THE STAINTNG OF COVER-GLASS PREPARATIONS. 91 



stained there. The latter method of making the so- 

 called dry cover-glass preparations is as a rule much 

 to be preferred, and is continually employed in all 

 bacteriological work. 



It is necessary to mention that the mere rubbing 

 with a cloth is not sufficient to thoroughly cleanse the 

 cover-glasses. The best plan is to dust them, and 

 then to heat them thoroughly for a few minutes in 

 concentrated sulphuric acid ; after rinsing them in dis- 

 tilled water, they should be immersed in a mixture of 

 equal quantities of ammonia and alcohol, and after that 

 polished on a linen rag which is quite free from grease. 

 These thoroughly cleansed glasses must be preserved 

 as carefully as possible from dust ; this is best done 

 by placing them in a glass dish with a sliding cover. 

 Before use, the cover-glass should be taken up by 

 its outside edges with a pair of forceps, and drawn a 

 few times slowly through the flame of a Bunsen burner 

 or of a spirit lamp. The last traces of grease, which 

 cling persistently to the slide, are removed by this 

 strong heating. 



We now take with a sterilised platinum wire a small 

 portion of one of our pure cultures, care being taken 

 to choose one that is not too old, and mix it carefully 

 with a drop of distilled water upon a slide. 



A small portion of this bacterium water is next 

 spread out as flat and evenly as possible upon a cover- 

 glass, and then allowed to dry in the air. It is esseu- 



