THE MICROSCOPE AND MICROSCOPIC METHODS 45 



(b) Gram's solution, one and one-half minutes: 



Iodine , i gram. 



Potassium iodide 2 grams. 



Water 300 c.c. 



In this solution the preparation becomes nearly black. 



(c) Wash in alcohol repeatedly; the alcohol becomes stained 

 with clouds of violet coloring matter; the alcohol is used as long 

 as the violet color continues to come away, and until the prepara- 

 tion is decolorized or has only a faint steel-blue color. 



(d) When desired, the specimens may be stained, by way of 

 contrast, with a watery solution of Bismarck brown, dilute 

 fuchsin or eosin. 



(e) Wash in water, and examine either in water directly or 

 after drying and mounting in Canada balsam. Gram's method 

 and its modifications should not be regarded as absolute means 

 of distinguishing between Gram-positive and Gram-negative bac- 

 teria in every case, as much depends upon the condition of the 

 bacteria, and very much upon the technic of staining. When the 

 Gram stain is used for diagnosis, it is well to put a smear of a 

 known Gram-negative and a smear of a known Gram-positive 

 organism on the same slide or cover-glass along with the un- 

 known, and subject them all to the same technic. 



Some bacteria that are stained by Gram's method : 

 Staphylococcus aureus, 

 Streptococcus pyogenes, 

 Micrococcus lanceolatus (of pneumonia), 

 Micrococcus tetragenus, 

 Bacillus of diphtheria, 

 Bacillus of tuberculosis, 

 Bacillus of leprosy, 

 Bacillus of anthrax, 

 Bacillus of tetanus, 



Bacillus welchii ( aerogenes capsulatus), 

 Ray fungus of actinomycosis. 



