COVER- GLASS PREPARATIONS 87 



-desirable to quicken the staining process, as is necessary in 

 the case of some organisms, by using hot staining solution, 

 the cover-glass, well covered with the staining reagent, is 

 held by means of a pair of forceps over a low gas-flame 

 until steam just begins to rise from the liquid ; when this 

 happens, the source of heat is removed. This treatment is 

 then repeated at frequent intervals. A better method is to 

 float the cover-glass face downwards upon the staining 

 liquid, which has just previously been heated in a small 

 dish, until the steam begins to rise. Great care must be 

 taken not to allow the staining solutions to boil, as this 

 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. 



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. 



