24 PRACTICAL BACTERIOLOGY. 



to be seen. T]\e water at the edge of the cover-glass 

 evaporates^ and thus different currents are continually 

 feeing created. The bacteria, which ard in the field of 

 vision, are swept along by these currents, and thus the 

 difiiculties of observation are much increased. In con- 

 sequence the following plan has been successfully 

 adopted for such observations : — A slide is chosen, in 

 the middle of which either a round hollow or depres- 

 sion has been ground, or better still, on to which a 

 glass ring has been cemented. A small drop of water 

 is now placed on a cover-glass, and the bacteria are 

 spread out in it ; the cover-glass is then turned over 

 and placed over the depression in the slide, so that the 

 drop of water hangs freely in the hollow space, and no- 

 where touches the bottom or the sides. A small drop 

 of olive-oil is then placed at the edges of the cover 

 glass, and this spreads itself out in a circle between it 

 and the slide, making the little hollow quite air-tight. 

 The drop of water is in this manner prevented from 

 evaporating, and in addition is protected from the 

 entrance of new germs from the outside, which, in 

 later experiments, is of great importance. In this 

 moist chwmber the development of the bacteria can be 

 very well observed ; and this method of culture, when 

 the drop of water has added to it a drop of nutrient 

 fluid, is called cultivation in the hanging drop. 



If some material from another colony, different per-, 

 haps in form or colour, be now placed in such a hang- 



