The Study of Living Bacteria 173 



While this method is simple, it cannot be recommended, 

 as evaporation at the edges 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. Should 

 it be desirable that such a specimen be kept for a time, 

 so much evaporation takes place that in the course of 

 an hour or two it has changed too much to be of further 

 use. 



The best way to examine living micro-organisms is in 

 what is called the hanging drop (Fig. 32). A hollow-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 center. A drop of the material to be ex- 

 amined is placed in the center of a large clean cover-glass 



Fig. 32. The " hanging drop " seen from above and in profile. 



and then placed upon the slide so that the drop hangs in, 

 but does not touch, the concavity. The micro-organisms 

 are thus hermetically sealed in an air chamber, and appear 

 under almost the same conditions as in the culture. Such a 

 specimen may be kept and examined from day to day, the 

 bacteria continuing to live until the oxygen or nutriment 

 is exhausted. By means of a special apparatus (Fig. 33), 

 in which the microscope is placed, the growing bacteria may 

 be watched at any temperature, and very exact observa 

 tions made. 



The hanging drop should always be examined at the 

 edge, as the center is too thick. 



In such a specimen it is possible to determine the shape, 

 size, grouping, division, sporulation, and motility of the 

 organism under observation. 



