188 B A CTERIOL OGY. 



reason the drop of bouillon should be small and the 

 air-chamber relatively large. 



The cover-slip and hollow-ground slide should be 

 carefully sterilized, and with a sterilized platinum loop 

 a very small drop of bouillon is placed in the centre 

 of the cover-slip. The slip is then inverted over 

 the hollow depression in the sterilized object-glass and 

 sealed with vaselin. The most convenient method of 

 performing this last step in the process is to paint a 

 ring of vaselin around the edges of the hollow in the 

 slide, and then, without taking the cover-slip from the 

 table upon which it rests, invert the hollow over the 

 drop and press it gently down upon the cover-slip. The 

 vaselin causes the slip to adhere to the slide, so that it 

 can be easily taken up. The drop now hangs in the 

 centre of the small air-tight chamber which exists be- 

 tween the depression in the slide and the cover-slip. 

 (See Fig. 35.) 



A very thin drop of sterilized agar-agar may be sub- 

 stituted for the bouillon. It serves to retain the organ- 

 isms in a fixed position, and the process may be more 

 easily followed. 



As soon as finished the preparation is to be examined 

 microscopically and the condition of the organisms 

 noted. It is then to be retained in a warm chamber 

 especially devised for the purpose, and kept under con- 

 tinuous observation. The form of chamber best adapted 

 to the purpose is one which envelops the whole micro- 

 scope. It is provided with a window through which 

 the light enters, and an arrangement by which the slide 

 may be moved from the outside. The formation of 

 spores requires a much longer time than the germina- 



