86 APPLIED BACTERIOLOGY 



causes a precipitation of colouring matter which renders 

 the preparation useless. 



The stain should, as a general rule, be filtered just before 

 using, particularly gentian violet. The staining reagent is 

 allowed to act for from three to ten minutes, the time 

 varying according to the organism or particular stain being 

 operated upon. The cover-glass is then well rinsed in 

 running water until no more colouring matter comes away. 

 When the washing is found to be complete, the cover-glass 

 is held between the fingers and dried by very gentle 

 warming over a low flame, or, better, it is allowed to dry 

 spontaneously. Some workers prefer to use the actual 

 glass slip instead of cover-glasses for the staining process. 



A small drop of a thick solution of Canada balsam in 

 xylol is placed in the centre of a clean glass microscopic 

 slip, and the cover-glass, preparation downwards, deposited 

 on the drop of balsam, which then spreads out, and finally 

 extends over the whole under- surface of the cover-glass. 

 The preparation can now be observed by placing a drop of 

 cedar-oil on the top of the cover-glass, and examining with 

 the oil immersion lens. After examining, the cedar-oil on 

 the cover-glass is carefully absorbed with filter-paper. 



After a few days the balsam will harden, and become 

 very hard after a few weeks. 



If a permanent preparation is not required, the cover- 

 glass can be examined immediately after washing off the 

 excess of stain by placing on a glass slip, taking care to dry 

 the top surface of the cover-glass before applying th6 drop 

 of cedar-oil. 



Smear Preparations. — In cases where micro-organisms 

 are found in the blood and tissues of the body, their 

 presence may be demonstrated by making a smear pre- 

 paration. A drop of the blood is spread in a very thin 

 layer over a perfectly clean cover-glass, or ^it may be 



