88 



PATHOGENIC BACTERIA. 



their shape, make their outlines less distinct, break up 

 their arrangement, and disturb them in a variety of other 

 ways. Because of the possible errors of appearance re- 

 sulting from these causes, as well as because it must be 

 determined whether or not the individual is motile, in 

 making a careful study of a bacterium it must always be 

 examined in the living, unstained condition. 



The simplest method of making such an examination 

 would be to take a drop of the liquid, place it upon a 

 slide, put on a cover, and examine. 



While this method is simple, it cannot be recommended, 

 for if the specimen should need to be kept for a time 

 much evaporation takes place at the edges of the cover- 

 glass, and in the course of an hour or two has changed it 

 too much for further use. The immediate occurrence of 

 evaporation at the edges also causes currents of liquid to 

 flow to and fro beneath the cover, carrying the bacteria 

 with them and making it almost impossible to determine 

 whether the organisms under examination are motile or 

 not. 



The best way to examine living micro-organisms is in 

 what is called the hanging drop (Fig. 6). A hollow- 



FIG. 6. The "hanging drop" seen from above and in profile. 



ground slide is used, and with the aid of a small camel's- 

 hair pencil a ring of vaselin is drawn on the slide about, 

 not in, the concavity at its centre. A drop of the mate- 

 rial to be examined is placed in the centre of a large 

 clean cover-glass, and then placed upon the slide so 



