is placed on the slide to keep the 

 air in the chamber moist, and to prevent evaporation of the drop 

 on the cover slip. The moist chamber is also employed in making 

 single cell cultures as follows : a miniature gelatine plate is made 

 on a special cover glass marked with numbered squares, and with 

 the aid of the microscope the most isolated cells are sought out 

 and their positions noted with respect to the markings on the 

 cover glass (see Fig. 23). The colonies which grow in these 

 positions may then be sown separately into suitable media. The 

 first yeast culture of undoubted purity was prepared in this vay 

 by Emil Christian Hansen, an achievement of far-reaching im- 

 portance in the brewing industry. The method is not adapted 



FIG. 24. A. Cornet's Forceps. B. Kilhne's Forceps. 



for the preparation of pure cultures of the bacteria, as these are so 

 much smaller than the yeasts. 



Bacteria may be seen more clearly if they have been stained 

 after fixation on the cover slip. Fixation is accomplished by 

 picking up the cover slip by the edges, between the thumb and 

 first finger, and twice drawing it slowly through a Bunsen flame ; 

 as long as the fingers are not burnt, the preparations will not be 

 overheated ; the bacteria are killed, and "on this account stain 

 better. If the culture medium is whole milk, the preparation 

 should be freed from fat by immersion in chloroform before 

 staining. The cover glass is immersed for a few minutes in the 

 staining solution in a small dish or watch glass, washed with pure 

 water, laid on the slide and dried on the surface by means of filter 

 paper ; it should be handled by means of the Kuhne's forceps 

 (Fig. 24). The stain may also be dropped on to the cover slip 

 held clamped by means of the Cornet forceps (Fig. 24). The 



