GRAM'S METHOD OF STAINING 95 



second and third drops on the slide by drawing the bent 

 needle once, lightly across the cover-glass. // there is any 

 tendency of the smear to roll or " gather in drops," the cover- 

 glass should be discarded and a clean one substituted. This 

 is imperative. 



7. Allow the preparation to dry for about five min- 

 utes. 



8. Filter on each of the fixed smears enough of the mor- 

 dant to cover the cover-glass. 



9. Allow to stand about five minutes at room tempera- 

 ture. 



10. Wash off the mordant in a small stream of 

 water. 



11. Draw off the excess water from the edge of the cover- 

 glass by means of filter paper. 



12. Stain with anilin-water fuchsin or anilin-water gen- 

 tian violet for about five minutes, either cold or by warm- 

 ing somewhat over a low flame. 



13. Wash off the excess stain with clean water. 



14. Mount on a slide in water. 



15. Absorb the excess water with filter paper. 



16. Examine under the microscope. If the prepara- 

 tion has been successful, it may be dried and mounted 

 in balsam, 



EXERCISE 32. GRAM'S METHOD OF STAINING 



Certain organisms, when stained with anilin-water 

 gentian violet and afterwards treated with a solution of 

 iodin and washed in alcohol or anilin oil, give up the stain; 

 others retain the color when subjected to this process. 

 These latter organisms are said to be Gram-positive, those 

 losing the stain are Gram-negative. 



This phenomenon is interpreted by Benians to be due 

 to the possession of a definite cell-envelope which by the 

 action of iogiin is rendered impermeable to alcohol. His 



