THE MICROSCOPE AND MICROSCOPIC METHODS 53 



placed, face down, in a dish containing the stain; it is then left 

 for one to five minutes, warming slightly. 



(d) Wash in water. 



(e) Stain with aniline-water fuchsin, or carbol-fuchsin. 

 (/) Wash in water. 



(g) Dry. 



(ti) Mount in Canada balsam. , 



(According to Loffler, certain bacteria require the addition of 

 an acid solution, and certain others an alkaline solution, but many 

 observers consider this unnecessary.) 



Another and very valuable method is that of Van Ermen- 



(a) Make and fix cover-glass preparations as in the preceding 

 method. 



(b) Use the following mordant for one-half hour at room 

 temperature or for five minutes at 50 to 60 C. 



Osmic acid 2 per cent solution i 



Tannic acid 10 to 25 per cent solution 2 



(c) Wash carefully in distilled water and then in alcohol. 



(d) Place for a few seconds in a 0.25 to 0.50 per cent solution of 

 nitrate of silver "the sensitizing bath." 



(e) Without washing transfer to the "reducing and reinforcing 

 bath": 



Gallic acid 5 grams. 



Tannic acid 3 grams. 



Fused potassium acetate 10 grams. 



Distilled water 350 c.c. 



(/) After a few seconds, replace the preparation in the nitrate 

 of silver solution, in which it is kept constantly moving, till the 

 solution begins to acquire a brown or black color. Some recom- 

 mend leaving the preparation in the nitrate of silver solution for 

 two minutes in the first place, and in the reducing bath for two 

 minutes, without using the nitrate of silver solution a second time. 



