146 A MANUAL OF BACTERIOLOGY 



pressure breaks the unsupported cover-glass. It often 

 saves time first to centre the drop with the low power 

 before examining with the immersion lens ; an ink or 

 pencil dot at the margin of the drop aids focussing. The 

 light must be diminished, and artificial light is generally 

 preferable to daylight. The central parts of the drop 

 only should be examined, not the margin. 



Instead of hollow slides, various devices may be 

 employed to form the cell. Metal, glass, or vulcanite 

 rings, or rings cut out of thin sheet lead, tin-foil, card- 

 board, or two or three thicknesses of paper or filter -paper 

 may be cemented on to slides with vaseline, Hollis's glue, 

 gold size, or Canada balsam, or a thick ring of vaseline, 

 paraffin, or plasticine may be used. 



The hanging drop is the only certain method for ascer- 

 taining whether an organism is motile or not often an 

 important clue to its identification. Actively motile 

 organisms may frequently assume a non-motile resting 

 stage, although still alive, and various factors may bring 

 about this condition, such as old age, exhaustion of nutri- 

 ment, excessive heat or cold, electric shocks, and the like. 

 The absence of movement of an organism in a specimen 

 prepared from an ordinary culture, particularly if more 

 than a day or two old, does not necessarily prove that it 

 is non-motile. A hanging drop should be prepared from 

 a fresh glucose broth culture a few hours old and ex- 

 amined. If no motility is now observed younger and 

 older cultures should be similarly examined ; it is only 

 by repeated examinations that absence of motility can 

 be definitely proved. It is necessary to beware of two 

 fallacies in connection with motility not to mistake for 

 it the so-called Brownian movement, which is an oscilla- 

 tory one around a point, and common to all fine particles 

 suspended in a fluid ; and not to be misled by a flotation 

 of the cells due to currents set up in the fluid from some 



